Patient-specific MU rate optimization to reduce lung dose rate in Volumetric Modulated Arc Therapy-based Total Body Irradiation: A feasibility study.

Abstract

Reducing the lung dose rate significantly decreases the risk of toxicity in high-dose total-body irradiation (TBI). This study investigated the feasibility of delivering Volumetric Modulated Arc Therapy (VMAT) TBI with a reduced monitor unit (MU) rate on a Versa HD linear accelerator. Treatment data for nine patients who underwent VMAT TBI were retrospectively analyzed. The mean lung dose per fraction and thorax VMAT field MU were used to calculate a patient-specific MU rate required to achieve a lung dose rate below 10 cGy/min. The patient-specific MU rate was implemented for thorax VMAT fields using the Mosaiq record-and-verification system and delivered in continuous variable dose-rate (CVDR) mode. Dosimetric consistency was evaluated using MatriXX Resolution detector-based planar dose verification with 3%/2 mm gamma analysis, and treatment log files were reviewed to assess MU rates and beam-on times. The mean lung dose was 782.3 ± 12.8 cGy. With a MU rate of 600 MU/min, the average lung dose rate was 16.1 ± 1.8 cGy/min. Application of patient-specific MU rates (95-196 MU/min) reduced the average lung dose rate to 8.8 ± 0.2 cGy/min in all patients (p < 0.001). Gamma pass rates exceeded 95% for all plans, confirming dosimetric consistency. Patient-specific MU-rate optimization is a feasible and reproducible strategy for reducing lung dose rate in VMAT-TBI on Elekta linacs. Further prospective studies with clinical outcome assessments are necessary to validate the impact of patient-specific lung dose rate reduction.