SU‐F‐T‐46: The Effect of Inter‐Seed Attenuation and Tissue Composition in Prostate 125I Brachytherapy Dose Calculations

Abstract

Purpose:To investigate the difference of dose distributions with/without the effect of inter‐seed attenuation and tissue compositions in prostate 125I brachytherapy dose calculations, using Monte Carlo simulations of Particle and Heavy Ion Transport code System (PHITS).Methods:The dose distributions in 125I prostate brachytherapy were calculated using PHITS for non‐simultaneous and simultaneous alignments of STM1251 sources in water or prostate phantom for six patients. The PHITS input file was created from DICOM‐RT file which includes source coordinates and structures for clinical target volume (CTV) and organs at risk (OARs) of urethra and rectum, using in‐house Matlab software. Photon and electron cutoff energies were set to 1 keV and 100 MeV, respectively. The dose distributions were calculated with the kerma approximation and the voxel size of 1 × 1 × 1 mm3. The number of incident photon was set to be the statistical uncertainty (1σ) of less than 1%. The effect of inter‐seed attenuation and prostate tissue compositions was evaluated from dose volume histograms (DVHs) for each structure, by comparing to results of the AAPM TG‐43 dose calculation (without the effect of inter‐seed attenuation and prostate tissue compositions).Results:The dose reduction due to the inter‐seed attenuation by source capsules was approximately 2% for CTV and OARs compared to those of TG‐43. In additions, by considering prostate tissue composition, the D90 and V100 of CTV reduced by 6% and 1%, respectively.Conclusion:It needs to consider the dose reduction due to the inter‐seed attenuation and tissue composition in prostate 125I brachytherapy dose calculations.