S278
Clinical - Breast
ESTRO 2026
toward poorer survival outcomes compared with both HER2-positive and IHC 2+ cases. In this era of RT field de-escalation for patients receiving neoadjuvant CT, caution is warranted, as HER2-low IHC 1+ patients may have a less favorable prognosis. References: [1] Modi S, Jacot W, Yamashita T, Sohn J, Vidal M, Tokunaga E, et al. Trastuzumab Deruxtecan in Previously Treated HER2-Low Advanced Breast Cancer. New England Journal of Medicine. 2022;387:9-20. Keywords: breast cancer, HER2-low, radiotherapy High-dose volume predicts fat necrosis after post- operative single fraction stereotactic partial breast irradiation (S-PBI) in a phase II trial Giuseppe Facondo 1 , Agnese Prisco 1 , Chiara Reverberi 1 , Carmen Giunco 2 , Mario Alexander Liuzzo 2 , Tino Ceschia 1 , Eleonora Festa 1 , Daniela Marfisi 2 , Sarah Bassi 2 , Claudio Foti 2 , Paolo Scalchi 2 , Marco Trovo 1 1 Radiation Oncology, Santa Maria della Misericordia, Udine, Italy. 2 Medical Physics, Santa Maria della Misericordia, Udine, Italy Mini-Oral 2446 Purpose/Objective: to evaluate predictive dosimetric factors for fat necrosis (FN) after post-operative single fraction stereotactic partial breast irradiation (S-PBI) in early- stage breast cancer. Material/Methods: This prospective, single-institution phase II trial enrolled 148 patients who received breast-conserving surgery followed by S-PBI. Treatment consisted of a single 17.5 Gy fraction prescribed to 95% of the planning target volume (PTV) using GammaPod™. The nominal dose was 18.4 Gy. FN was evaluated clinically at each follow-up. FN was classified into two categories: FN detected only by imaging (mammography or contrast-enhanced mammography, CEM) and symptomatic FN detected by imaging and associated with clinical symptoms. To describe the cumulative incidence of FN over time after S-PBI we used the Kaplan Meier method. Univariate and stepwise logistic regression analyses were conducted to predictive factors associated with FN. Results: Between January 2022 and May 2024, 148 patients were enrolled. Median age at diagnosis was 68 years (IQR 58-74). Median follow-up was 29.9 months (IQR 23.5-38.8). Median whole breast volume was 1242.63cc (IQR 722.69-1394.42). Median PTV volume was 43.6cc (IQR 32.6-58.5). MedianV19.3(105%) was 0.55cc. 18 patients (12.1 %) developed RN and 6 patients (4.1%) symptomatic FN.The incidence rate FN at 6-12-18-24 months were 1.4%, 6.8%, 9.5% and11.3%
At the last follow-up, 161 patients (82.1%) were alive w/o disease, 7 patients (3.6%) were alive with disease, 26 patients (13.3%) had died of disease, and 2 patients (1%) had died of other causes. At a median follow-up of 46.8 months (0–261 months), the 5- and 10-year OS rates were 83% and 59% for IHC 1+ cases, 92% and 92% for IHC 2+ cases, and 85% and 69% for HER2- positive cases (p = 0.3) (Figure 1).
The corresponding 5- and 10-year locoregional recurrence–free survival (LRRFS) rates were 85% and 60% for IHC 1+, 92% and 92% for IHC 2+, and 84% and 68% for HER2-positive cases (p = 0.3), respectively. Conclusion: Patients with the HER2-low IHC 2+ subtype exhibited higher prevalence of grade 3 histology and greater ER & PR positivity. Also, patients with HER2-low IHC 1+ & 2+ tumors demonstrated poorer responses to neoadjuvant CT. IHC 1+ cases also showed a trend
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