Efficacy and Safety of Neoadjuvant Stereotactic Body Radiotherapy (SBRT) Combined with Chemoimmunotherapy in Locally Advanced Breast Cancer: A Single-Center, Retrospective Pilot Study.

[PURPOSE] The combination of radiotherapy with immunotherapy holds synergistic potential, yet its role in the neoadjuvant treatment of breast cancer remains underexplored. This single-center, retrospective pilot study aimed to explore the preliminary efficacy and safety of integrating stereotactic body radiotherapy (SBRT) with chemoimmunotherapy as a novel neoadjuvant regimen for a small cohort of patients with high-risk, locally advanced breast cancer.

[PATIENTS AND METHODS] Between June 2023 and August 2025, 20 patients received neoadjuvant SBRT (18Gy/3 fractions for node-positive; 24Gy/3 fractions for node-negative disease) concurrently with chemoimmunotherapy (various anti-PD-1/bispecific antibodies plus chemotherapy), followed by surgery. Key endpoints were pathological complete response (pCR, ypT0/Tis ypN0), near-pCR (Residual Cancer Burden [RCB] class 0 or I), objective response rate (ORR), and safety.

[RESULTS] The cohort included triple-negative breast cancer (TNBC, 35%) and hormone receptor-positive/HER2-negative breast cancer of the Luminal B subtype (Luminal B/HER2-, 50%). Most had cT2 (75%) and node-positive (75%) disease. The median number of chemoimmunotherapy cycles were 8, with 80% receiving an AC-T backbone. The overall pCR rate was 45% (9/20). Efficacy varied by subtype: the pCR rate was 85.7% (6/7) in TNBC and 20% (2/10) in Luminal B/HER2- disease. Notably, 100% of TNBC and 40% of Luminal B/HER2- patients achieved near-pCR (RCB 0/I). ORR was 90%. Grade 3-4 adverse events occurred in 25% of patients, with no treatment-related mortality.

[CONCLUSION] In this single-center, retrospective pilot study, early integration of SBRT with chemoimmunotherapy demonstrated promising antitumor activity, particularly high pCR rates in TNBC and induction of deep pathological responses (RCB 0/I) in Luminal B/HER2- disease, with a manageable safety profile. Observed in a heterogeneous cohort, these preliminary findings highlight potential efficacy but require cautious interpretation and warrant validation in larger, prospective trials.