Pre-Operative Single Fraction Stereotactic Partial Breast Irradiation (S-PBI) for Early-Stage Breast Cancer Patients with Gammapod Technology: Pathological Findings and Ki-67 Evaluation.

Abstract

Purpose: The purpose of this Phase II clinical study is to investigate the safety and feasibility of a single fraction Stereotactic Partial Breast Irradiation (S-PBI) for early-stage breast cancer (BC) in the pre-operative setting and to evaluate tumor response to a single large radiation dose through pathological examination and immunohistochemistry analysis of the surgical specimen.

Materials and methods: This single arm, phase II clinical trial includes patients in post-menopausal status, over the age of 50, with early-stage (cT1-T2 cN0) BC, luminal type, any grade, unifocal tumor, suitable for breast-conserving surgery (BCS). The gross tumor volume (GTV) includes the tumor. The clinical target volume (CTV) corresponds to GTV. The planning target volume (PTV) is created by adding 3 mm symmetrical margins from the CTV. Treatment is delivered through GammaPod technology as single fraction radiosurgery, to a total dose of 30-36 Gy. Surgery is performed 8 to 28 weeks after S-PBI. Pathological response is classified as complete response (pCR), near complete response (nCR) with less than 10% of residual disease, partial response (pPR) with 10% to 90% of residual disease, or stable disease with more than 90% of residual disease. We further group pCR and nCR together as "Major Response".

Results: From January 2022 to November 2023, 49 patients were enrolled and underwent S-PBI followed by BCS. 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.

Conclusion: Pre-operative single fraction S-PBI appears to be associated with a promising rate of 'Major Response,' including cases of complete response.