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Breast cancer treatment typically involves a combined modality approach, with surgical treatment playing the central role. However, due to various reasons, some patients are not able to undergo surgery. In the absence of resection, it is paramount to improve local control of the primary tumor. To that end, we explored the role of ablative radiation dose delivered with SBRT. 

Between 2015-2022, 28 patients underwent definitive SBRT for primary breast cancer without planned surgical intervention. SBRT dose was 25-40 Gy in 5 fractions (40 Gy in 5 fractions in 80% of cases). Post-treatment imaging (PET-CT, MRI, MMG and ultrasound) as well as clinical exams were used to evaluate the response. Toxicity rates and response rates were assessed as primary endpoints of the study.  Kaplan-Meier curves were used to estimate local control (LC) and overall survival (OS).

The reasons for breast SBRT included the need for aggressive local palliation, medical comorbidities precluding surgery, unresectable disease, and patient preference. Median age was 69 (36-97) years. Median follow-up was 32 (3.4-70.4) months after RT completion. Initial post-treatment imaging was available in 96% of the cases and was performed at a median 2.2 (0.6-8.1) months after SBRT. Radiographically, complete response (CR) was seen in 28% and partial response (PR) in 64% of the cases. Among those with PR, median decrease in diameter was 44% (16.1-68.8%). Median SUV reduction was 65.2% (27.9-69.7%). Acute toxicity occurred in 93% of the cases, including grade 1 (n=16), grade 2 (n=8), and grade 4 (necrosis; n=2). Late toxicity included grade 2 edema (n=2) and grade 4 skin toxicity (necrosis; n=2, including 1 case of sequential late effect). Acute and late grade 4 toxicities were successfully managed with debridement or steroids.
There have been 3 (11%) cases of local progression in the index lesion. Estimated LC after SBRT was 100% at 6 months and 89% at 1, 2 and 3 years. Median OS was not reached. The estimated median OS was 80% at 1 year, and 60% at 2 and 3 years.

Definitive SBRT to primary tumor in the breast appears to be well tolerated with high rates of radiographic and clinical response as well as high local control rates. To our knowledge, the current study represents the largest series of definitive SBRT in non-surgical breast cancer patients. Larger prospective studies and longer follow up are needed to assess late toxicity and durability of response.