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

Purpose: We sought to develop an activity nomogram for magnetic resonance (MR)-planned 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: 22-35). Lines of best fit for implant activity were U = 4.344 × (vol) + 54.13 (R ² : 95%) for ¹⁰³ Pd monotherapy, U = 3.202 (vol) + 39.72 (R ² : 96%) for ¹⁰³ Pd boost and U = 0.684 (vol) + 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: 92-96%). 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.
(Copyright © 2020 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.)