The organ equivalent dose to quantify secondary cancer induction in breast after VMAT treatments

Introduction: During volumetric arc therapy (VMAT) therapy, the dose delivered involves significant area and organs with low dose. In order to study the Radiation Therapy (RT) cancer induction probability, the risk for contralateral breast and lung secondary tumor was evaluated, estimating the organ equivalent dose (OED). Materials and Methods: The dose distribution of 30 breast cancer patients treated with VMAT techniques were analyzed in this retrospective study. The cohort was divided for treatment side (right/left) and the OED for each organ was calculated from the dose volume histogram (DVH). The bell shaped model formula was applied in a MATLAB® toolbox to estimate secondary cancer induction in breast and lung tissues. Contralateral and ipsilateral statistic outcomes were assessed using SPSS®. Results: A mean ODE of 2.09 ± 0.32, 1.94 ± 0.32 and 2.55 ± 0.61 was obtained for contralateral lung, ipsilateral lung and contralateral breast respectively. For contralateral organs, an ANOVA analysis (sign. > 0.30) confirmed that these results were independent to the side of treatment, with an intra-group variability of [2.1 ÷ 3.1 Gy] and [2.2 ÷ 3.8 Gy] for the right and left side. For ipsilateral organs, in agreement with other scientific works in literature, the OED showed a less variability (±0.16 Gy). Conclusions: The ODE value is a simple and useful estimation of radiation secondary cancer induction of VMAT treatments based on a DVH analysis. With a cross correlation between contralateral and ipsilateral structures, an independence from the side of treatment was detected, highlighting a lower intra-organ variability in case of ipsilateral comparison. To increase the robustness of the study, the bell shaped model used could be related to the time patterns of cancer induction. Moreover, absolute risk should be investigated in a larger patients' cohort or in national data mining.