Abstract PS6-04: Eliminating breast surgery for invasive, hormone-positive breast cancers with an exceptional response to endocrine therapy and ablative radiotherapy: a single-arm, phase 2 trial

Abstract Objective(s): To define pathologic response rates to endocrine therapy and ablative radiotherapy, with omission of breast surgery, for early-stage, hormone receptor (HR)+ breast cancer in a prospective, phase II trial (NCT02945579). Methods: Twenty eligible patients with HR+, HER2-, clinical stage I, unicentric, non-lobular breast cancers with no lymphovascular space invasion, Oncotype ≤25 and age ≥50 were accrued to an IRB approved-protocol. Enrolled patients received three months of endocrine therapy followed by restaging ultrasound and ablative radiotherapy, 37.5Gy/5 fractions every other day. MR LINAC was used when feasible. After radiotherapy, patients continued on endocrine therapy and underwent percutaneous vacuum-assisted, image-guided core biopsy (VAIGCB) of the tumor 6-12 months following radiation, with a minimum of 12 9G cores. Near complete response (nCR) was defined as Miller-Payne 4 and pCR as 5. Patients with a pathologic complete response (pCR) were followed every 6 months with imaging; those without a pCR were recommended for standard-of-care surgery. Miller-Payne score was evaluated on core biopsy and surgical specimens. Co-primary endpoints are pCR on VAIGCB and tumor control at 3 years. We report here the former co-primary endpoint of pCR along with the 95% credible interval (CI). Results: 19 of 20 (95%) of patients underwent VAIGCB; 1 declined and elected continued observation. Of the 19 biopsies, 10 (52.6%) demonstrated pCR (Miller-Payne 5), 7 (36.8%) nCR (Miller-Payne 4) and 2 (10.5%) Miller-Payne 3. Of patients who had a VAIGCB 6 months after radiotherapy, 5/11 (45.4%, 95% CI 18.9%-71.5%) had pCR; of those with VAIGCB 12 months after RT, 5/8 (62.5%, 95% CI 27.4%-86.6%) had pCR. 7/9 patients with residual disease (Miller-Payne <5) underwent surgery, one of whom had pCR in the surgical specimen, consistent with complete removal at VAIGCB. There were no postoperative complications. Two patients with nCR declined surgery: one underwent cryoablation and one continued endocrine therapy and underwent repeat biopsy 4 months later with pCR. In total, 17/19 pts who underwent VAIGCB (89.5%) had pCR or nCR. The 1 patient who declined VAIGCB has no residual disease on imaging 2 years after RT. Median follow-up time for all patients who did not have surgery is 26 (range 18 to 38 mo months), with none (0/12) experiencing progression or recurrence. Conclusion: This is the first study to demonstrate a high rate of VAIGCB pCR and nCR following endocrine therapy and ablative radiotherapy for early stage, HR+, HER2- breast cancers. This may be an appealing approach for patients with breast cancer interested in non-surgical approaches to definitively treat their tumors and highlights the efficacy for non-surgical candidates. Citation Format: Simona Shaitelman, Savitri Krishnamurthy, Gaiane M. Rauch, Yu Shen, PhD, Yan H. Lin, Benjamin D. Smith, Melissa P. Mitchell, Karen E. Hoffman, Chelain R. Goodman, Vicente Valero, Helen M. Johnson, Wendy A. Woodward, Henry Kuerer. Eliminating breast surgery for invasive, hormone-positive breast cancers with an exceptional response to endocrine therapy and ablative radiotherapy: a single-arm, phase 2 trial [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr PS6-04.