Definitive hypofractionated radiation therapy for early stage breast cancer: Dosimetric feasibility of stereotactic ablative radiotherapy and proton beam therapy for intact breast tumors

Purpose: Few definitive treatment options exist for elderly patients diagnosed with early stage breast cancer who are medically inoperable or refuse surgery. Historical data suggest very poor local control with hormone therapy alone. We examined the dosimetric feasibility of hypofractionated radiation therapy using stereotactic ablative radiotherapy (SABR) and proton beam therapy (PBT) as a means of definitive treatment for early stage breast cancer. Methods and Materials: Fifteen patients with biopsy-proven early stage breast cancer with a clinically visible tumor on preoperative computed tomography scans were identified. Gross tumor volumes were contoured and correlated with known biopsy-proven malignancy on prior imaging. Treatment margins were created on the basis of set-up uncertainty and image guidance capabilities of the three radiation modalities analyzed (3-dimensional conformal radiation therapy [3D-CRT], SABR, and PBT) to deliver a total dose of 50 Gy in 5 fractions. Dose volume histograms were analyzed and compared between treatment techniques. Results: The median planning target volume (PTV) for SABR, PBT, and 3-dimensional CRT was 11.91, 21.03, and 45.08 cm(3), respectively, and were significantly different (P < .0001) between treatment modalities. Overall target coverage of gross tumor and clinical target volumes was excellent with all three modalities. Both SABR and PBT demonstrated significant dosimetric improvements, each in its own unique manner, relative to 3D-CRT. Dose constraints to normal structures including ipsilateral/contralateral breast, bilateral lungs, and heart were all consistently achieved using SABR and PBT. However, skin or chest wall dose constraints were exceeded in some cases for both SABR and PBT plans and was dictated by the anatomic location of the tumor. Conclusions: Definitive hypofractionated radiation therapy using SABR and PBT appears to be dosimetrically feasible for the treatment of early stage breast cancer. The anatomical location of the tumor relative to the skin and chest wall appears to be the primary limiting dosimetric factor. (C) 2018 The Author(s). Published by Elsevier Inc. on behalf of the American Society for Radiation Oncology.