Relative biological effectiveness-weighted dose sparing utilizing a linear energy transfer optimization function for organs-at-risk in left breast proton treatment plans.

Current proton treatment plans employ a constant relative biological effectiveness (RBE) of 1.1, which may underestimate organs-at-risk (OAR) doses due to high linear energy transfer (LET) at the distal end of the proton beam. When 2 anterior fields are used for left breast treatment using pencil beam scanning (PBS), the ipsilateral lung and heart are susceptible to high LET. Although studies have indicated a potential increase in OAR toxicity linked to LET effects, the strategy of optimizing LET to RBE-weighted doses (D_vRBE) to spare OAR is still underexplored in treatment planning research. Addressing this gap is crucial for improving treatment outcomes and minimizing toxicities. To evaluate the effectiveness of LET optimization in reducing doses to OAR, we aimed to achieve more than a 10% reduction in the mean or maximum D_vRBE for the heart, ipsilateral lung, and humeral joint, while ensuring adequate target coverage. In this study, 12 cases of left-sided breast cancer with locally advanced invasive carcinoma were randomly selected to assess the dose reduction of OAR using LET optimization. The OAR doses were compared to those of clinically accepted PBS plans that did not incorporate LET optimization. To validate the hypothesis, a statistical analysis was implemented using the Wilcoxon-Signed Rank test. The results demonstrated a significant reduction in mean D_vRBE for both the heart and ipsilateral lung, as well as the maximum D_vRBE for humeral joint (p < 0.05). A decrease in both maximum and mean LET was observed in the ipsilateral lung (p < 0.05). These findings indicate that optimizing LET has the potential to effectively reduce D_vRBE for OAR, which can lead to minimizing organ toxicity.