Can low risk, early stage breast cancer be treated with ablative radiation alone? In this phase 2 trial, 20 patients with unicentric, T1N0 (1.5cm or smaller), ER/PR+, HER2- breast cancer received 3 months of endocrine therapy followed by SABR to the primary breast tumor. The target was the gross tumor on MRI plus clip and a 2 cm margin cropped from muscle and 5 mm from the skin. A 5 mm PTV margin was used around the GTV and CTV for two different prescription dose levels of 37.5 Gy and 28.5 Gy in 5 fractions. Endocrine therapy was continued for 5 years. A biopsy was done 6-12 months after radiation. The pathologic complete response rate was 53%, and another 37% had near-complete response. Of those who did not have surgery, the rate of PFS was 92% at 3 years, and local control was 100%. Over half (58%) had mild radiation effects including fibrosis, edema, and induration. | Shaitelman, Radiother Oncol 2026
