Monte Carlo calculation of dosimetry parameters for the IR08-103Pd brachytherapy source.

Purpose: For the treatment of some cancerous tumors using brachytherapy methods and low-energy photon sources, such as 125I and 103Pd, the American Association of Physicists in Medicine Task Group No. 43U1 report recommends that the dosimetric parameters of a new brachytherapy source must be determined in two experimental and Monte Carlo theoretical methods before using each new source clinically. This study presents the results of Monte Carlo calculations of the dosimetric parameters for IR08-103Pd brachytherapy source design. IR08-103Pd seed has been manufactured at the Agricultural, Medical and Industrial Research School.

Methods: Version 5 of the (MCNP) Monte Carlo radiation transport code was used to calculate the dosimetry parameters around the source. Three geometric models of the seed, based on different locations of beads inside the titanium capsule, were simulated. The seed contains five resin beads of 0.6 mm diameter having 103Pd uniformly absorbed in the bead volume, which were contained within a cylindrical titanium capsule having 0.8 mm outside diameter and 4.8 mm length.

Results: The Monte Carlo calculated dose rate constant of the IR08-103Pd seed was found to be 0.695 +/- 0.021 cGyU(-1) h(-1). Also in this study, the geometry function G(r, theta), line and point-source radial dose functions gL(r) and gP(r), and the anisotropy function F(r, theta), have been calculated at distances from 0.25 to 7 cm. The results of these calculations have been compared with measured values for an actual IR08-103Pd seed.

Conclusions: There are no statistical significant dosimetric differences among the three seed orientations in this study (i.e., ideal, vertical, and diagonal). However, the observed differences between the calculated and measured values could be explained by the measurement uncertainty and the configuration of the resin beads within the capsule and capsule orientation.