MRI-assisted radiosurgery: A quality assurance nomogram for palladium-103 and iodine-125 prostate brachytherapy

Abstract

We sought to develop an activity nomogram for magnetic resonance (MR)eplanned permanent seed prostate brachytherapy to improve quality assurance through a secondary dosimetric check. METHODS AND MATERIALS: Patients undergoing MRI-assisted radiosurgery (MARS), whereby MRI is used for preoperative planning and postimplant dosimetry, were reviewed from May 2016 to September 2018. Planned activity (U) was fitted by MR-prostate volume (cc) via simple linear regression. Resulting monotherapy nomograms were compared with institutional nomograms from an ultrasound-planned cohort. Dosimetric coverage and external urinary sphincter (EUS) dose were also assessed for MR-planned patients. RESULTS: We identified 183 patients treated with MARS: 146 patients received palladium-103 (103 Pd; 102 monotherapy and 44 boost), and 37 received iodine-125 (125 I) monotherapy. Median prostate volume was 28 cc (interquartile range: 22e35). Lines of best fit for implant activity were U 5 4.344 Â (vol) þ 54.13 (R 2 : 95%) for 103 Pd monotherapy, U 5 3.202 (vol) þ 39.72 (R 2 : 96%) for 103 Pd boost and U 5 0.684 (vol) þ 13.38 (R 2 : 96%) for 125 I monotherapy. Compared with ultrasound, MR-planned nomograms had lower activity per volume (p ! 0.05) for both 103 Pd monotherapy (~6%) and 125 I monotherapy (~11%), given a median size (30 cc) prostate. Across all MARS implants, postimplant dosimetry revealed a median V100% of 94% (interquartile range: 92e96%). Median EUS V125 was !1 cc for all patients, regardless of isotope. CONCLUSIONS: We developed a quality assurance nomogram for MR-planned prostate brachytherapy. When compared with ultrasound-planned, MR-planned monotherapy resulted in a lower activity-to-volume ratio while maintaining dosimetric coverage, likely secondary to EUS-sparing and reduced planning target margins.