The benefit of DIBH on irradiated lung volume is overestimated in right-sided breast cancer VMAT with node involvement

Deep inspiration breath-hold (DIBH) is the preferred technique over free-breathing (FB) to reduce the irradiated lung volume, based on direct comparisons of dose-volume histogram (DVH) values. This study employs deformable image registration (DIR) to account for the non-uniform lung expansion in assessing the actual benefit of DIBH in volumetric modulated arc therapy (VMAT) of right-sided breast cancer with regional node involvement. The study included 25 patients, divided between two treatment planners, with retrospective VMAT plans for right-sided breast cancer with regional node involvement in both DIBH and FB anatomy. The irradiated ipsilateral lung volumes of V16Gy, V8Gy and V4Gy were transferred from DIBH CT onto the FB CT image using DIR. DVH parameters were calculated based on the transferred irradiated lung volumes and compared to the planned values in the FB image anatomy. The irradiated V16Gy, V8Gy and V4Gy volumes of the ipsilateral lung increased after transferring from DIBH CT to FB CT (21.3 % to 23.1 %, 35.6 % to 38.9 % and 53.4 % to 57.3 % by median, respectively, p < 0.001), regardless of planner (p > 0.49). To achieve dosimetric benefits with DIBH compared to FB, reductions in lung DVH parameters between -7.2 % and -4.9 %, relative to planned FB values, are required. The apparent dosimetric benefit of DIBH on the irradiated lung volume over FB is overestimated at the planning stage. However, DIBH can reduce the dose to all organs at risk through rigorous treatment planning.