Radiologic Evaluation of Malignant Pleural and Peritoneal Mesothelioma

Mesothelioma usually presents in the fifth to seventh decades, and 70-80 % of cases occur in men (Moore et al., 2008). Malignant pleural mesothelioma (MPM) is the most widely form of mesothelioma. Patients frequently present with dyspnea, chest pain, cough, and weight loss (Moore et al., 2008, Wang et al., 2004). Although most of the mesotheliomas cover the pleural surface, approximately 35% arise only from peritoneum. Patients with malignant peritoneal mesothelioma may present with abdominal pain, distention, anorexia, and weight loss (Park et al., 2008).


Introduction
Malignant mesothelioma is an asbestos-associated malignancy arising from the mesothelial cells of the pleural and peritoneal cavities, as well as the pericardium and the tunica vaginalis.
Mesothelioma usually presents in the fifth to seventh decades, and 70-80 % of cases occur in men (Moore et al., 2008).Malignant pleural mesothelioma (MPM) is the most widely form of mesothelioma.Patients frequently present with dyspnea, chest pain, cough, and weight loss (Moore et al., 2008, Wang et al., 2004).Although most of the mesotheliomas cover the pleural surface, approximately 35% arise only from peritoneum.Patients with malignant peritoneal mesothelioma may present with abdominal pain, distention, anorexia, and weight loss (Park et al., 2008).
Radiologic modalities play a crucial role in the evaluation of malignant mesothelioma.Computed tomography is the primary imaging method used for the diagnosis and the staging of malignant mesothelioma, but also for guiding biopsy for tissue diagnosis.Magnetic resonans imaging (MRI) is useful for detection of extension of disease, especially to the chest wall and diaphragm (Moore et al., 2008, Wang et al., 2004).In this article we review radiologic findings of malignant pleural and peritoneal mesothelioma with our patient archives.We also wants to give some information about differential diagnosis malignant pleural and peritoneal mesothelioma.

Material and methods
We scanned our patient archive of mesothelioma between 2008-2011 years.We accepted patients who had CT or MRI at their initial diagnosis.We have had 135 patient who suffered from mesothelioma but only 35 patient had CT or MRI at the time of diagnosis.Twenty seven of them were pleural mesothelioma, and 8 of them peritoneal mesothelioma.

Discussion
The association of history, examination, radiology and pathology is essential in the diagnosis of mesothelioma.Radiological imaging is important for the diagnosis, staging and management of mesothelioma.

Pleural mesothelioma
Intravenous contrast-enhanced CT is the primary imaging modality for suspected malignant mesothelioma.CT can show the whole pleural surface and diaphragm.CT

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Mesotheliomas -Synonyms and Definition, Epidemiology, Etiology, Pathogenesis, Cyto-Histopathological Features, Clinic, Diagnosis, Treatment, Prognosis 34 findings that is seen mostly are nodular pleural thickening, unilateral pleural effusion, pleural calcification, thickening of interlobar fissur, reduction of thoracic volume (Wang et al., 2004, Ismail-Khan et al., 2006).Pleural calcification is seen approximately 20% of cases (Moore et al., 2008, Wang et al., 2004).Typically, both the visceral and parietal pleurae are involved.Malignant pleural thickening characteristically is circumferantial, nodular and > 1 cm.Also, mediastinal pleural involvement is often detected (Ismail-Khan et al., 2006).Malignant pleural mesothelioma is locally aggressive with invasion of the chest wall, mediastinum and diaphragm.Obliteration of extrapleural fat planes, invasion of intercostal muscles, displacement of ribs, and bone destruction are findings of chest wall involvement.Heart, esophagus, trachea and major vascular structures of mediastinum may be involved by tumor.Nodular pericardial thickening and pericardial effusion refers to pericardial invasion by malignant pleural mesothelioma.Obliteration of surrounding fat planes of mediastinal organs, covering of vascular structure more than 50% is a strong evidence of invasion (Moore et al., 2008, Wang et al., 2004, Miller et al., 1996, Patz et al., 1992).
Pulmonary metastases of MPM presenting as nodules and masses and, rarely, diffuse miliary nodules may be seen at CT. Chest CT may also rarely demonstrate extrathoracic spread of MPM.Metastasis to the hilar and mediastinal lymph nodes is present at autopsy in approximately 40-45% of patients with MPM ( Miller et al., 1996, Patz et al., 1992, Dynes et al., 1992).MRI screening is not used routinely in the assessment of malignant mesothelioma, however in patients with potentially resectable disease, MRI can help to provide additional staging information over and above CT.Using gadolinium enhancement, MRI can advance the identification of tumor extension into the diaphragm or chest wall.MRI also is preferred in some patients whom intravenous iodinated contrast is contraindicated.
Malignant pleural mesothelioma is typically isointense or slightly hyperintense on T1weighted images and moderately hyperintense on T2-weighted images relative to adjacent chest wall muscle.After the gadolinium injection, MPM shows enhancement.MR imaging is superior to CT for showing invasion of the diaphragm and invasion of endothoracic fascia or a single chest wall focus (Moore et al., 2008, Miller et al., 1996, Patz et al., 1992).
The radiologic differential diagnosis includes metastatic pleural disease, pleural lymphoma, asbestos releated benign pleural disease, and tuberculous empyema.Pleural rind, nodular pleural thickening, pleural thickening greater than 1 cm, and mediastinal pleural involvement favor malignant pleural disease.Pleural calsification is usually seen in benign process.Mesothelioma can not be distinguished from metastatic pleural disease on CT.Discrimination between epithelial types of mesothelioma and metastatic adenocarcinoma requires histochemical, immunohistochemical, and ultrastructural analysis.The presence of hilar-mediastinal adenopathy may be helpful in differentiating metastases and lymphoma from mesothelioma.The radiologic criteria for unresectability are tumor encasing diaphragm, invasion of extrapleural soft tissue, infiltration, displacement, or seperation of ribs by tumor, or bone destruction (Moore et al., 2008, Dynes et al., 1992, Barreiro et al., 2006, Jeong et al., 2008).
Morphologically malignant pleural mesothelioma can be seen in three forms: epithelial, sarcomatous, and mixed.The mixed form is usually mentioned as biphasic or bimorphic.Mixed tumors are composed of both epithelial and sarcomatous components.Epithelial mesotheliomas have a better diagnosis than sarcomatous and mixed tumors so differential diagnosis is very important for determining the prognosis.Epithelial malignant mesotheliomas consist of cells that are similar to normal mesothelial cells.The cells form a tubulopapillary or trabecular pattern.Epithelial malignant mesothelioma may also show prominent secretory changes, microglandular patterns, signet cell structure, or desmoplastic responses that make these tumors difficult to differentiate from adenocarcinomas based on routine histologic analysis alone.The sarcomatous pattern of malignant mesothelioma is typically consist of closely packed spindle cells.No immunohistochemical markers are spesific for malignant mesotheliomas and so there are some immunohistochemical markers such as calretinin thrombomodulin, and cytokeratin 5/6 to differentiate from metastatic adenocarcinomas and soft tissue sarcomas that have similar to histologic appearances (Levy et al., 2008).(Fig. 10).

Peritoneal mesothelioma
Approximately 35% of all mesotheliomas arise only from the peritoneum.There are three pathologic subtypes of peritoneal mesothelioma: Malignant mesothelioma, cystic mesothelioma, or well-differentiated papillary mesothelioma.CT findings of these subtypes are different from each other (Park et al., 2008).
Malignant peritoneal mesothelioma is seen at fifth and sixth decades.Asbestos exposure is a predisposing factor.We can see two different apperance at CT. Dry apparence is characterized with peritoneal based masses and wet apparence is characterized ascites, irregular or nodular peritoneal thickening and omental mass may be seen at CT. Peritoneal carcinomatosis, serous papillary carcinoma of peritoneum, tuberculous peritonitis and peritoneal lymphomatosis should be thought in differential diagnosis.It is very difficult to do differential diagnosis by using only CT.Prominent ascites and less severe peritoneal thickening is seen in peritoneal carcinomatosis.The incidence of liver metastasis and lymphadenopathy is also higher in peritoneal carcinomatosis.Serous papillary carcinoma is found predominantly in elderly women and postmenopausal women.We must think tuberculous peritonitis if we see smooth peritoneal thickening, mesenteric lymphadenopathy with central necrosis, ascites with high attenuation, and splenomegaly at CT. Diffuse retroperitoneal and mesenteric lympadenopathy and the lack of omental involvement may misgive about lymphomatosis (Park et al., 2008, Levy et al., 2008).
Cystic mesothelioma is a benign tumor that is occur mainly in young to middle-aged women.It is usually associated with a history of previous abdominal surgery or pelvic inflammatory disease.Relationship between asbestos exposure and cystic mesothelioma has not been reported.Involvement of pelvic region is typical.Hormonal therapy is usually useful for treatment of cystic mesothelioma.Multilocular cystic mass, multiple unilocular cystic thin-walled cysts, or a unilocular cystic mass.Cystic lymphangioma, cystic epithelial neoplasms of the ovaries and endometriosis is thought in the differantial diagnosis.Cystic lymphangioma is seen in younger patients than cystic mesothelioma.It does not show regional predilection.Thick-walled cysts, thick internal septa, and high-attenuation internal debris favor the diagnosis of endometriosis.Well-differentiated papillary mesotheliomas is found reproductive-age women.Peritoneal thickening, multiple peritoneal nodules, omental infiltration and ascites may be seen at CT.It should be thought as the same disease that is thought in malignant peritoneal mesothelioma in differential diagnosis (Park et al., 2008, Levy et al., 2008., Pickhardt et al., 2005).

Conclusion
Malignant mesothelioma can be difficult to diagnose.Neither CT scanning nor MRI provides an unequivocal diagnosis of mesothelioma; tissue biopsy is required for the definitive diagnosis (Wang et al., 2004, Miller et al., 1996, Patz et al., 1992, Pickhardt et al., 2005, Zahid I et al. 2011).

Fig. 1 .
Fig. 1.Axial contrast enhanced CT parenchymal (a.) and mediastinal sections (b.) shows nodular, irregular and circumferantial right sided pleural thickening in 55 year-old man.Note that contracted right hemithorax and anterior mediastinal lymph node (arrow head).We can see pleural calcification on left sided pleural surface (arrow head).

Fig. 6 .
Fig. 6.Coronal (a) and axial (b) postcontrast T1 weighted images show a solitary mass with central necrosis into left retrocrural space at a patient with malignant pleural mesothelioma.
Fig. 7. Malignant peritoneal mesothelioma.a. Contrast enhanced CT scan shows nodular peritoneal thickening.b.Axial contrast enhanced CT shows perisplenic and perihepatic large amount of ascites.c.Axial contrast enhanced CT shows diffuse peritoneal thickening with omental cake.
Fig. 10.a. Malignant mesothelioma that shows papillary formation and desmoplastic stromal reaction, b.Biphasic malignant mesothelioma which consists of epitheloid and spindle cells, c.Malignant mesothelioma cells that show immunreactive with calretinen, d.Pleomorphic mesothelial cells (May Gruwald Giemsa)