Contour‐based lung dose prediction for breast proton therapy

PURPOSE: This study evaluates the feasibility of lung dose prediction based on target contour and patient anatomy for breast patients treated with proton therapy. METHODS: Fifty‐two randomly selected patients were included in the cohort, who were treated to 50.4–66.4 Gy(RBE) to the left (36), right (15), or bilateral (1) breast with uniform scanning (32) or pencil beam scanning (20). Anterior‐oblique beams were used for each patient. The prescription doses were all scaled to 50.4 Gy(RBE) for the current analysis. Isotropic expansions of the planning target volume of various margins m were retrospectively generated and compared with isodose volumes in the ipsilateral lung. The fractional volume V of each expansion contour within the ipsilateral lung was compared with dose–volume data of clinical plans to establish the relationship between the margin m and dose D for the ipsilateral lung such that V (D) = V(m). This relationship enables prediction of dose–volume V(D) from V(m), which could be derived from contours before any plan is generated, providing a goal of plan quality. Lung V (20 Gy() (RBE) ()) and V (5 Gy() (RBE) ()) were considered for this pilot study, while the results could be generalized to other dose levels and/or other organs. RESULTS: The actual V (20 Gy() (RBE) ()) ranged from 6% to 23%. No statistically significant difference in V (20 Gy() (RBE) ()) was found between breast irradiation and chest wall irradiation (P = 0.8) or between left‐side and right‐side treatment (P = 0.9). It was found that V(1.1 cm) predicted V (20 Gy() (RBE) ()) to within 5% root‐mean‐square deviation (RMSD) and V(2.2 cm) predicted V (5 Gy() (RBE) ()) to within 6% RMSD. CONCLUSION: A contour‐based model was established to predict dose to ipsilateral lung in breast treatment. Clinically relevant accuracy was demonstrated. This model facilitates dose prediction before treatment planning. It could serve as a guide toward realistic clinical goals in the planning stage.