Safety of pembrolizumab with concurrent radiotherapy in early stage breast cancer: preliminary prospective real-world evidence

Abstract INTRODUCTION Data on the feasibility and potential synergy of concomitant immunotherapy with radiation therapy (RT) is still scarce for early breast cancer (BC). We aimed to assess the safety profile of adjuvant pembrolizumab with concomitant radiotherapy (RT) in breast cancer patients. MATERIAL AND METHODS: We prospectively collected data from ten patients with triple-negative breast cancer treated with neoadjuvant chemotherapy with pembrolizumab (as per Keynote-522 criteria) followed by surgery and at least 1 cycle of adjuvant pembrolizumab with concomitant RT. A total dose of 40.05 Gy delivered in 15 fractions was prescribed to the breast or chest wall, with regional nodes and tumor bed boost administered as clinically indicated. The study endpoint was to assess acute toxicity according to the Radiation Therapy Oncology Group scale, and to report the rate of discontinuation of RT and/or pembrolizumab. RESULTS A median of 8 (range 4 – 9) cycles of neoadjuvant Pembrolizumab were prescribed. Before RT, patients received a median of 3 (range 2 – 4) cycles of adjuvant Pembrolizumab. Severe acute toxicity occurred in 2 patients. Specifically, G4 myositis led to permanent discontinuation of adjuvant pembrolizumab in one case, whereas G3 electrolyte imbalance caused definitive RT interruption and temporary discontinuation of adjuvant pembrolizumab in the second case. Among the remaining 8 patients, only 2 patients experienced G2 skin erythema with no treatment discontinuation. CONCLUSIONS The combination of adjuvant pembrolizumab with concurrent radiotherapy proved feasible in the real world, as toxicity was minimal and consistent with existing literature.