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Background and Importance Pembrolizumab, an anti–PD-1 monoclonal antibody, has demonstrated a significant improvement in pathological complete response (pCR) rates, assessed by the residual cancer burden (RCB) index, when used in the neoadjuvant treatment of triple-negative breast cancer (TNBC) at a fixed dose (200 mg every 3 weeks) in pivotal trials such as KEYNOTE-522.In our routine clinical practice, pembrolizumab is administered using a weight-based regimen (2 mg/kg every 3 weeks), a strategy that may optimise drug exposure and healthcare resource use while maintaining clinical efficacy and safety.However, comparative real-world data on this dosing strategy are remain limited.Aim and Objectives To assess the effectiveness and safety of weight-based pembrolizumab in combination with chemotherapy as neoadjuvant treatment for locally advanced or early-stage TNBC with high recurrence risk, and to compare these real-world outcomes with those reported in pivotal fixed dose trials.Material and Methods A retrospective, observational study was conducted at a tertiary hospital between October 2023 and September 2025. All patients receiving neoadjuvant pembrolizumab plus chemotherapy for TNBC were included. Clinical data were obtained from electronic medical records and the hospital prescribing system.Effectiveness was evaluated by pCR rate using the RCB index. Safety was assessed by recording adverse events and grading them according to CTCAE v5.0.Results Forty-three female patients were included, with a median age of 55 years (range 27–81). At data cut-off, 34 patients had completed neoadjuvant therapy and surgery.The pCR rate (RCB 0) was 65%; 6% had minimal residual disease (RCB 1), 20% moderate (RCB 2) and 9% extensive (RCB 3).Grade ≥3 toxicities occurred in 65% of patients, mainly neutropenia, thrombocytopenia, anaemia and mucositis, leading to cycle delays in 59% of patients and dose reductions in 20%.Conclusion and Relevance Weight-based pembrolizumab dosing achieved pCR and safety results consistent with those from the KEYNOTE-522 trial using fixed dosing. This supports weight-based dosing as a clinically effective and economically sustainable approach for neoadjuvant TNBC treatment in European real-world settings.Conflict of Interest No conflict of interest