Energy-dependent OAR sparing and dose conformity for total marrow irradiation of obese patients

Abstract

Purpose: To investigate the effect on target coverage and organs at risk sparing by using 10 versus 6 MV for VMAT total marrow irradiation of obese patients. Methods and Materials: Twenty-six total marrow irradiation, TMI, treatment plans delivered between December 2014 and June 2017 were reviewed and 10 were chosen for replanning based on patient characteristics and plan metrics. Beam geometry and isocenter placement were conserved, energy was changed from 6 to 10 MV and plans were reoptimized. Resulting dose distributions were compared to original plans to evaluate any potential advantage of choosing one energy over the other. Results: Target coverage and total monitor units were consistent between the 6 and 10 MV plans when averaged over all ten patients. Improvement in the conformity index (−11.0%, P = 0.009) when using 10 MV was statistically significant compared to the 6 MV plans. Volumes of normal tissue receiving 50%, 75%, and 90% Rx all decreased for the 10 MV plans compared to the original 6 MV plans. The mean dose to individual OARs decreased significantly for all investigated structures except for the lenses, oral cavity, and genitalia. The largest decreases in Dmean were found for the rectum (22.4%, P = 0.004) and bladder (18.1%, P = 0.005). The three highest priorities for sparing during plan optimization (lungs, liver, and heart), showed decreases of 7.6%, 16.1%, and 13.0%. Conclusions: Use of a higher energy 10 MV beam provided similar dose to target while achieving increased OAR and normal tissue sparing for the patients reviewed in this study.