Evaluation of the effect of prostate volume change on tumor control probability in LDR brachytherapy

Purpose: This study evaluates low dose-rate brachytherapy (LDR) prostate plans to determine the biological effectof dose degradation due to prostate volume changes. Material and methods: In this study, 39 patients were evaluated. Pre-implant prostate volume was determinedusing ultrasound. These images were used with the treatment planning system (Nucletron Spot Pro 3.1 ) to create treatmentplans using 103Pd seeds. Following the implant, patients were imaged using CT for post-implant dosimetry. Fromthe pre and post-implant DVHs, the biologically equivalent dose and the tumor control probability (TCP) were determinedusing the biologically effective uniform dose. The model used RBE = 1.75 and α/β = 2 Gy. Results: The prostate volume changed between pre and post implant image sets ranged from –8% to 110%. TCP andthe mean dose were reduced up to 21% and 56%, respectively. TCP is observed to decrease as the mean dose decreasesto the prostate. The post-implant tumor dose was generally observed to decrease, compared to the planned dose.A critical uniform dose of 130 Gy was established. Below this dose, TCP begins to fall-off. It was also determined thatpatients with a small prostates were more likely to suffer TCP decrease. Conclusions: The biological effect of post operative prostate growth due to operative trauma in LDR was evaluatedusing the concept. The post-implant dose was lower than the planned dose due to an increase of prostate volumepost-implant. A critical uniform dose of 130 Gy was determined, below which TCP begun to decline.