240 - Long-Term Results of a Phase II Prospective Clinical Trial of Single-Fraction, High-Gradient Adjuvant Partial Breast Irradiation in Early-Stage, Hormone Sensitive Breast Cancer

D. S. Lakomy¹, E. Laugeman², L. L. Ochoa¹, M. A. Thomas¹, and I. Zoberi¹; ¹Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO, ²Department of Radiation Oncology, Washington University School of Medicine in St. Louis, St. Louis, MO

Purpose/Objective(s): To evaluate long-term cancer control, toxicity, and cosmesis of adjuvant single-fraction, high-gradient partial breast irradiation for early-stage, hormone sensitive breast cancer. Materials/

Methods: Eligible patients had stage 0-1, hormone-sensitive, node-negative breast cancer. Patients were treated with adjuvant, single fraction, partial breast irradiation. Radiation dose was 20 Gy to the surgical bed decreasing with sharp gradient to 5 Gy at 1 cm from the surgical bed. Patients were seen for routine follow up and assessed for ipsilateral breast, regional, and distant recurrence. Treatment-related acute and late adverse events were prospectively measured by CTCAE v 4.0. Cosmesis was evaluated quantitatively by percent breast retraction assessment (pBRA) and qualitatively by physician and patient assessment.
Results: From 2015-2018, 50 patients were enrolled with a median age at diagnosis of 64 years (interquartile range 52-70); 44 were White (88%). The majority of patients had stage I (n=39, 78%) disease and median largest tumor dimension 1.0 cm (interquartile range 0.8-1.2 cm). Median follow-up was 74 months. Ipsilateral breast recurrence-free survival at 2 and 5 years was 98% and 96%. Both regional and distant metastasis recurrence-free survival at 2 and 5 years were 100% and 98%. Two patients developed ipsilateral breast tumor recurrences remote from the index cancer; no recurrent disease occurred within the PTV. One patient developed isolated axillary recurrence and proceeded to salvage treatment including radiation to the chest wall and regional nodes but later progressed with distant metastases. Radiation-associated acute toxicity was seen in 40% of patients and largely consisted of grade 1 skin erythema (26%) or other grade 1 soft-tissue changes (10%). Acute breast and/or chest wall pain was seen in 8% of patients. Radiation-associated late toxicity was seen in 68% of patients and was predominated by grade 1-2 fibrosis of the deep connective tissue (50%) and skin hyperpigmentation (36%). One instance of grade 3 fibrosis was observed. Median pBRA at baseline measured 6.8% and improved to 4.7% at 5 years. Overall breast cosmesis was deemed good-to-excellent in >93% of patients across all time points by both physician and patient evaluations.
Conclusion: Single fraction, partial breast irradiation planned with a high dose gradient for early-stage, hormone-sensitive patients results in excellent loco-regional control rates at 5 years, with low rates of acute and long-term toxicity and favorable cosmetic outcomes.