Can Proton Therapy Improve the Therapeutic Ratio in Breast Cancer Patients at Risk for Nodal Disease?

OBJECTIVES: Regional node irradiation in patients with invasive breast cancer often results in increased radiation exposure to organs at risk. We evaluated the potential advantages of 3-dimensional conformal photon+proton therapy (3DCX+PT) in treating regional nodes versus photon-electron (3DCRT) or intensity-modulated radiotherapy (IMRT). MATERIALS AND METHODS: Ten left-sided breast cancer patients underwent radiation treatment planning. 3DCX+PT, 3DCRT, and IMRT plans were generated for each patient. RESULTS: There was no significant difference in target coverage between 3DCX+PT and IMRT. However, coverage of level II axillary lymph nodes was inferior with 3DCRT with a median D95 of 45 versus 50 Gy with either IMRT (P=0.0006) or 49.5 CGE with 3DCX+PT (P=0.0033). Internal mammary nodes coverage was also inferior with 3DCRT (median D95 was 42 Gy) compared with 47 Gy with IMRT (P=0.043) or 48.5 CGE with 3DCX+PT (P=0.0068).With 3DCX+PT, left lung V20 and V5 were 31% and 50% versus 36% (P=0.0368) and 70% (P=0.0007) with 3DCRT and 30% (P=0.7328) and 81% (P=0.0002) for IMRT. 3DCX+PT resulted in heart V20 and heart V5 of 0% and 7%, respectively, versus 4% (P=0.0067) and 24% (P=0.0002) with 3DCRT and 21% (P=0.0001), 50% (P=0.0001) with IMRT. The IMRT plans produced significantly higher integral, contralateral lung, and breast doses. CONCLUSIONS: Regional node target coverage was inferior with 3DCRT compared with either IMRT or 3DCX+PT. Organs at risk were exposed to less radiation with 3DCX+PT compared with 3DCRT or IMRT. Proton treatment offered both improved coverage of the regional lymph nodes and decreased dose to the heart, lung, and contralateral normal tissue.