PP04 Presentation Time: 9:30 AM: 169Yb-Based High Dose Rate Intensity Modulated Brachytherapy for Focal Treatment of Prostate Cancer

Abstract

<h3>Purpose</h3> Intensity-modulated brachytherapy (IMBT) is a developing application of brachytherapy where high-density metallic shields located inside the catheters are used to modulate radiations allowing the production of anisotropic dose distributions at each source dwell position. By rotating the shields during the treatment, the radiation is directed towards the tumour, resulting in better tumour conformity, while sparing the surrounding radiation-sensitive healthy tissues. This study compares conventional ¹⁹²Ir-based high dose rate (HDR) brachytherapy to ¹⁶⁹Yb-based HDR IMBT for focal treatment of prostate cancer and evaluates less invasive IMBT plans that use fewer catheters. <h3>Materials and Methods</h3> RapidBrachyMCTPS, an in-house treatment planning system (TPS) that takes into consideration the different organs' tissue composition and mass densities, was used to conduct a retrospective dosimetric study for two patients previously treated with conventional ¹⁹²Ir-based focal HDR brachytherapy and 5 catheters. The initial HDR ¹⁹²Ir-based treatment plan was first simulated and optimized in RapidBrachyMCTPS, which utilizes Fast Mixed Integer Optimization (FMIO), followed by IMBT treatment plans using the AIM-Brachy's shielded ¹⁶⁹Yb source. Furthermore, dose distributions were investigated when decreasing the number of catheters down from 5 to 2 ensuring the same coverage of dose to the target volume to produce less invasive treatment plans. <h3>Results</h3> The resulting dosimetric indices calculated with RapidBrachyMCTPS and the clinical TPS were compared. For the same planning treatment volume (PTV) D₉₀ coverage, IMBT results show a reduction in the urethra D₁₀ for the first and second patients of 11.75% and 25.44% for 5 catheters, of 13.37% and 22.77% for 4 catheters, of 9.98% and 21.71% for 3 catheters and 8.68% and 25.13% for 2 catheters. A decrease in the rectum D_2cc by up to 27.44% and 0.77% for the first and second patients respectively were recorded. The decrease of the dose to the urethra resulted in an increase of the PTV V₁₅₀ between 2.05-4.41% for patient 1 and 11.53-17.00% for patient 2. Similarly, the PTV V₂₀₀ increased between 0.92-4.54% for patient 1 and 9.28-25.69% for the second patient. All dosimetric indices of interest are presented in Table 1. <h3>Conclusions</h3> IMBT has the potential to successfully deliver less invasive focal prostate brachytherapy treatments with reduced dose spillage in healthy tissues leading to fewer side effects improving patients' quality of life.