Combined Whole Prostate Dose Escalation With Focal Gross Disease Boost Using 169Yb-Based Rotating Shield Brachytherapy

Abstract

<h3>Purpose/Objective(s)</h3> To assess the capability of ¹⁹²Ir-based high-dose-rate brachytherapy (HDR-BT) versus ¹⁶⁹Yb-based rotating shield brachytherapy (RSBT) for simultaneous whole prostate dose escalation with focal boost to dominant intraprostatic lesions (DILs) in a treatment planning study. <h3>Materials/Methods</h3> Twenty-six patients were considered. For each patient, 27 DILs were programmatically generated at locations sampled over a grid of 27 sectors within the planning target volume with a range of volumes sampled from a gaussian probability density function with a mean of 2.4 ± 0.66 cm³, based on the literature. For HDR-BT there were 20-24 clinically placed needles which were supplemented by up to two additional needle positions placed within the DIL at dosimetrically beneficial positions, resulting in a total of 21-26 total needles used. For RSBT a total of 20 needles were used for all patient cases and the needle configuration was not changed based on DIL case. Prescription dose was 20.5 Gy with HDR-BT dose scaled such that 90% of the PTV received 110% of the prescription dose. RSBT dose was scaled to a maximum PTV D_90% while matching the urethra D_10% of HDR-BT. The maximum focal boost dose was defined by normal tissue constraints. <h3>Results</h3> A separate treatment plan was generated for each DIL for each patient, for both RSBT and HDR-BT, a total of 702 unique DIL treatment plans with 26 reference plans. For all combinations of patient and DIL cases, whole prostate dose escalation was 22.6 ± 0.0 Gy (average ± standard deviation) for HDR-BT and 29.9 ± 0.2 Gy for RSBT. The DIL D_90% for HDR-BT was 37.9 ± 4.8 Gy and RSBT DIL D_90% was 60.9 ± 7.2 Gy. The relative difference in dose escalation between whole prostate and focal boost for HDR-BT was 72.7% ± 24.1% and was 102.8% ± 24.3% for RSBT. <h3>Conclusion</h3> Compared to conventional HDR-BT, ¹⁶⁹Yb-based RSBT can provide a relative increase in dose escalation to the whole prostate of 32.6% ± 1.0% with a simultaneous focal gross disease boost of 35.8% ± 11.8%. Without altering the number or positioning of implanted needles, RSBT can achieve simultaneous dose escalation and focal boost for a variety of clinically relevant DIL volumes and locations.