Preoperative Single Fraction Stereotactic Partial Breast Irradiation for Early-Stage Breast Cancer Patients With GammaPod Technology: Pathologic Findings and Ki-67 Evaluation.

PURPOSE: The purpose of this phase 2 clinical study is to investigate the safety and feasibility of a single fraction stereotactic partial breast irradiation (S-PBI) for early-stage breast cancer in the preoperative setting and to evaluate tumor response to a single large radiation dose through pathologic examination and immunohistochemistry analysis of the surgical specimen.

METHODS AND MATERIALS: This single arm, phase 2 clinical trial includes patients in postmenopausal status, over the age of 50 years, with early-stage (cT1-T2 cN0) breast cancer, luminal type, any grade, unifocal tumor, and suitable for breast conserving surgery. The gross tumor volume includes the tumor. The clinical target volume corresponds to gross tumor volume. The planning target volume is created by adding 3 mm symmetrical margins from the clinical target volume. Treatment is delivered through GammaPod technology as single fraction radiosurgery, to a total dose of 30 to 36 Gy. Surgery is performed 8 to 28 weeks after S-PBI. Pathologic response is classified as pathologic complete response (pCR), near complete response with <10% of residual disease, pathologic partial response with 10% to 90% of residual disease, or stable disease with >90% of residual disease. We further group pCR and near complete response together as "Major Response."

RESULTS

From January 2022 to November 2023, 49 patients were enrolled and underwent S-PBI followed by breast conserving surgery. The rate of Major Response was 37%, including pCR in 18% of cases. The mean Ki-67 index was reduced from 9.5% pre-S-PBI to 2% post-S-PBI.

CONCLUSIONS

Preoperative single fraction S-PBI appears to be associated with a promising rate of "Major Response," including cases of complete response.