Utility of Stereotactic Body Radiation Therapy in Establishing Local Control for Patients With Invasive Breast Cancer Not Undergoing Definitive Surgery.

Purpose: Surgery is the backbone of breast cancer (BC) treatment. For patients who cannot undergo surgery, improving local control (LC) of the primary tumor is paramount. To that end, this study explored the role of stereotactic body radiation therapy (SBRT).

Methods And Materials: Between 2015 and 2022, 21 nonsurgical candidates (10 metastatic, 11 stage IA-IIIC) received 23 SBRT courses to primary BC. Seven were analyzed retrospectively; 15 are currently enrolled in a prospective study. SBRT (40 Gy/5 fractions) was delivered every other day. Follow-up imaging was reviewed. Acute (≤3 months) and late toxicities were evaluated using Common Terminology Criteria for Adverse Events, version 5. LC and overall survival (OS) were estimated using Kaplan-Meier curves.

Results: Median age was 78.4 years (45.9-97.3). Median follow-up was 14.7 months (3.3-70.3). Median pre-SBRT index lesion size was 3.1 cm (0.5-14.5) and planning treatment volume was 32.4 cc (11.5-522.4). Initial posttreatment imaging performed at a median 4.0 months (0.6-11.9) post-SBRT demonstrated median decrease in index lesion size of 20.8% (0%-100%); SUV reduction of 65.2% (20.8%-100%). Second follow-up scans at a median 7.8 months post-SBRT showed 62% (0%-100%) and 88% (33.3%-100%) median reduction in tumor size and SUV, respectively, compared with pre-SBRT values. The estimated LC rate was 100% at 6 months and 93.3% at 12, 24, and 36 months. Local progression occurred in 1 case 9.5 months after SBRT, after an initial response. Regional progression occurred in 4 cases (17.4%) at a median 18.6 months (5.2-22.7) post-SBRT. Six patients (35.3%) developed distant progression at a median 2.7 months (0.9-16.2). The estimated OS was 85.7% at 6 months, 69.6% at 12 months, and 63.8% at 24 and 36 months. The rates of acute toxicity were G1: 47.8%, G2: 4.3%, G3: 8.7%, and G4: 0%.

Conclusions: Definitive SBRT for primary BC resulted in good LC in nonsurgical patients and was well-tolerated. Considering the pattern of progression, additional approaches to improve regional/distant control should be investigated.