ESTRO 2024 - Abstract Book

S608

Clinical - Breast

ESTRO 2024

Nazionale dei Tumori di Milano, Medical Physics Unit, Milan, Italy. 4 Fondazione IRCCS Istituto Nazionale dei Tumori di Milano, Medical Oncology Department, Milan, Italy. 5 Fondazione IRCCS Istituto Nazionale dei Tumori di Milano, Breast Unit, Milan, Italy

Purpose/Objective:

With the advent of novel drugs for Breast Cancer, quantifying their interactions with radiation treatment (RT) is paramount. We aim to assess the safety of concomitant RT and Pembrolizumab in early breast cancer.

Material/Methods:

We prospectively analyzed female patients’ toxicity outcomes with diagnosis of triple negative breast cancer, who received neoadjuvant chemoimmunotherapy with Pembrolizumab as per Keynote 522 criteria, breast cancer surgery, at least 1 cycle of adjuvant pembrolizumab with concomintant adjuvant RT. Prescribed dose was 40.05Gy in 15 fractions to the breast or chest wall, and eventual regional nodal irradiation; boost to the tumor bed was allowed if required. Left-sided BC patients were treated with deep inspiration breath hold. The study's endpoint was the feasibility of the treatment in terms of acute toxicity, evaluated according to the RTOG scale, and discontinuation of RT and/or Pembrolizumab.

Results:

From February to July 2023, we treated a total of 8 patients. Median age was 54 (range: 27 – 68); six patients had left-sided breast cancer; clinical staging was II and III in 6 and 2 women, respectively. All patients underwent neoadjuvant chemoimmunotherapy with Pembrolizumab, of which a median of 8 (range 4 – 9) cycles were prescribed. Six patients received breast conservative surgery, while two had mastectomy. Biopsy of the sentinel lymph node without any further axillary surgery was performed for every patient. Major pathologic response was reached in all of the cases, with 6 complete pathologic response. Before RT, patients received a median of 3 (range 2 – 4) cycles of adjuvant Pembrolizumab. Volumes of RT consisted of residual breast + tumoral bed boost after breast conservative surgery (6 patients) and chest wall after mastectomy (2 patients), We prescribed regional nodal irradiation in three patients: one to the III and IV axillary level, one to the internal mammary chain, and one to all axillary levels + internal mammary chain. Two patients experienced grade (G) ≥ 3 acute toxicities with interruption of treatments: one had G4 hepatotoxicity one month after RT despite no side effects during the entire course of neoadjuvant chemoimmunotherapy, with consequent definitive interruption of adjuvant Pembrolizumab. The other one had G3 electrolyte alteration due to dysphagia, leading to a definitive RT interruption at 14 out of 15 fractions and a temporary discontinuation of Pembrolizumab. Among the remaining six patients, the worst toxicity registered was G2 skin erythema, which occurred in two patients

Conclusion:

Despite the shortage of patients included, our study confirms that the concomitant use of RT and Pembrolizumab is safe, with no increase of local toxicity. Nevertheless, the systemic synergy of this association needs to be further investigated, considering not only the possible Abscopal antitumor effect but also the results of RT-induced immune