¹⁶⁹Yb-based rotating shield brachytherapy for prostate cancer

Purpose: To present a system for the treatment of prostate cancer in a single-fraction regimen using ¹⁶⁹Yb-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 ¹⁹²Ir-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 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) ¹⁶⁹Yb source and one 370 GBq (10 Ci) ¹⁹²Ir 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 D_10%, PTV D_90% for RSBT vs 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 min for RSBT and 15.8 ± 2.3 min for 10 Ci ¹⁹²Ir-based HDR-BT. For urethra sparing, with equivalent PTV D_90%, urethra D_10% for RSBT vs HDR-BT decreased for RSBT vs 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 min for RSBT and 12.3 ± 1.8 min for HDR-BT. Differences between measured vs predicted rotating catheter positions (corresponding to source position) were within 0.18 mm ± 0.12 mm longitudinally and 0.07° ± 0.78°. Conclusion: ¹⁶⁹Yb-based RSBT can increase PTV D_90% or decrease urethral D_10% relative to HDR-BT with treatment times of less than 1 h 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.