Dosimetric study of GZP6 60Co high dose rate brachytherapy source

Abstract The purpose of this study was to obtain dosimetric parameters of GZP6 60Co brachytherapy source number 3. The Geant4 MC code has been used to obtain the dose rate distribution following the American Association of Physicists in Medicine (AAPM) TG‐43U1 dosimetric formalism. In the simulation, the source was centered in a 50 cm radius water phantom. The cylindrical ring voxels were 0.1 mm thick for r ≤ 1 cm, 0.5 mm for 1 cm < r ≤ 5 cm, and 1 mm for r > 5 cm. The kerma‐dose approximation was performed for r > 0.75 cm to increase the simulation efficiency. Based on the numerical results, the dosimetric datasets were obtained. These results were compared with the available data of the similar 60Co high dose rate sources and the detailed dosimetric characterization was discussed.

The GZP6 60 Co afterloading high dose rate (HDR) unit (Nuclear Power Institute of China) is widely used at home and abroad. It is comprised of six different source designs with a stepping source (source number 6) and five nonstepping sources (source number [1][2][3][4][5]. It mainly addresses intracavitary and interstitial applications and is considered an integral part of the treatment of cervical, vaginal, rectal, and esophageal cancers. In the literature, several investigations have been performed on the GZP6 60 Co HDR unit. In the study of Mesbahi et al. 1 , air kerma strengths of source numbers 1, 2, and 5 were obtained by in-air measurements and a Farmer-type ionization chamber. In a separate investigation, the radial dose functions of the three sources were calculated by Mesbahi et al. 2 using the MC method and GZP6 TPS. Toossi et al. 3 estimated the air kerma strength of GZP6 60 Co source number 3 by Monte Carlo simulation and in-air measurements. The dose distribution for GZP6 60 Co stepping source was also calculated using the matrix shift method by Toossi et al. 4 For the purpose of quality assurance, the dose distributions generated by GZP6 TPS were verified in another investigation. 5 The dosimetric parameters of radioactive sources are crucial elements in clinical practice as they are important input data in Monte Carlo code to obtain the dosimetric parameters of GZP6 60 Co sources. However, in the study of Vijande et al. 6  photon-emitting brachytherapy sources. [7][8][9] The calculated dosimetric parameters were compared with the available data of the similar 60 Co high dose rate (HDR) sources 6,10,11 and the detailed dosimetric characterization of GZP6 60 Co source number 3 was discussed.

| MATERIAL AND METHODS
GZP6 60 Co brachytherapy source number 3 is composed of an active cylindrical 60 Co pellet with a 3.5 mm length and a 1.5 mm diameter covered with a titanium layer with a thickness of 0.1 mm. The radioactive 60 Co is uniformly distributed in the core. Several nonactive steel pellets 1.5 mm in diameter line up with the active cylindrical 60 Co pellet. All the pellets described above were packaged in a steel spring cover with a thickness of 0.5 mm. The detailed information of this source is taken from published studies 2,12 and is illustrated in Fig. 1. The mass density and chemical composition of the materials are shown in Table 1.
The 60 Co brachytherapy source emits two gamma rays with energies of 1.33 MeV and 1.17 MeV, mixed with b rays whose maximum energy and average energy is 0.318 MeV and 0.096 MeV, respectively. 8 In the simulation, the b rays are neglected because of absorption in the source steel cover. 10 In this study, the Monte Carlo code Geant4 13 (Geant4.9.6.P02 development version) is used to simulate transport and interaction of gamma rays emitted from the GZP6 60 Co HDR brachytherapy source in water. The Evaluated Photon Data Library, 1997 Version (EPDL97) and Evaluated Electron Data Library (EEDL) cross-section libraries were used for photons and electrons, respectively. 14,15 In the simulation, a spherical liquid water phantom with a 50-cm radius was utilized to approximate an infinite water environment. The density used for the liquid water has been 0.998 g/cm 3 as recommended in the TG-43 U1 report. 7 The GZP6 60 Co HDR source was F I G . 1. A schematic view of the GZP6 60 Co HDR source number 3. Dimensions are given in mm. Source type The new BEBIG 60 Co 10 | 315 accommodated in the center of the water phantom. Electronic equilibrium within 1% was reached for 60 Co at distances greater than 7 mm from the source center. 16 Thus, the kerma-dose approximation was performed for r > 0.75 cm to speed up calculations. 17 Dose and collisional kerma rate distributions were used to derive the final dosimetric parameters as described in the AAPM TG-43U1 report. 7 The dose distribution of the source was calculated within the radial distance of 20 cm. The cylindrical ring voxels were 0.1 mm thick for r ≤ 1 cm, 0.5 mm for 1 cm < r ≤ 5 cm, and 1 mm for r > 5 cm, which can provide high-resolution dosimetry. The cutoff energy was set to 10 keV for both photons and electrons. Collisional kerma and absorbed dose were obtained in cylindrical (y, z) and spherical (r, h) coordinates. The coordinate axes used are shown in which recommended air at 40% relative humidity. 6 9 10 9 photon histories (r ≤ 0.75 cm) were simulated to score dose. 10 9 photon histories (r > 0.75 cm) were simulated to score kerma.

| RESULTS AND DISCUSSION
The air kerma strength calculated for GZP6 60 Co source number 3 is 3.004 9 10 À7 cGy cm 2 h À1 Bq À1 with a statistical uncertainty of 0.14%. The dose rate constant of GZP6 60 Co source number 3 obtained is 1.088 AE 0.002 cGy h À1 U À1 (with k = 1), which is comparable to the available data of the similar 60 Co HDR sources (see Table 2). The radial dose function values and the anisotropy function values of the GZP6 60 Co source number 3 are provided in Table 3 and Table 4, respectively. In Fig. 3, the anisotropy function results of GZP6 60 Co source number 3 are plotted vs polar angle at the selected radial distances. In addition, the along-away data are shown in Table 5.
The radial dose function values of the GZP6 60 Co source number 3 were compared with corresponding data from the relevant literature (see Fig. 2). It is observed that the curves of the radial dose functions of the source models match well for r > 1 cm and small differences exist for r < 1 cm. These differences are caused by varying degrees of photon absorption and scattering in the sources. 8 In general, the radial dose functions do not depend significantly on source dimensions and encapsulation designs. 11 As shown in Table 4 and Fig. 3, the anisotropy function values of GZP6 60 Co source number 3 are nearly uniform for polar angles 30°≤ h ≤ 90°. For example, the anisotropy function values are around 0.998 for h = 70°and around 0.994 for h = 50°. However, a strong dependence on radial distance was observed for h < 30°. As described in Ref. [8], the anisotropy function values decrease for polar angles close to the long axis (see Fig. 3), which is caused by the oblique filtration within the source structure.

CONF LICTS OF INTEREST
None declared.