Examples of radioisotopes commonly used in brachytherapy.
This study will briefly explain the production of 103Pd via cyclotron for brachytherapy use. The excitation functions of 103Rh(p,n)103Pd and 103Rh(d,2n)103Pd reactions were calculated using ALICE/91, ALICE/ASH, and TALYS-1.2 codes and compared with published data. Production of 103Pd was done via 103Rh(p,n)103Pd nuclear reaction. The target was bombarded with 18 MeV protons at 200 μA beam current for 15 h. After irradiation and radiochemical separation of the electroplated rhodium target, at the optimum condition, 103Pd was absorbed into Amberlite®IR-93 resin. The preparation of the brachytherapy seed, which is loaded by the resin beads, has also been presented. At least, the method to determine the dosimetric parameters for the seed by experimental measurement has been presented.
- excitation function
Cyclotrons are charged particle circular accelerators. They are a type of particle accelerator that has many applications in nuclear physics, industry, technology, and medicine. They play an important role in medicine; for example, they are used for radiation therapy, production of medical radioisotopes, and biomedical research . As a particle accelerator, one of the important uses of the cyclotron in medicine is radioisotope production .
For a long period, radioisotope production is basically done in nuclear reactors, but their availability is slowly decreasing, and due to some advantages of radioisotope production with the cyclotron, the development of particle accelerators started in the past century, so accelerator-based production facilities are growing, and various radioisotopes suitable for medical applications are produced.
In this chapter, the production method for the radioisotope, Palladium-103 (103Pd), via cyclotron is discussed. Palladium-103 with energy emission about 20 keV results in the rapid dose falloff with the distance which is suitable for low-dose-rate (LDR) brachytherapy . For nearly 25 years, brachytherapy sources containing 103Pd have been clinically introduced and are in use [4, 5]. Sources containing 103Pd are most commonly used in the treatment of prostate and eye cancer [6, 7, 8].
Brachytherapy is a form of treatment where a sealed radioactive source placed on or in the tissue/tumor is to be irradiated. With this method, a high dose can be delivered to the tumor with a rapid decrease in dose in the surrounding normal tissue. Brachytherapy sources are usually encapsulated; the encapsulated sources are placed within the body cavities close to the tumor, in a lumen of organs, or implanted into the tumor or placed over the tissue to be radiated.
Depending on the source dose rate, brachytherapy sources are classified into three categories :
High-dose-rate sources (HDR): >12 Gy/h radioisotopes with high-energy photon emitters like 137Cs, 60Co, 192Ir, and 198Au are used.
Medium-dose-rate sources (MDR): 2–12 Gy/h radioisotopes are used; this category is not commonly used in brachytherapy.
Low-dose-rate sources (LDR): Less than 2 Gy/h radioisotopes with low-energy photon emitters like 103Pd and 125I are used.
|Radionuclide||T1/2||Mode of decay||The energy of the emitted particle (keV)|
|125I||59.4 d||EC (100)||28|
|169Yb||32.02 d||EC (100)||93|
|103Pd||16.99 d||EC (100)||21|
This study will briefly explain the production of the 103Pd via cyclotron and will shortly describe some detail of 103Pd production such as production process, targetry, radiochemical separation, seed fabrication, and seed dosimetry method.
2. Materials and methods
2.1 Cyclotron production of 103Pd
There are different methods for the production of 103Pd via cyclotron. Two of them are 103Rh(p,n)103Pd and 103Rh(d,2n)103Pd . In this study, the excitation functions for both reactions have been calculated, and the optimum condition for each reaction has been obtained. Experimental data for the proton bombardment on rhodium metal as the target, via 103Rh(p,n)103Pd reaction, has also been measured. Rhodium target has been bombarded by proton in a cyclotron (Cyclone-30, IBA) with 18 MeV energy and a beam current intensity of 200 μA at the Agricultural, Medical and Industrial Research School (AMIRS) .
To achieve the 103Pd from the irradiated target, the radiochemical separation stage is started. The problem in this stage is the dissolution of target material due to the extremely low chemical reactivity of rhodium metal. The other problem is the high quantity of rhodium in the solution. A well-known palladium extractor is dimethylglyoxime, but to prevent the decrease of extraction yield, α-furyldioxime has been used . Purely obtained 103Pd is then absorbed into resin; the active resins are encapsulated inside the titanium casing.
2.2 Calculation of excitation function
Excitation functions of the 103Rh(p,n)103Pd and 103Rh(d,2n)103Pd reactions were calculated using ALICE/ASH, EMPIRE (version 3.1 Rivoli) and TALYS-1.2 nuclear codes, and the TENDL-2010. Using the codes simultaneously increases the accuracy of calculations. The calculated results were compared to the existing data of references [13, 14, 15, 16, 17, 18].
2.3 Nuclear models applied for cross-section calculations
The ALICE/ASH code: This code is a modified version of the ALICE code, and to describe the pre-equilibrium particle emission from nuclei, the geometry-dependent hybrid model (GDH) is used. Calculations were carried out based on the Fermi gas model with the nuclear level density parameter a = A/y and the generalized superfluid nuclear model. The default value of y is equal to [17, 19].
The TALYS code: TALYS is a computer code developed at NRG Petten and CEA to predict and analyze the nuclear reactions. TALYS models the nuclear reactions that involve protons, deuterons, neutrons, alpha particles, gamma rays, tritons, and hellions. The code simulated the reactions in the energy range from 1 keV to 200 MeV, for target nuclides of mass 12 and heavier [14, 17].
EMPIRE: EMPIRE (version 3.1 Rivoli) simulates various nuclear reactions, over a broad range of energies and incident particles. This system can be used for nuclear data evaluation and also for the theoretical calculation of nuclear reactions. A projectile can be a photon, a nucleon, and light or heavy ion. There is a broad range of energy in the system; the energy range starts just above the resonance region in the case of a neutron projectile and extends up to a few hundred MeV for heavy ion-induced reactions .
2.4 The thickness of the target
The required thickness of the target has been calculated via the stopping and range of ions in matter (SRIM) code . Based on the code results, to take full advantage of the excitation function and also to avoid the production of the radioisotope impurity, the entrance energy of the proton should be 18 MeV. The physical thickness of the rhodium layer is chosen in such a way that for a given beam/target angle geometry, the particle exit energy should be 6 MeV. The thickness of the rhodium target has to be 475 μm for 90° geometry. To minimize the thickness of the rhodium layer (and hence lower the cost price per target), a 6° geometry is preferred; in this case, the thickness of the target decreases, and a 45–50 μm layer is sufficient.
Identification of the gamma ray emitting from the radionuclides is performed by using gamma-ray spectroscopy with a high-purity germanium HP(Ge) detector (Canberra™ model GC1020-7500SL).
2.5 Preparation of 103Pd brachytherapy seeds
After irradiation, the radiochemical separation phase has been started. In this phase, the PdCl2 solution has been separated from rhodium, zinc, and copper.
According to the brachytherapy seed model (in case of using a sphere made of resin), resin beads and marker are encapsulated inside the titanium capsule. The end caps of the capsule are welded precisely to prevent source leakage . Regarding the physical design and configuration of the source internal component, two types of designs have been used: (a) rod/wire/cylinder made of ceramic, glass, or high-Z materials and (b) sphere made of resin. In this study sphere design of the source has been discussed.
2.6 Dosimetry of the seed
According to the American Association of Physicists in Medicine (AAPM) Radiation Therapy Committee recommendation, the dosimetry characteristics for all new interstitial brachytherapy seeds with energies less than 50 keV should be investigated by two independent investigators, theoretical calculations and experimental measurements. This work presents the method for thermoluminescent dosimeter (TLD) measurements to determine the dosimetric characteristics of the brachytherapy seed containing resin beads. The TLD-GR200A thermoluminescent dosimeters and two Perspex phantoms have been used, one Perspex phantom for the anisotropy function, F(r, θ), and the other for the radial dose function, gL(r).
3. Results and discussion
3.1 Excitation function
3.1.1 Excitation function study of 103Rh(p,n)103Pd reaction
The evaluation of the acquired data from the codes showed that the best range of the energy for proton in the 103Rh(p,n)103Pd reaction is 18–8 MeV. The maximum cross-section by EMPIRE (version 3.1 Rivoli) code is at Ep = 10 MeV, and the value is 574.44 mb. To evaluate the obtained results, Figure 2 shows the comparison between the calculated results in this study and measured data by others. For the 103Rh(p,n)103Pd reaction, there are five cross-section measurements that exist in the literature by authors of references [26, 27, 28, 29].
The calculated results from the TENDL-201, TALYS-1.2, and ALICE/ASH codes are in acceptable agreement with the measured data from , and calculated results from EMPIRE (version 3.1 Rivoli) code are in good agreement with the Hermanne et al. measured data (Figure 2).
3.1.2 Excitation function study of 103Rh(d,2n)103Pd reaction
According to the results from the codes, the optimum range of energy of the deuteron particle to produce 103Pd from 103Rh target for the 103Rh(d,2n)103Pd reaction is 22 to 8 MeV. The obtained results from ALICE/ASH hybrid model (a = A/9) show that the maximum cross-section is 1158.795 mb (Ed = 13 MeV). Comparison between the calculated results of this study and the measured data obtained by [30, 31] is presented in Figure 3.
The obtained results from ALICE/ASH code are in good agreement with the measured data by Hermanne et al. up to 20 MeV, whereas TALYS-1.2 and EMPIRE (version 3.1 Rivoli) calculated results have lower values than ALICE/ASH results and also experimental data.
3.2 Production of 103Pd
To prepare the rhodium target for irradiation, via the electrodeposition process, a thick layer of rhodium has been placed of the copper backing. According to Sadeghi et al. study [11, 12], the following conditions are the optimum conditions for the electrodeposition:
4.8 g rhodium (as Rh2(SO4)3)
pH = 2
DC current density of 8.5 mA cm−2
1% sulfamic acid (w/v)
After the electrodeposition process, the adhesion quality of the rhodium layer on the target backing has been tested by the thermal shock. The thermal shock has been carried out by heating the target up to 500°C for 1 h (The temperature that the Rh layer experience during high current irradiation). Thereafter, the hot target is submerged in cold water in a temperature of about 15°C. Observation of neither crack formation nor peeling off of the rhodium layers indicated a good adhesion for the purpose.
Afterward, the rhodium target was bombarded with 18 MeV protons at 200 μA beam current for 15 h (3000 μAh) . At the end of the bombardment (EOB), the activity of 103Pd and the yield of production are 685 mCi and 8.44 MBq/lAh, respectively. After the proton bombardment, the dissolution process has been started.
The optimum conditions of the electro-dissolution are as follows:
12 N HCl solution
AC current density > 1.8 A cm2
After the dissolution of the target from copper backing, the residual contains PdCl2, rhodium, zinc, and copper, so during the radiochemical separation phase, the PdCl2 solution should be separated from rhodium, zinc, and copper. According to the data in Figure 4, the purity of the obtained radio-palladium is about 99%. Thereafter, the obtained filtrate solution was loaded onto the column of size Ø 0.5 cm × 2 cm packed with Amberlite®IR-93 resin with 0.6 mm diameter. The summarized results in Figure 5 show that 0.05 M HCl is the most suitable concentration for adsorption of 103Pd on the Amberlite®IR-93 resin.
The dosimetric parameters of the seeds have been determined by theoretical calculation and experimental measurement, according to TG-43 U1 report. The theoretical method to obtain the dosimetric parameters of the brachytherapy seeds has been discussed and explained in Refs. [6, 9].
The following is the method to determine the dosimetric parameters by experimental measurements.
3.3 Dosimetry method
3.3.1 Thermoluminescent dosimeters
The TLD-GR200A (PTW, Freiburg, Germany) circular chips  of the following specifications have been used in this study:
0.8 mm thickness
4.5 mm in diameter
For the TLD calibration, before each experimental measurement, the entire batch of TLDs is exposed to a calibrated Cobalt-60 standard beam. The variation of response of the TLDs to the same exposure is tracked by normalizing the individual TLD readings to the average value.
The irradiated TLDs (irradiated by the brachytherapy seed in the phantom) are read using a KFKI RMKI TLD reader (KFKI Research Institute of the Hungarian Academy of Sciences, Budapest, Hungary), and then they are annealed by heating at 240°C for 10 min followed by fast cooling. The responses of the TLD have to be corrected for background. This is done by subtracting the average response of background TLDs from the responses of all other TLDs in each measurement [25, 32].
To determine the dosimetric parameters of the seeds by experimental measurement, the phantom of Perspex slabs (Figure 6), by the following specification, has been used.
Dimension: 30 cm × 30 cm × 15 cm
Composition (by weight percent): H, 8%; C, 60%; and O, 32%
Density: 1.19 g/cm3
The design of the two phantoms to measure the radial dose and anisotropy functions are based on those of .
Figure 7 shows the phantom slab which is used for the experimental measurements of the brachytherapy seed radial dose function values. As shown in Figure 7, in the central phantom slaps, the holes are drilled to place the TLD circular chips. The circular surface of the TLDs is parallel to the seed long axis and is perpendicular to the slab plane [33, 34].
The measurements are carried out at distances of r = 0.5, 1, 1.5, 2, 3, 4, and 5 cm relative to the seed center. To minimize the interference of any of TLDs with regard to the response by other TLD chips, the measurement is performed in a spiral configuration [6, 8, 35, 36].
In this study, 28 TLDs (4 at each radial distance) were used for every single experiment to prevent the shadowing effect due to the configuration and the design of the phantom. To improve the statistical quality of the data, the experiment was repeated several times.
The other phantom is shown in Figure 8. This phantom has been used for the measurement of the anisotropy function of the brachytherapy seed. It has the same dimensions as the first phantom but differs in the configuration of the source in that. The source is placed parallel to the central slab plane with its long axis.
The TLDs are placed at radial distances of r = 1.5, 2, 3, and 5 cm relative to the seed center and also lie at the polar angles θ ranging from 0 to 330° in 30° increments with respect to the seed long axis. The measurements were performed with 48 holes containing TLDs since it was found that for the experimental anisotropy function determination at a specific point, shadowing effects due to the TLDs that lie at the same polar angle do not affect results. This is due to the definition of anisotropy function that normalizes dose rate at a particular (r, θ) point to the dose rate at the corresponding point along the transverse source bisector, (r, 90°). Therefore, since shadowing was found similar at any polar angle for the same radial distance, the overall effect is canceled out in the calculation of an anisotropy function .
This chapter presents the application of the cyclotron in brachytherapy by the production of radioisotopes such as Palladium-103.
In this chapter, production of the 103Pd via cyclotron has been presented. 103Pd is used in permanent low-dose radiation brachytherapy. So preparation of the brachytherapy source having 103Pd radioisotope has also been discussed.
103Pd production is performed via the 103Rh(p,n)103Pd reaction by 18 MeV protons for 15 h at 200 μA beam current. The optimum energy range and the thickness of the rhodium target are calculated by the several computer codes (ALIS/ASH, TALYS, EMPIRE). Several codes have been used to increase the accuracy of the calculations. To use the 103Pd as brachytherapy source, the resin beads which are loaded by 103Pd are encapsulated inside the titanium capsule, and then the capsules are implanted into the cancerous area. So, after the chemical separation process, 103Pd radioisotope is absorbed uniformly in the resin Amberlite®IR-93, (20–50 mesh) bead to encapsulate them inside the titanium casing.
According to the American Association of Physicists in Medicine (AAPM) Radiation Therapy Committee recommendation, the dosimetric parameters of all new interstitial brachytherapy sources with energies less than 50 keV should be determined by two independent verifications, experimental measurements and theoretical calculations. The method for the theoretical calculation of the brachytherapy seed has been previously explained in Refs. [6, 9]. In this study, the experimental measurement method, the design, and dimension of the phantom and configuration of the TLDs have also been explained.