169 Yb-Based Rotating Shield Brachytherapy for Prostate Cancer.

PURPOSE To present a system for the treatment of prostate cancer in a single-fraction regimen using 169 Yb-based rotating shield brachytherapy (RSBT) with a single-catheter robotic delivery system. The proposed system is innovative because it can deliver RSBT through multiple implanted needles independently, in serial, using flexible catheters, with no inter-needle shielding effects and without the need to rotate multiple shielded catheters inside the needles simultaneously, resulting in a simple, mechanically robust, delivery approach. RSBT was compared to conventional 192 Ir-based high-dose-rate brachytherapy (HDR-BT) in a treatment planning study with dose escalation and urethral sparing goals, representing single-fraction brachytherapy monotherapy and brachytherapy as a boost to external beam radiotherapy, respectively. A prototype mechanical delivery system was constructed and quantitatively evaluated as a proof of concept. METHODS Treatment plans for twenty-six patients with single fraction prescriptions of 20.5 Gy and 15 Gy, were created for dose escalation and urethral sparing, respectively. The RSBT and HDR-BT delivery systems were modeled with one partially-shielded 999 GBq (27 Ci) 169 Yb source and one 370 GBq (10 Ci) 192 Ir source, respectively. A prototype angular drive system for helical source delivery was constructed. Mechanical accuracy measurements of source translational position and angular orientation in a simulated treatment delivery setup were obtained using the prototype system. RESULTS For dose escalation, with equivalent urethra D10% , PTV D90% for RSBT versus HDR-BT increased from 22.6 ± 0.0 Gy (average ± standard deviation) to 29.3 ± 0.9 Gy, or 29.9 % ± 3.0%, with treatment times of 51.4 ± 6.1 minutes for RSBT and 15.8 ± 2.3 minutes for 10 Ci 192 Ir-based HDR-BT. For urethra sparing, with equivalent PTV D90% , urethra D10% for RSBT versus HDR-BT decreased for RSBT versus HDR-BT from 15.6 ± 0.4 Gy to 12.0 ± 0.4 Gy, or 23.1% ± 3.5%, with treatment times of 30.0 ± 3.7 minutes for RSBT and 12.3 ± 1.8 minutes for HDR-BT. Differences between measured versus predicted rotating catheter positions (corresponding to source position) were within 0.18 mm ± 0.12 mm longitudinally and 0.07° ± 0.78°. CONCLUSION 169 Yb-based RSBT can increase PTV D90% or decrease urethral D10% relative to HDR-BT with treatment times of less than one hour using a single source robotic delivery system with treatment delivered in a single fraction. The prototype helical delivery system was able to demonstrate adequate mechanical accuracy.