Purpose: Line-source consideration is appropriate for dose calculations in low-dose-rate prostate seed implant (LDR-PSI) cases when stranded sources are used, as the orientation of the seed is known. Some RTOG protocols, however, recommend use of a point-source approximation. The purpose of this study is to evaluate the differences in target and organs-at-risk (OAR) dosimetry of point-source as compared to line-source approximations on intra-operative planning of ultrasound-guided LDR-PSI.
Methods: Out of 30 randomly selected PSI patients, 15 utilized I-125 (Theragenics AgX100), 15 had Pd-103 (Theragenics TheraSeed-200), and 15 patients who were treated with I-125 were replanned using Cs-131 (IsoRay CS-1 model). Dosimetric computation was performed on TRUS images using MIM Symphony TPS for both point-source and line-source approximations to evaluate dosimetric differences in OARs and target.
Results: In most cases, point-source underestimated dose to OARs. For rectum, mean dose difference between line-source and point-source for D0.1cc D1cc, and D2cc were -1.9% (+/-1.6), -3.1% (+/-1.3), and -3.3% (+/-1.9), respectively. Similarly, D10 of urethra was underestimated by point-source, with mean difference of -3.4% (+/-1.9). For prostate, mean difference for D90 was -0.3% (+/-1.7), but for V150 and V200, point-source dosimetry overestimated in comparison to line-source by 2.3% and 9.0%, respectively. I-125, Pd-103, and Cs-131 isotopes exhibited mixed results on OARs. Rectum D1cc and D2cc for Pd-103 and Cs-131 were approximately 1% lower for point-source compared to I-125, while point-source overestimated for Cs-131 target parameters. The largest noted difference was found in V100 for urethra, with a mean difference for I-125 of -23.1% (+/-79.9%), and for Pd-103 -4.5 (+/-5.6).
Conclusion: Overall, point-source approximation, compared to line-source, underestimated dose. Dose difference between I-125, Pd-103, and Cs-131 showed variation, depending on OAR parameters considered. When orientation of the implanted source is known, it is reasonable to use line-source approximation to achieve accurate dosimetry for LDR-PSI.