Purpose: To compare target coverage and normal tissue sparing of state of the art photon whole breast irradiation (WBI) and proton partial breast irradiation (PPBI).
Methods: Consecutive women with node negative breast cancer treated with lumpectomy and adjuvant WBI, without regional nodal irradiation, or PPBI were included. WBI was delivered with 3-dimensional conformal tangential fields, targeting the breast CTV with a 5 mm expansion to PTV to a median dose of 40.05 Gy in 15 fractions. Left-sided WBI patients were treated in deep-inspiratory breath hold. PPBI was delivered with a median of 2 multi-field optimized beams, targeting the lumpectomy cavity plus 1 cm, to a median dose of 21.9 Gy in 3 daily fractions. Setup uncertainty analyses of +/- 3 mm isocenter shifts in each translational axis and 3% beam range uncertainty were performed to ensure robust target coverage and normal tissue sparing. Dosimetric parameters, collected prospectively, are primarily presented as % prescription and we excluded the boost component of WBI for uniformity of plan comparisons.
Results: 836 women were treated between 2015-2018; 762 received WBI (274 [36%] with boost to the lumpectomy cavity), and 74 with PPBI. Patients treated with PPBI were older and had more favorable breast cancer (Table 1). Patients treated with PPBI had comparable target coverage but significantly lower heart and lung doses (Table 2).
Conclusions: PPBI reduces exposure to the heart and lungs. Follow-up is needed to determine if these dosimetric advantages translate into improved clinical outcomes.