Soft Tissue Tumors: Molecular Pathology and Diagnosis

Abstract

Tumors of mesenchymal origin, also called soft tissue tumors, include tumor from muscle, fat, fibrous tissue, vessels and nerves, which are a group of heterogeneous neoplasms, and accounts for about 1% of all malignant tumors. They are uncommon tumors in routine practice, with complex tumorigenesis. Due to the recent advance in molecular pathology, we got a major achievement in the understanding of these tumors at the gene level, which makes the diagnosis and prognosis of this type of tumor more accurate and comfortable. This chapter will cover some molecular pathology and diagnosis of soft tissue and bone tumors.

1. Introduction

Compare to the epithelium-originated carcinomas, the incidence of malignant mesenchymal tumors, refers to as sarcomas, are much less, which account for approximately 1% of human malignancies. The terminology to descript those tumors is usually related to the tissue origin of those tumors, such as the malignant tumors from fibrous tissue is called fibrosarcoma, while the malignant tumor from bone is called osteosarcomas. Each sarcoma usually has some histological subtypes, which may present different clinical courses and prognoses. Like other tumors, pathological diagnosis is the key point for the management of those tumors. However, pathology diagnoses of mesenchymal tumors sometimes are challenge because of the rarity and the histological diversity of this group of tumors [1], and most of the time, the diagnoses are performed by senior pathologists or specialty-fellowship-trained pathologists, in some cases, outside expert consultations are necessary. In addition, clinical information is important for diagnosis, like patient’s age, tumor location, and especially the radiology imaging studies. Some sarcomas have their favorite age group and location, for example, liposarcoma often occurs in elderly patients’ deep thighs, and retroperitoneum is a favorite location for at least three sarcomas, they are well-differentiated liposarcoma, dedifferentiated liposarcoma, and fibrosarcoma. This information is very helpful for making correct diagnoses. Since some soft tissue and bone tumors are with the characteristic radiological presentation. It also provides a very important diagnostic clue for pathologists. For example, radiology for osteosarcoma shows mixed lytic (bone destructive) and blastic (new bone formation) features with invasive and destructive intraosseous mass and classic Codman triangle, while chondrosarcomas likely occur on pelvic bones, femurs and humerus, and show on radiology a unique popcorn-like calcification (Figure 1). Thanks to the recent massive advance in molecular biology, our understanding of these tumors has moved to a new level, which significantly affects both diagnosis and treatment of those neoplasms. For example, 30 years ago, the treatment of choice for osteosarcoma, a malignant bone tumor primarily affects teenagers and young adults, was amputation of the leg in order to prevent fatal lung metastasis. However, currently, the first-line treatment for this tumor is chemotherapy, which not only controls the tumor growth by preventing lung metastasis but also saves the patient’s leg with improved patient’s quality of life. Molecular pathology of soft tissue tumors includes tumor suppressor genes, oncogenes, growth factors, and their receptors as well as DNA methylation, which are closely associated with the behavior of the soft tissue tumors. Chromosomal analyses, molecular cytogenetics (Fluorescent in situ hybridization, [FISH]) and molecular assays (RT-PCR and NGS) may become increasingly useful in our routine practice, providing important diagnostic, prognostic, and even therapeutic information and leading to new insights and approaches into the classification and treatment of those tumors. A small group of morphologically designed tumors called “small blue round cell tumors”, include Ewing/peripheral neuroectodermal family of tumors, rhabdomyosarcoma, neuroblastoma, and lymphoma. The accurate diagnosis can be made by genotypic technique, such as FISH; by using specific probes of interphase nuclei can allow identification of tumor-specific chromosome changes in those sarcomas. We hope that this new knowledge of genetic events will guide us toward the more rational and successful development of new therapies for soft tissue tumors [2]. Gene mutation and alteration may produce some specific biomarkers, which can be detected by commonly used methods, like IHC, FISH, PCR and NGS. Understanding these genetic abnormalities and the methods to detect them, are very useful for making an accurate diagnosis in our practice. In addition, the diagnoses of some common and uncommon soft tissue and bone tumors were reviewed here.

Figure 1.

Chondrosarcoma’s characteristic popcorn-like calcification (A). A unique Codman’s triangle in osteosarcoma on imaging study is helpful for making a diagnosis (B, arrows).

2. Molecular studies of tumorigenesis of soft tissue tumors

Similar to most neoplasms, inherited and environmental risk factors are the major players in the development of soft tissue and bone tumors. The etiology of soft tissue and bone tumors is multifactorial and largely unknown. Some tumors have a genetic background, while other tumors may have both environmental and preexisting conditions as major etiologic factors. In addition, many soft tissue and bone tumors have both genetic and environmental factors interact to play a synergistic role to cause the neoplasm. For example, patients with familiar retinoblastoma have a higher incidence of bone and soft tissue sarcomas. Familiar osteochondromas and fibrous dysplasia can also be complicated by osteosarcoma. Both are due to Rb gene mutation. Rb gene is located in 13q14, and its mutation is associated with the development of at least following tumors, such as malignant fibrous histiocytoma (MFH), leiomyosarcoma, rhabdomyosarcoma, fibrosarcoma, liposarcoma, Ewing sarcoma, osteosarcoma, and chondrosarcoma [2]. The other tumor suppressor gene, p53, when it becomes mutated, has similar results for developing those sarcomas listed above [2].

Several chemicals can induce soft tissue sarcoma, but those chemicals’ tumorigenesis effect is sometimes under genetic control. Ionizing radiation represents a prototype carcinogenic agent and was first recognized for the induction of osteosarcoma [2]. Other soft tissue tumors associated with radiation include MFH, extraskeletal tumors of bone and cartilage, fibrosarcoma, hemangiosarcoma, and neurofibrosarcoma [2] as well as meningiomas and gliosarcomas. In addition, soft tissue tumors may also develop during the period of immunosuppression, which can be associated with chemotherapy for hematological disorders, genetic diseases, or iatrogenic (pharmacologic) treatments in order to prevent graft rejection after organ transplantation, such as the steroid treatment after the organ transplantations. Moreover, those preexisting bone diseases, like bone infarcts, chronic osteomyelitis, and Paget’s disease are all associated with an increased incidence of bone tumors [2].

3. Tumor suppressor genes

The p53 gene, the first identified tumor suppressor gene, is located in 17p13.1. Mutations generating defective P53 may represent early steps of carcinogenesis in many neoplasms, including soft tissue and bone tumors. For example, mutations of p53 have been detected in MFH, liposarcoma, leiomyosarcoma, angiosarcomas, fibrosarcoma, synovial sarcomas, Ewing’s sarcoma, rhabdomyosarcoma, chondrosarcomas, osteosarcomas, and other tumors of bone, and even some brain tumor like astrocytomas. It has been further suggested that overexpression of mutated p53 (P53) is associated with a less-differentiated phenotype and more aggressive behavior in tumors of soft tissue and bone tumors. p53 mutation can be easily detected by immunohistochemical (IHC) stain as nuclear stain, which is assumed to be the result of p53 gene mutation leading to a prolonged P53 half-time [3, 4].

The retinoblastoma (Rb) tumor suppressor gene is located on chromosome 13q14, and encodes a 105-kd nuclear phosphoprotein, which plays an important role in the regulation of cell proliferation. Mutation at the Rb locus or genetic alterations that lead to the production of malfunctioning Rb protein has been detected in osteosarcoma, MFH, liposarcoma, leiomyosarcoma, fibrosarcoma, and spindle cell sarcoma. It has also been the primary event during sarcoma development. Again, immunohistochemical analysis of Rb expression could serve as a screening step for a more specific analysis of molecular alteration of the Rb gene, since the complexity of the Rb gene and its product, as well as the random pattern of its point mutation, prevents us from fully understanding their role in tumorigenesis [5].

In addition, other tumor suppressor genes involved in the development of bone and soft tissue tumors include the p21, p16, and p18 genes. Those genes’ products have primarily involved in the regulations of cyclin-dependent protein kinases (Cdk). The mutations of these louses have been detected in osteosarcomas, leiomyosarcoma, and other soft tissue tumors. However, the specific role of p21, p16, and p18 genes in the development of bone and soft tissue tumors requires further research [2].

4. Oncogenes and related genes

Overexpression of several oncogenes has been reported in tumors of soft tissue and bone. The genes coding for nuclear transcription factors includes myc, myb, gli, and fos [6, 7, 8, 9]. The c-myc and c-myb are nuclear phosphoproteins that stimulate cell proliferation (c-myc) or inhibit cell differentiation (c-myb) by binding to specific DNA sequence [9]. Myc protein has a transcriptional activation domain, a DNA binding domain, a nuclear localization signal, a helix-loop-helix motif and a leucine-zipper motif, which allow for the formation of dimers necessary for transcriptional activity [9]. Amplification, alteration, or increased expression of c-myc oncogene has been detected in osteosarcomas, soft tissue sarcomas, and MFH [6, 7, 8, 9]. Increased expression of c-myb gene was found in some soft tissue sarcomas [8, 9], while c-fos DNA amplification was detected in liposarcomas but not in other types of soft tissue tumors [8].

The oncogene of ras family is another most common group of oncogene involved in the development of both carcinomas and sarcomas. It contains isoforms of HRAS, KRAS, and NRAS [6, 7]. Oncogenetic activation of these proteins owing to missense mutations and these proteins control a complex molecular circuitry that consists of a wide array of interconnecting signal pathways to affect multiple cellular processes and drive tumorigenesis. KRAS mutations are most frequently detected in colorectal tumors, lung cancer (mostly non-small cell lung cancer (NSCLC) and was detected in at least 30% of adenocarcinomas of the lung), and in pancreatic carcinomas; HRAS mutation are associated with tumors of the skin and the head and neck; and NRAS mutations are common in hematopoietic malignancies. In addition, mutations or changes in expression of ras genes were detected in a few sarcomas, including embryonal, alveolar, and pleomorphic rhabdomyosarcomas, leiomyosarcoma, MFH, and angiosarcoma [7, 10].

5. Growth factors and their receptors

Proliferation and differentiation of mesenchyme or epithelium-derived cells are coordinated by peptide growth factors that activate corresponding receptors expressing tyrosine kinase function in multiple patterns [11, 12]. Within tumor cells, growth factors can be produced by different cells, including malignant cells themselves; by stromal elements, including fibroblasts, endothelial cells, and immune cells; or they can be released from carrier circulating cells, such as platelets. Since both malignant and normal cellular components of tumor express growth factors and corresponding receptors, a complex regulatory network is formed, which include the function of regulation of tumor cell growth and proliferation [2].

The growth factor and their receptors protein are transmembrane growth factor receptors that function to activate intracellular signaling pathways in response to extracellular signals. They include platelet-derived growth factor (PDGF), epidermal growth factor (EGF), transforming growth factor (TGF)-α and –β, fibroblast growth factor (FGH), insulin, and insulin-like growth factor (IGF), vascular endothelial growth factor (VEGF), and Hepatocyte growth factor/scatter factor (HGF/SF) [12, 13, 14, 15, 16]. The common traits of these receptors are the presence of an extracellular ligand-binding domain, a single transmembrane region, a cytoplasmic portion with a conserved protein tyrosine kinase domain, and regulatory sequences that are subject to autophosphorylation or phosphorylation and dephosphorylation by exogenous kinase and phosphatases [10, 12, 14, 15]. Regulation of signal transduction by receptor tyrosine kinase involves ligand-induced receptor reorganization, which increases the ligand binding and signals transduction capacity [14]. Altered growth factor signaling caused by genomic amplification or mutations is oncogenic and had been observed in multiple malignancies.

Insulin, besides its well-known metabolic effects, can act as a growth factor directly or synergistically with other growth factors. The insulin signal is translated into phenotypic effect by the insulin receptor, which is a disulfide-linked tetramer with 2 α and 2 β subunits. The insulin receptor has serine, threonine, and tyrosine kinase activities and can autophosphorylate the tyrosine residues on the C-terminal of the β subunit [17].

Insulin-like growth factor-I (IGF-I) and IGH-II belong to a small family and are related proteins, composed of single-chain polypeptides that play important roles in growth, development, and metabolism. They are produced mainly by the liver and regulated by the pituitary growth hormone (GH). The IGF-I receptor is structurally similar to the insulin receptor and is composed of two α and two β subunits that have tyrosine kinase activities [15, 18]. The IGF-II receptor is a transmembrane protein of 270 kd with a small cytoplasmic domain that lacks kinase activity [9, 17]. Insulin and IGFs can cross-interact with corresponding receptors, although with lower affinity than the native ligand [18]. Insulin-like growth factors are produced by the liver and peripheral tissues, including tumor stroma and malignant cells [10, 18]. IGF-1 and IGF-II were detected in some sarcomas, like Ewing sarcoma, PNET, and osteosarcoma [13, 12, 18].

Human epidermal growth factors (EGFs) and their receptors are well known to be involved in many neoplasms, from lung and breast cancers to malignant brain tumor like primary glioblastoma.

The EGF family includes EGF and TGF-α [10, 12]. EGF is a peptide growth factor of approximately 6 kd, which is synthesized as part of a large protein that is processed to mature EGF and EGF-like polypeptides. These growth factors and their receptors can regulate the proliferation of mesenchymal and epithelial cells, including corresponding sarcomas and carcinomas, through a signal transduction pathway [8]. Another example of EGF-R mutation in the development of malignant neoplasm can be seen in primary glioblastoma. A WHO grade 4 (highest-grade) malignant brain tumor often occur in elderly patients without previously existing low-grade tumor. Studies show in this tumor, the EGF-R gene is often mutated and amplified in primary glioblastomas, referred to as EGFRvIII (EGFR variant III), which results in over-expression of the EGF-R protein. EGF-RvIII plays an important role in tumorigenesis by activating Mitogen Active Protein Kinase (MAPK) and Phosphoinositide-3-Kinase (PI3K-Akt) pathways, EGF-RvIII mutation is characterized by in-frame deletion of exons 2–7, resulting in a truncated extracellular domain with the inability to bind a ligand but retains ligand-independent constitutive activity and produce tonic activation of the pathway to promote tumor cell proliferation, which makes this tumor one of the most challenge neoplasms in the treatment by neuro-oncologists.

The role of PDGF in the growth of soft tissue and bone tumors is well recognized [12]. PDGF can be transported to the tumor site by circulating platelets or can be produced locally by endothelial or immune cells in the tumor. PDGF consists of a group of disulfide-bonded homodimers and heterodimers of A and B chains. During tumoral transformation, the requirement for PDGF diminishes significantly, because in some systems the tumor cells can produce PDGF-like molecules [10]. For example, fibrosarcoma, osteosarcomas, and glioblastomas produce PDGF-like factors [12].

Fibroblast growth factor receptors (FGFRs) have a family of 4 (FGFR1–4) highly conserved transmembrane receptor tyrosine kinases, and an additional receptor (FGFR5, also known as FGFRL1) binds fibroblast growth factor (FGF) ligands but without an intracellular kinase domain. The FGF signal pathway has been implicated in oncogenesis, tumor progression, and resistance to chemotherapy in many tumors [12]. They share significant sequence homology and overlap in their binding specificities for different FGFRs.

The VEGF/VPFs and SF/HGFs play a crucial role in normal and tumor tissue’s angiogenesis [15, 16, 19]. The degree of vascularization in a tumor has a profound effect on its growth and behavior, such as in benign tumors of hemangioblastoma and malignant tumors of renal cell carcinomas and glioblastomas [15]. It has been proposed that activation of the SF-c-met receptor system is important for the development of Kaposi sarcoma, a human herpesvirus 8 induced, an HIV and AIDS-related angiosarcoma [16, 20, 21]. The important role of vascularization in tumor development is emphasized by a potent antitumor effect of angiogenesis inhibitors, including angiostatin and endostatin [19, 22]. Some VEGF-target inhibitors have been developed, such as sorafenib and sunitinib, and applied clinically to treat those tumors with prominent vascular supply.

Oncogenes coding for growth factors or their receptors, or non-receptor tyrosine kinases, might correlate in some tumors with increased malignant potential and poor prognosis. A large fraction of osteosarcoma, approximately 42%, express the Erb-B2 protein and its expression correlates with malignant behavior, although no significant alteration in the erb-B2 gene was found in those tumors so far [8].

6. DNA methylation in soft tissue and bone tumors

DNA epigenetic modifications (post-translational modifications), such as methylation are important regulations of tissue differentiation, contributing to processes of development in both normal tissue and malignant tissue. That is strongly associated with gene expression regulation. In that way, DNA methylation patterns reflect both the cell of origin and gene expression changes associated with different tumor types [23]. Current studies also indicate that DNA methylation status may affect prognosis in tumors treated with chemotherapy, such as glioblastomas, a type of malignant brain tumor. Furthermore, a new classification of sarcomas based on their DNA methylation has been proposed [24, 25].

6.1 Ewing’s sarcomas and small round cell neoplasms and their chromosomal translocations

With traditional histological approaches, it is often difficult to classify a group of tumors that have been categorized by morphology as “small round cell tumors” or “small round blue cell tumors” (Figure 2). Those tumors include Ewing’s sarcoma, peripheral primitive neuroectodermal tumors (pPNETs), rhabdomyosarcoma, lymphoma, desmoplastic small round cell tumor, small cell lung cancer, and neuroblastoma, characterized histologically by small round nuclei with scanty cytoplasm, active mitoses and apoptosis, some are with nuclear molding, suggestive of neuronal origin, because their homogeneous appearance by light microscopy examination and their frequent lack of organ specificity, making a diagnosis a challenging work during routine practice. An accurate diagnosis is essentially important for treatment and prognosis because each tumor is biologically different. Fortunately, with the greatest advance in molecular biology technique and each of those above tumor’s distinctive chromosomal abnormalities as well as special protein production, identification of those abnormalities became an important method for establishing an accurate diagnosis. Those chromosomal anomalies can be detected by conventional karyotyping, FISH with specific probes from loci spanning or flanking the translocation breakpoints and RT-PCR analysis, and even NGS [26, 27, 28, 29].

Figure 2.

Gross picture of Ewing’s sarcoma shows the tumor stars from bone marrow and extends to the soft tissue around the bone (A). Microscopically, Ewing’s sarcoma is a classic small round blue cell tumor (B).

Ewing’s sarcomas, a primarily children’s bone tumor, while in adults, it occurs more commonly in soft tissue. Ewing’s sarcoma is the second most common primary bone sarcoma, after osteosarcoma. Extraskeletal Ewing’s sarcoma usually affects the deep soft tissue of the trunk and extremity, but unusual sites such as head and neck or retroperitoneum have also been reported [30].

Ewing’s sarcoma affects children, adolescents, and young adults, with most cases occurring in the second and third decades of life. The medium age at diagnosis ranges from 13 to 19 years. The few patients older than 30 years have a similar spectrum of EWSR1-ETS fusions. The femur is the most common location of this tumor and on gross examination, the tumor usually occupies the medullary (central portion of bone/bone marrow) space with an expending into periosteal soft tissue. Radiologically, the tumor shows a prominent periosteal reaction and forms an “onion skin” like reaction (Figure 2).

Microscopically, most tumors are composed of a striking uniform cytomorphology, with round nuclei showing smooth nuclear contour and vesicular fine chromatin. The tumor cells are arranged in solid sheets and show ill-defined cell borders with scant, often clear cytoplasm, nuclear molding is hard to see (Figure 2). The tumor cells are positive for CD99 by immunohistochemical (IHC) stain that is usually used as the first line of screening test of diagnostic workup.

Ewing’s sarcoma (a tumor of unknown histopathogenesis) is with a unique chromosomal rearrangement, t (11; 22)(q24;q12)(EWS/FLI1), which was also found in peripheral primitive neuroectodermal tumors (pPNET, a tumor of neural origin), suggesting a similar mechanism of tumorigenesis and a tissue of origin for these distinct clinicopathological entities. The 11;22 translocation or variants, therefore, are detectable cytogenetically or by molecular approaches in more than 95% of Ewing’s sarcoma and pPNETs [28, 29]. Approximately 5% Ewing’s sarcoma and pPNETs are with slightly differently chromosomal translocation and are referred to as “cytogenetic variant translocation”, which is not uncommon due to the tumors’ heterogenesis. These variants exhibit rearrangements of 22q12 with a chromosomal partner other than 11q24. An example is the 21;22 translocation [t(21;22)9q22;q12)(EWS/ERG)]. At least four variant translocations (including EWS/ERG) have been identified cytogenetically and molecularly [26, 27, 28, 29].

Recent studies suggest that identification of the 11; 22 translocation of EWS/FLI1 fusion transcript is not only useful diagnostically in Ewing’s sarcoma and pPNETs but also affects the prognosis of tumors. Two large clinical studies have been shown that the most common type of EWS/FLI1 fusion, seen in approximately two-thirds of cases, is associated with significantly better survival [5, 28, 29].

Specific nucleic acid sequences can be shown in individual metaphase and interphase cells by specially designed chromosome-specific probes and fluorescence in situ hybridization (FISH). This technique can be applied on fresh or aged samples (such as blood smear or cytological touch preparations), paraffin-embedded tissue sections, and disaggregated cells retrieved from fresh, frozen or paraffin-embedded material. FISH is usually the same day or overnight procedure with rapid turn-around time (TAT), depending on the probes utilized. Paraffin-embedded material, however, may require more prolonged pretreatment for deparaffin, which may be resulting in a slightly longer turn-around time [26].

FISH is a valuable technique for detecting chromosomal rearrangements in bone and soft tissue tumors. Those tumors/sarcomas have prominent and unique chromosomal translocations. These chromosomal translocations indicate an exchange of chromosomal material between two or more chromosomes, can be visualized in interphase cells by the use of site-specific probes labeled with different colored fluorescent dyes. Bicolor FISH with translocation breakpoint” flanking” or “spanning” cosmid probes (labeled unique sequences using large-insert probes) or whole chromosome paint probe can be used diagnostically. A significant advantage of FISH is it is not dependent on the procurement of metaphase cells and can be performed on the tissue of limited quality such as cytological touch preparations. With these approaches, cryptic or masked translocations can be identified. This highly sensitive and reliable technology has been adopted into the routine practice of many clinical pathology laboratories, and as an effective alternative to reverse transcriptase polymerase chain reaction (RT-PCR) analysis. TAT usually takes a few days [26].

7. Reverse trabscriptase polymerase chain reaction (RT-PCR)

RT-PCR makes it possible to amplify RNA, usually mRNA. For TR-PCR, RNA extracted from the tissue is purified and then converted into cDNA by reverse transcriptase, an enzyme that transcribes RNA into single-stranded complementary DNA. The cDNA then serves as the template in a conventional PCR.

This technique can be used to detect chimeric or fusion genes created by translocation events such as the 11;22 translocation [t(11;22)(q24;q12)] in Ewing’s sarcoma. Ideally, snap-frozen tissue is preferred for RNA extraction and RT-PCR analysis. However, this procedure can also be performed on archival (formalin-fixed and paraffin-embedded) material if RNA is of sufficient quality [26].

RT-PCR analysis is a very sensitive method. It may allow for the detection of abnormalities present in cells too few to be detectable by traditional cytogenetic or FISH methods. It can detect the sarcoma-specific fusion transcripts, and is also capable of consistently detecting one t(11;22)-bearing tumor cell among 10⁶ non-t(11;22)-bearing cells [31]. Therefore, the RT-PCR technique has been considered a potential method for monitoring minimal residual disease clinically in patients undergoing sarcoma therapy or in identifying a micro-metastatic disease by testing circulating tumor cells in the bloodstream [32, 33, 34, 35, 36].

This assay could become a clinically useful test for the early identification of patients who may benefit from alternative therapy or who may be spared possible overtreatment [32]. For example, several studies have been conducted at examining peripheral blood and/or bone marrow specimens of patients with Ewing’s sarcoma/pPNET, rhabdomyosarcoma and myxoid liposarcoma for molecular evidence of circulating tumor cells at the time of diagnosis [32, 33, 34, 35, 36, 37]. RT-PCR detection of circulating Ewing’s sarcoma or pPNET cells in 23 patients (all with the clinically localized disease in a study performed by de Alava [34]) is in accordance with the markedly poor outcome of surgery alone for Ewing’s sarcoma or pPNET. In addition, preliminary studies suggest that the minimal marrow involvement by Ewing’s sarcoma or pPNET cells can be detected by RT-PCR is it may be associated with a poor clinical prognosis [35, 36].

Alveolar rhabdomyosarcomas are another tumor characterized by specific chromosomal translocations that appear to have a relationship with clinical behavior [38]. Most alveolar rhabdomyosarcomas exhibit one of two chromosomal translocations: t(2;13)(q35;q14) associated with a PAX3/FKHR fusion transcript or t(1;13)(q36;q14) associated with a PAX7/FKHR fusion transcript [23]. The 2;13 translocations has been observed in approximately 75% of alveolar rhabdomyosarcomas examined and the 1;13 translocations in 10%. A comparison study of the clinical features of 18 patients with PAX3/FKHR alveolar rhabdomyosarcomas and 16 patients with PAX7/FKHR alveolar rhabdomyosarcomas showed that PAX7/FKHR group had better overall survival and significant longer tumor-free survival time. These findings suggest that, similar to Ewing’s sarcoma and pPNET, an association with fusion transcript type and distinct tumor clinical behavior may exist in alveolar rhabdomyosarcomas [38, 39].

In our routine practice, immunohistochemistry (IHC) plays an important role in diagnoses. For example, Ewing’s sarcoma typically shows strong and diffuse membrane expression of CD99 by IHC stain, which recognizes the p30/32 MIC2 glycoprotein on the membrane of the tumor cells, which is generally not observed to this extent in other tumors; this pattern is therefore relatively specific for Ewing’s sarcoma. So CD99 IHC stain is considered highly sensitive for Ewing’s sarcoma, which is used as a first step screening test in practice. However, as shown recently, nuclear expression of the transcription factor NKX2.2 is found in about 95% of Ewing’s sarcomas but is also expressed in a subset of histologically mimics such as mesenchymal chondrosarcoma (in 75% of cases) and is therefore not specific for Ewing’s sarcoma any more. However, the combination of CD99 and NKX2.2 is still useful in diagnosis [1]. However, these tiny IHC stain differences (perinuclear vs. nuclear stain) can be recognized by experienced pathologists.

7.1 Solitary fibrous tumor (SFT) vs. Hemangiopericytoma (HPC)

Used to be two separate dura-based mesenchymal tumors, which were considered as two subtypes of meningiomas, and they are nowadays considered as one tumor with different presentations. As SFT is at the end of benign tumor and hemangiopericytoma (HPC) as a more aggressive tumor. Both tumors are CD34 and STAT-6 immunoreactive spindle cell tumors origin from the dura, the cover of the brain and spinal cord, with only minor histologically difference, as SFT is with small and thin-walled vessels and HPC with large and branched vessels, with “staghorn” features.

SFT is a typically well-circumscribed and CD-34 immunoreactive fibroblastic tumor, origins from the pleura as solitary fibroblastic mesothelioma. Although it is a benign tumor, SFT has a high propensity for recurrence and metastasis.

SFTs affect men and women equally and are most common in adults between 40 and 70 years old. SFTs may occur at any anatomic location, including superficial and deep soft tissue and visceral organs and bone; they are more common at the extrapleural location, like the abdominal cavity, the pelvis, or the retroperitoneum. Most SFTs present as a mass lesion with associated clinical presentation. Abdominopelvic tumors may present with distention, constipation, urinary retention, or early satiety, whereas head and neck SFTs may present with nasal obstruction, voice changes, or bleeding. In rare cases of SFT occurs intraventricularly, in that cases, hydrocephaly may be present. Radiology examination shows SFTs a well-demarcated, multi-lobular mass lesion, with heterogeneous contrast enhancement by MRI scan due to the extensive vascular component in the tumor.

SFTs, microscopically, the tumor cells consist of uniform spindled cells in a variably collagenous deposition or occasionally myxoid matrix, in which cellular and hyalinized areas alternate in irregular patterns (Figure 3). A hemangiopericytoma-like vascular feature with a complex or staghorn-like profile is present in most cases, at least in some areas. The tumor cells have indistinct cytoplasm and oval nuclei, usually with inconspicuous nucleoli. Mitotic activity is usually low in most cases. Put here as the World Health Organization (WHO) grading system currently used. The current WHO classification gave SFTs 3 grades by histological evaluation based on mitotic activity. Specifically, SFTs with fewer than five mitoses per 10 high-power fields are considered grade 1, while a mitotic count of five or more justifies a grade two designation. If, in addition to this elevated mitotic count, necrosis is present, then a grade three designation is assigned. There have been reported some rare histological patterns in SFTs, including lipomatous, papillary, giant cell, and myxoid, but these changes do not affect prognosis.

Figure 3.

SFT low power view shows spindle cell neoplasm H&E stain ×100 (A) SFT, small thin-walled vessels are characteristic for SFT, H&E stain ×100 (B). While larger and branched vessels are unique for HPC, H&E stain ×100 (C). SFT on high power view shows collagen deposition around tumor cells, H&E stain ×400 (D), SFT immunoreactive for CD34 and STAT-6 immunohistochemical stain ×100 (E and F).

Final diagnosis requires IHC staining to show the tumor cells are positive for both CD34 and STAT-6 (Figure 3) in order to confirm the diagnosis of SFT and HPC.

The NAB2 and STAT6 genes are adjacent genes on chromosome 12q13 that are transcribed in opposite directions. This intrachromosomal fusion results from a genomic inversion at 12q13 locus, fusing NAB2 and STAT6 in a common direction of transcriptio. Require immunohistochemical staining showing the tumor cells are both positive for CD34 and STAT-6 (Figure 2). Confirmative diagnosis of SFT and HPC requires immunohistochemical staining support of tumor. Confirmative diagnosis of SFT and HPC requires immunohistochemical staining support that tumor cells are both positive for CD34 and STAT-6 by IHC staining (Figure 3), which are the confirmative diagnosis for SFT and HPC. Careful microscopic evaluation is necessary to distinguish those two tumors, in which SFT is with thin-walled and smaller vessels while HPC is usually with larger and branched vessels (Figure 3). Those are the only microscopic difference between these two tumors.

7.2 Phosphaturic mesenchymal tumor (PMT)

Phosphaturic mesenchymal tumor (PMT) is a rare mesenchymal tumor of unknown etiology, which can produce excessive fibroblast growth factor 23 (FGF23) and lead to tumor-induced osteomalacia (TIO), a rare paraneoplastic syndrome [40, 41, 42, 43].

PMT most commonly affects middle-aged adults but has also been reported in children and the elderly. Men and women are equally affected. The age and gender distribution of patients may be related to tumor location. PMT located in the alveolar bone occurs more frequently in younger males. Approximately half of cases occur in the extremities including femur, foot, and thigh soft tissue. The head and neck including the sinonasal area and the mandible and maxilla are also commonly involved. PMT rarely affects the trunk, pelvis, and spine [40].

Grossly, most PMT presents as non-specific soft tissue or bone masses and may contain calcified or hemorrhagic areas. PMT typically focally infiltrates into surrounding tissues (Figure 4A), probably accounting for their high local recurrence rate. The neoplastic cells typically have a low nuclear grade with absent or minimal nuclear pleomorphism, absent to rare mitotic figures, and low Ki-67 proliferative index (<5%). The tumor typically produces a characteristic “smudgy” matrix that calcifies in a peculiar “grungy” or flocculent fashion, and sometimes osteoid, chondroid, and/or myxoid matrix (Figure 5A–C). A variable component of osteoclast-like giant cells and mature adipose tissue are also common findings in PMT (Figure 5D). The tumor contains a small, arborizing network of capillaries. Prominent hyalinized and branching HPC-like vasculature may be also found (Figure 5E–I). Due to the wide histological spectrum, the tumor had been previously diagnosed as a variety of diseases including fibrosarcomas, osteosarcomas, osteoblastoma, giant cell tumors, or other mesenchymal tumors, and was finally categorized by Folpe et al. [42] in 2004 as PMT, mixed connective tissue type (PMTMCT). PMT in sinonasal and craniofacial bone may show some unique histopathological features. PMT arising from alveolar bone is characterized by haphazardly and diffusely distributed small, irregular odontogenic epithelial nests, which has been named “Phosphaturic mesenchymal tumors” of the mixed epithelial and connective tissue type (PMTMECT)” (Figure 4A–F) [40].

Figure 4.

The histology of PMT arising from alveolar bone was characterized by admixed epithelial components, which resemble odontogenic epithelium and may be easily overlooked or misinterpreted as multinucleated giant cells. (A) PMT arising from alveolar bone typically focally infiltrates into surrounding mucosa. (B, C) The neoplastic mesenchymal components are composed of spindle cells and arranged in a fascicular or storiform pattern. (D-F) The epithelial cell nests highly resemble odontogenic epithelium.

Figure 5.

The wide histological spectrum of PMT. (A) Focal osteoid matrix. (B) “Grungy” calcification. (C) Slate-gray crystals. (D) Osteoclast-like giant cell. (E) Prominent “staghorn” blood vessels. (F) Perivascular hyalinization. (G) Dilated thin-walled blood vessels. (H) “Patternless” arrangement with elaborate microvasculature. (I) Malignant PMT.

Although the histological criteria for malignant PMT have not been well developed, frankly sarcomatous features (high cellularity, marked nuclear atypia, elevated mitotic activity and Ki-67 proliferative index, and necrosis) support the diagnosis of malignant PMT (Figure 5I). Malignant PMT typically appears as a recurrent or metastatic tumor [41].

By immunohistochemistry, FGF23, SSTR2A, NSE, CD99, CD56, Bcl-2, D2-40, CD56, CD68, SATB2, and ERG has also been demonstrated to be frequently expressed in PMT (Figure 6A–J) [40, 44]. Other mesenchymal markers including FLI-1, SMA, and CD34 were also expressed to varying degrees (Figure 6K and L). Although immunohistochemistry is non-specific and thus of limited value, the polyimmunophenotypic profile favors the diagnosis of PMT. Although previous studies have used immunohistochemistry for detecting FGF23 expression, some pathologists believe that commercially available antibodies to FGF23 have questionable specificity and are not widely available, and prefer chromogenic in situ hybridization (CISH) for FGF23 expression detection in PMT [41]. However, CISH is not commonly used in routine pathology practice. Besides, detecting the characteristic FN1/FGFR1 or FN1–FGF1 gene fusions by FISH or next-generation sequencing (NGS) can be of great value in the diagnosis of morphologically ambiguous cases, cases without a given history of TIO or so-called “Non-phosphaturic PMT” (tumors showing morphological features of PMT without TIO).

Figure 6.

The polyimmunophenotypic profile of PMT. AE1/AE3 (A) and vimentin (B) positivity in the epithelial cell nests and mesenchymal components of PMT arising from the alveolar bone, respectively. (C) FGF23. (D) SSTR2A. (E) NSE. (F) CD99. (G) CD56. (H) Bcl-2. (I) D2-40. (J) CD68. (K) SMA. (L) CD34.

Limited data have been obtained in regarding to TIO-associated mesenchymal tumors other than PMT. The histopathological, immunohistochemical, and molecular features of these tumors remain unclarified. Due to the apparent difference in the clinical implications, great caution is recommended when diagnosing any other specific type of mesenchymal tumor as the cause of TIO.

8. Atypical lipomatous tumor/well-differentiated liposarcoma

Liposarcomas account for approximately 20% of mesenchymal malignancies [45]. Atypical lipomatous tumor/well-differentiated liposarcoma (ALT/WDLPS) is a locally aggressive adipocytic malignancy. It is one of the most common subtypes of liposarcoma and accounts for approximately 40% - 45% of liposarcomas [46, 47]. ALT and WDLPS are exchangeable terms with ALT used when tumors in the extremities site, and WDLPS used when tumors in deeper sites, such as retroperitoneum, paratesticular region, or mediastinum. Amplification of a chromosomal region (12q13–15) that covers MDM2 and CDK4 cell cycle oncogenes is the pathognomonic genetic change and can be detected in most cases.

8.1 Clinical features and presentations

ALT/WDLPS affects middle age to elderly adults and is rare in children [47]. ALT/WDLPS has been associated with Li-Fraumeni syndrome in childhood [48] patients with ALT/WDLPS often present with slow-growing, painless mass in proximal extremities, trunk, and retroperitoneum. Other less common anatomic sites include paratesticular region, mediastinum, skin, and head/neck region. ALT/WDLPS in retroperitoneum is usually large and asymptomatic.

8.2 Histopathology

The size of ALT/WDLPS is usually larger than 5 cm and can grow very large to more than 20 cm, especially when located in deep sites, such as retroperitoneum. Grossly, ALT/WDLPS is well circumscribed with a tan-gray to the yellow cut surface. Fat necrosis or focal hemorrhages can be seen in large lesions. Thickened fibrous bands also may be noticed on cut surfaces (Figure 7B).

Figure 7.

Representative well-differentiated liposarcoma (WDLPS) (A, B, C) and myxoid liposarcoma (MLPS) (D, E). A, B. Gross picture and cut suface of WDLPS. C. High power view shows mature lipocytes and atypical lipoblasts in WDLPS (denoted in blue arrow). D, E. Different power views show high-grade round cell morphology in MLPS (denoted in green arrow).

ALT/WDLPS appears close to mature fat in histology. It is composed of sheets of well-differentiated adipocytes and lipoblasts. Adipocytes are in variable size and shape in the tumor. Nuclear hyperchromasia and atypia can be seen in both adipocytes and stromal cells. Stromal cells may have one to multiple nuclei and when multiple nuclei are present, floret-like morphology can be seen. Lipoblasts are usually hyperchromatic and vacuolated. However, no significant nuclear pleomorphism or severe atypia should be seen in ALT/WDLPS. The number of lipoblasts varies, or even rare to none (Figure 7). Lipoblasts are not required to make the diagnosis for ALT/WDLPS. ALT/WDLPS usually has cellular fibrous septa or even extensive sclerosing fibrosis. Lipogenic cells in the fibrous septa are hypocellular and mitotic activity is rare or absent, which are histologic features of ALT/WDLPS and help differentiate ALT/WDLPS from dedifferentiated liposarcoma (DDLPS). ALT/WDLPS may have mild myxoid stromal changes, mimicking myxoid liposarcoma. In rare cases, heterologous components, such as bone, cartilage, or muscle can be seen. However, the presence of heterologous components does not change the prognosis. In some ALT/WDLPS cases, fat necrosis and atrophic skeletal muscle cells are present, which possess a certain degree of nuclear atypia and they should not be confused with lipoblasts.

WDLPS has three histological subtypes (from most common to least): adipocytic (lipoma-like), sclerosing (common in retroperitoneum and paratesticular region) and inflammatory (common in retroperitoneum) [49, 50]. Adipocytic subtype WDLPS processes lobules of adipocytes with a mature appearance and irregular fibrous septa of variable thickness. There is nuclear atypia in adipocytes and stromal cells. Lipoblasts in WDL are rare and sometimes can be difficult to find. The sclerosing subtype is composed of hypocellular adipocytic components and extensive stromal fibrosis. Nuclear hyperchromasia and atypia of adipocytic components are present. Stromal collagenization or hyalinization can be seen in this subtype. The inflammatory subtype contains dense lymphoplasmacytic infiltrate in the stroma.

Ancillary tests, such as cytogenetic testing to detect the amplification of MDM2 and CDK4 and IHC stain for nuclear MDM2 and/or CDK4 are required for diagnosis.

8.3 Prognosis and treatment

ALT/WDLPS is a locally aggressive malignancy. ALT/WDLPS has a recurrent rate of up to 50%, depending on the site [50]. ALT/WDLPS does not metastasis unless dedifferentiation occurs. Both recurrence and dedifferentiation are anatomic location dependent. Recurrence after complete surgical excision occurs more often from the intra-abdominal cavity, retroperitoneum, mediastinum, or paratesticular regions. The risk for ALT/WDLPS to dedifferentiate is very low in extremities (<2%), while much higher in retroperitoneum (>20%). Radical surgical excision with a negative margin is the mainstay of management. For tumors that occur in deep central anatomic sites, such as retroperitoneum, paratesticular region or mediastinum, radical excision with removal of multiple visceral structures is recommended, which can increase relapse-free survival [50]. The median time to death is 6–11 years. Ten-year to 20-year mortality rates range from essentially 0% for ALT of the extremities to >80% for WDLPS located in the retroperitoneum (see Figure 7).

9. Dedifferentiated liposarcoma

Dedifferentiated liposarcoma is typically a non-lipogenic sarcoma with variable histologic grades. DDLPS can either be progressed from ALT/WDLPS (10%) or arised de novo (90%). Dedifferentiation from WDLPS can occur at any site of the body. The anatomic site of DDLPS is closely associated with prognosis and DDLPS from retroperitoneum predicts the worse prognosis. Same as ALT/WDLPS, DDLPS is characterized by amplification of chromosomal sequence from 12q13–15 region [51, 52, 53].

9.1 Clinical features and presentations

Same as ALT/WDLPS, DDLPS occurs in middle-aged to elderly adults (peak incidence in 60s–80s) with equal frequency in males and females. Dedifferentiation happens in up to 10% of ALT/WDLPS and the rest of DDLPS cases arise de novo. DDLPS is most common in deep soft tissue, such as retroperitoneum and abdominal cavity, less common in paratesticular region, trunk, extremities, head/neck, and least in the subcutaneous region [49]. The dedifferentiation risk of ALT/WDLPS is location dependent. It is higher in deeper sites (such as retroperitoneum) and lower in other sites (approximately 28% vs. up to 10%) [49, 51, 52, 53]. Patients with DDLPS typically present as a large, painless mass within slow-growing and lasts a couple of years. Patients can also present with symptoms due to organs impinged by tumors. The symptoms include organ obstructions of the intestinal or urinary system.

9.2 Histopathology

Patients with DDLPS present with a well-demarcated, large and lobulated mass. The cut surface reveals yellow to gray-white consistency and focal necrosis and hemorrhage can be seen. DDLPS usually contains both ALT/WDLPS components and non-lipogenic sarcoma components. An abrupt or gradual transition from these two components can be appreciated microscopically. The amount of lipogenic component/ALT/WDLPS in DDLPS differs and can be as little as minimal to none. The diagnosis of DDLPS can be challenging in the absence of the ALT/WDLPS part. Lipogenic components can be any histologic subtype(s) of ALT/WDLPS. Non-lipogenic sarcoma component shows variable morphology and histologic grade, depending on different dedifferentiated mesenchymal component(s). Non-lipogenic sarcoma components can show variable morphology, including spindled, fascicular, and storiform in the background of myxoid stroma, which can lead to broad differential diagnoses, mimicking from fibrosarcoma to undifferentiated pleomorphic sarcoma (UPS). DDLPS with the myxoid background may resemble a low- to high-grade myxofibrosarcoma. However, myxofibrosarcoma is not as commonly seen in retroperitoneum as DDLPS. Low nuclear grade DDLPS with myxoid background could resemble myxoid liposarcoma or low-grade fibromyxoid sarcoma. Spindle or fascicular morphology in DDLPS can mimic fibromatosis, fibrosarcoma, leiomyosarcoma, or malignant peripheral nerve sheath tumor. Sclerotic stroma in DDLPS can mimic sclerosing subtype of ALT/WDLPS and lack of fat tissue in DDLPS can help tell them apart. DDLPS with predominant storiform morphology can mimic dermatofibrosarcoma protuberans. The whorled growth pattern in DDLPS can mimic a meningothelial tumor. Inflammatory infiltrates in DDLPS may mimic inflammatory myofibroblastic tumor (IMT) or inflammatory subtype of ALT/WDLPS. Amplification of MDM and CDK4 and/or immunohistochemical staining of nuclear MDM and CDK4 can be detected in DDLPS and ALT/WDLPS, while they are negative in all other abovementioned differential diagnoses. Proliferation usually increases in DDLPS (more than five mitotic figures in ten high power fields (HPF)). Heterologous differentiation has been reported in 5–10% of DDLPS, including myogenic and osteo/chondrosarcomatous elements [54]. Angiosarcomatous differentiation is uncommon in DDLPS.

DDLPS and ALT/WDLPS share the same genetic alterations - amplifications of the chromosomal sequence from 12q13–15 region, which cover MDM2 and CDK4oncogenes. In addition, studies have demonstrated that DDLPS harbors more genetic changes, such as co-amplifications of 6q23 and 1p32 [47].

9.3 Prognosis and treatment

DDLPS is more aggressive (up to 40% local recurrence rate and 15–30% metastatic rate) than ALT/WDLPS. However, it has a lower local recurrence or metastatic rate than morphologically similar undifferentiated pleomorphic sarcoma [5]. DDLPS in retroperitoneum shows a worst prognosis than DDPLS in other anatomic sites. DDLPS can metastasize to distant sites, such as the liver, lungs, brain, bone, and other soft tissue sites [49, 51, 52, 53]. Overall, 5-year survival rate for DDLPS is approximately 25–30%. The percentage and histologic grade of the non-lipogenic sarcoma component usually does not change the survival rate, except high-grade DDLPS in retroperitoneum. The most important adverse prognostic factor for DDLPS has been reported to be retroperitoneal location [55]. A recent study found that DDLPS with myogenic differentiation has a better prognosis than DDLPS without myogenic differentiation [56]. Radical resection remains the mainstay of therapy and debulking surgery is an alternative solution when complete resection is impossible. After surgery, other conventional treatment options, such as chemotherapy or radiotherapy, are available for local recurrence or metastasis. The study showed that radiotherapy can lower the local recurrence rate, but not overall survival [51, 52, 53, 55]. Novel targeted therapies against gene products from amplification of the 12q13–15 region are under investigation and might provide new therapeutic options in the future.

10. Myxoid liposarcoma

Myxoid liposarcoma (MLPS) is the second most common subtype of liposarcoma and accounts for approximately 20–35% of liposarcoma [46, 57]. More cellular and high-grade MLPS was used to call round cell liposarcoma (RCLPS). However, this term is now obsolete and not recommended by the World Health Organization (WHO) Classification of Tumors: Soft tissue and Bone Tumors, 5th edition [46]. MLPS carries a characteristic FUS-DDIT3(CHOP) fusion oncogene (95% of cases), resulting from a t(12;16) (q13;p11) chromosomal translocation. EWSR1-DDIT3(CHOP) fusion is observed in the rest 5% of MLPS cases.

10.1 Clinical features and presentations

MLPS occurs in young to middle-aged adults, with the peak incidence between 40s and 50s. It is the most common subtype of liposarcoma in children and adolescents [58, 59]. MLPS occurs equally in males and females. MLPSs typically present more commonly in the deep tissue of the lower extremities. Primary retroperitoneal MLPS are extremely rare [60].

10.2 Histopathology

Patients with MLPS present with circumscribed, painless, lobulated masses, which are usually larger than 10 cm (median 10–12 cm). MLPS can be unifocal or multifocal. The cut surface is usually tan-brown to red, glistening, and gelatinous. More cellular and high-grade areas on the cut surface appear more firmer and more whitish. Microscopically, low-grade MLPS is composed of non-lipogenic neoplastic cells (usually at the peripheral of the tumor), scattered signet ring lipoblasts, elaborate capillary vasculature (chicken-wire pattern), and abundant myxoid stroma. Non-lipogenic neoplastic cells usually are monomorphic, small, round to oval, spindled, or stellate shaped. Lipoblasts can be rare to none in number and are not required for diagnosis. Mitotic figures usually do not increase. Extracellular mucin can be noticed in some cases. In rare cases, a pulmonary edema-like pattern can be appreciated, which is a useful clue for diagnosing MLPS. Mature lipomatous tissue can be present with the variable amount in tumor. Chondroid and osseous metaplasia can be present in rare cases.

High-grade MLPS (formally called RCLPS) possesses more neoplastic cells, less myxoid stroma and capillary vasculature than low-grade MLPS. Neoplastic cells in high-grade MLPS are hyperchromatic, round cells with larger nuclei and prominent nucleoli. The amount of round cells in MLPS is essential to report since more than 5% of round cells in MLPS are closely correlated with increased risk for metastasis and death [57, 58]. Thorough sampling is recommended. Rare multinucleated cells can be seen in some cases and it is unclear if the presence of multinucleated cells is associated with prognosis.

MLPS possesses FUS-DDIT3 fusion oncogene, resulting from t(12;16) (q13;p11) chromosome translocation. Amplification of MDM2/CDK4 and IHC stains for MDM2/CDK4 are usually negative.

10.3 Prognosis and treatment

MLPS is an aggressive liposarcoma. The local recurrence rate is up to 25% of all MLPS cases. Distant metastatic rate varies by histologic grade. Histologic grade is an important prognostic factor. Metastatic risk of low-grade MLPS is less than 10%, while metastatic risk of high-grade MLPS is much higher (up to 60%). FUS-DDIT3 and EWSR1-DDIT3 fusion gene products do not associate with differences in histological grade or prognosis. MLPS is prone to metastasize to other fat-bearing areas, such as the retroperitoneum, abdomen, chest, trunk, as well as occasionally to extremities and bone. Lungs have been reported as distant metastatic sites [61]. Adverse prognostic factors include age (> 45 years), necrosis, TP53 and CDKN2A mutations, and P53 overexpression. A study with 418 primary MLPS and RCLPS cases showed that the overall 10-year local control rate was 93%. The 5- and 10- year metastatic-free survivals were 84% and 77% for MLPS and 69% and 46% for RCLPS [62]. Surgery is still the mainstay of the treatment of MLPS. Compared to other soft tissue sarcomas, MLPS is sensitive to radiotherapy and chemotherapy. RCLPS is usually treated with surgery combined with radiotherapy, which can significantly prevent local relapse and reduce tumor diameter [57, 63]. Clinical study has shown that patients with RCLPS treated with trabectedin have an encouraging response rate [58, 64].

11. Pleomorphic liposarcoma

Pleomorphic liposarcoma (PLPS) is the least common, but most aggressive subtype of liposarcoma. It is characterized by the presence of pleomorphic lipoblasts. PLPS has no specific molecular or IHC features so far, and the only diagnostic criteria are the presence of pleomorphic lipoblasts.

11.1 Clinical features and presentations

PLPS accounts for less than 5% of all liposarcomas. It occurs in elderly adults with a peak incidence in the 70s and slightly male predominant. Patients present with a rapidly growing mass and tumor site-related compression symptoms. Three-quarters of PLPS cases occur in the extremities (more in low extremities than upper extremities). Other less affected sites include the trunk, retroperitoneum, head/neck, abdomen/pelvis, and paratesticular region [16]. Rarely affected sites are the mediastinum, breast, and colon. Same as other subtypes of liposarcoma, most PLPS tumors arise in deep soft tissue (90%) and the rest 10% of tumors arise in subcutaneous fat.

11.2 Histopathology

Most PLPS tumors are large in size, ranging from a few centimeters to up to 23 cm. Patients with PLPS usually present with fast-growing mass within the last few months. Tumors are nodular and well-circumscribed with myxoid changes. Cut surface shows firm, tan-white to yellow with focal necrosis. Microscopically, PLPS is composed of both lipogenic component (lipoblasts) and non-lipogenic component (non-lipogenic sarcoma). Lipoblasts are hyperchromatic and have extremely large and remarkably bizarre nuclei, along with cytoplasmic vacuoles. Increased atypical mitotic figures are readily appreciated in PLPS. The number of lipoblasts is variable case by case and the presence of pleomorphic lipoblasts is the only diagnostic criteria required to diagnose PLPS. A non-lipogenic component is composed of high-grade, pleomorphic undifferentiated sarcomatous cells.

PLPS has three basic morphologic patterns: cellular pleomorphic, myxofibrosarcoma-like and epithelioid pattern. Cellular PLPS is the most common pattern, containing sheets of pleomorphic spindled, round, or polygonal cells. Nuclei are hyperchromatic, pleomorphic with severe atypia. Numerous atypical mitoses are present. Intracellular and extracellular eosinophilic globules or nuclear pseudoinclusions may be present. PLPS with pleomorphic tumor cells and capillary vasculature in the background of the myxoid stroma is considered as the myxofibrosarcoma-like pattern. Multinucleated floret-like cells can be present in this pattern. Epithelioid pattern contains sheets of epithelioid cells. The cytoplasm of epithelioid cells can be clear or eosinophilic, which mimic carcinomas with clear cell features, such as clear cell renal cell carcinoma and adrenocortical carcinoma. Necrosis is not uncommon in PLPS.

Lipoblasts are positive for S100 protein; however, they are negative for the nuclear stain of MDM2 and CDK4 by IHC stains.

11.3 Prognosis and treatment

PLPS is high-grade sarcoma with high local recurrence (30–50%) and metastatic rates (50%). The overall 5-year survival rate is about 60%, which is similar to other high-grade sarcomas, such as myxofibrosarcoma, leiomyosarcoma, and undifferentiated pleomorphic sarcoma [65]. Metastases are most common in the lungs and pleura. Adverse prognostic factors include a central location, large tumor size (>10 cm), and brisk mitotic rate (>10 per 10 HPF). Patients with superficial tumors usually have a better outcome, due to complete surgical excision. Surgical resection with or without radiotherapy is the mainstay of treatment for PLPS. Incomplete excision of the deep tumor will require postoperative radiotherapy (See Figure 8).

Figure 8.

Representative hematoxylin and eosin stains of well-differentiated liposarcoma (WDLPS) (A, B) and myxoid liposarcoma (MLPS) (C, D). A, B. Low power views show mature lipocytes and rare atypical lipoblasts in WDLPS (denoted in blue arrow). C, D. Low and high-power views show high–grade round cell morphology in MLPS (denoted in green arrow).

In conclusion, our understanding and knowledge of new molecular genetic alterations in human soft tissue and bone tumors are rapidly evolving and ever-changing. Table 1 summarized the most common human sarcomas’ chromosomal translocations with corresponding fusion genes, which can be detected by FISH techniques as long as the specific probes are available in the laboratory. In addition, DNA methylation-based sarcoma classification may provide new insight for diagnosis, prognosis, and even therapeutic information for this type of malignancy [5]. A challenging area for future research is the production of monoclonal and polyclonal antibodies against the actual proteins produced by the chimeric transcripts. Such antibodies would help in the immunocytochemical and/or immunohistochemical diagnosis of sarcomas, which will make our daily practice becoming more reliable and comfortable [2].

Ewing sarcoma/PNET	t(11;22)	EWS-LLI1
	t(21;22)	EWS-ERG
	t(7;22)	EWS-ETV1
Clear cell sarcoma	t(12;22)	EWSR1-ATF1
Alveolar rhabdomyosacoma	t(2;13)	PAX3-FAHR
	t(1;13)	PAX7-FAHR
Synovial sarcoma	t(X; 18)	SYT-SSX1
		SYT-SSX2
Extraskeletal myxoid chondrosarcoma	t(9;22)	EWS-CHN
	t(9;17)	TAF2N-CHN
	4(9;15)	TCF12-CHN
Dermatofibrosarcoma protuberant	t(17;22)	COLIA1-PDGFB

Table 1.

Most common chromosomal translocation and its fusion genes in human sarcomas.

Abbreviations

SFT	solitary fibrous tumor
HPC	hemangiopericytoma
FISH	fluorescent in situ hybridization
CNS	central nerve system
t	translocation
TAT	turn-around time
RT-PCR	reverse transcriptase polymerase chain reaction
MFH	malignant fibrous histiocytoma
PMT	phosphaturic mesenchymal tumor
pPNETs	peripheral primitive neuroectodermal tumors
Cdk	cyclin-dependent protein kinases
IHC	Immunohistochemistry
NGS	Next generation sequencing
HIV	human immunodefiency virus
AIDS	acquired immunodefiency syndrome
ALT/WDLPS	Atypical Lipomatous Tumor/Well-Differentiated Liposarcoma
(DDLPS	dedifferntiated liposarcoma
MLPS	myxoid liposarcoma
PLPS	pleomorphic liposarcoma