Fast dose optimization for rotating shield brachytherapy

Purpose: To provide a fast computational method, based on the proximal graph solver (POGS) - A convex optimization solver using the alternating direction method of multipliers (ADMM), for calculating an optimal treatment plan in rotating shield brachytherapy (RSBT). RSBT treatment planning has more degrees of freedom than conventional high-dose-rate brachytherapy due to the addition of emission direction, and this necessitates a fast optimization technique to enable clinical usage. Methods: The multi-helix RSBT (H-RSBT) delivery technique was investigated for five representative cervical cancer patients. Treatment plans were generated for all patients using the POGS method and the commercially available solver IBM ILOG CPLEX. The rectum, bladder, sigmoid colon, high-risk clinical target volume (HR-CTV), and HR-CTV boundary were the structures included in our optimization, which applied an asymmetric dose-volume optimization with smoothness control. Dose calculation resolution was 1 x 1 x 3 mm(3) for all cases. The H-RSBT applicator had 6 helices, with 33.3 mm of translation along the applicator per helical rotation and 1.7 mm spacing between dwell positions, yielding 17.5 degrees emission angle spacing per 5 mm along the applicator. Results: For each patient, HR-CTV D-90, HR-CTV D-100, rectum D-2cc, sigmoid D-2cc, and bladder D-2cc matched within 1% for CPLEX and POGS methods. Also, similar EQD2 values between CPLEX and POGS methods were obtained. POGS was around 18 times faster than CPLEX. For all patients, total optimization times were 32.1-65.4 s for CPLEX and 2.1-3.9 s for POGS. Conclusions: POGS reduced treatment plan optimization time approximately 18 times for RSBT with similar HR-CTV D-90, organ at risk (OAR) D-2cc values, and EQD2 values compared to CPLEX, which is significant progress toward clinical translation of RSBT. (C) 2017 American Association of Physicists in Medicine