CEBCC-101: an expanded multicenter real-world registry of first-line pembrolizumab plus chemotherapy in PD-L1-positive advanced triple-negative breast cancer in Central Europe

This project is an expanded version of the Central European Breast Cancer Collaboration registry CEBCC-101, designed to generate real-world evidence on the effectiveness and safety of first-line pembrolizumab plus chemotherapy in patients with PD-L1-positive (CPS at least 10) advanced or metastatic triple-negative breast cancer treated outside clinical trials in Poland, the Czech Republic, and Slovakia. Primary objectives are to describe overall effectiveness (progression-free survival, overall survival, objective response where available) and safety in an international multicenter population, with reporting of overall survival where feasible given follow-up maturity. Prespecified secondary analyses include outcomes in older patients, radiotherapy delivered during chemoimmunotherapy (including detailed radiotherapy parameters), PET imaging used for treatment monitoring (subject to case numbers), outcomes in metaplastic breast cancer (requiring accurate histology coding), outcomes in patients with central nervous system metastases (subject to case numbers), practical aspects of PD-L1 CPS assessment (primary versus metastatic tissue; metastatic site; local testing versus external vendor), and effectiveness analyses in the context of concomitant medications (e.g., proton pump inhibitors, antibiotics), using predefined exposure definitions. Eligibility includes patients who initiated pembrolizumab plus chemotherapy as first-line systemic therapy for advanced or metastatic disease up to 15 June 2025. The registry data cutoff (last date through which data are collected) is 30 November 2025. Participating centers are asked to include all eligible patients to minimize selection bias and maximize the analytical value of the dataset. Planned analyses include descriptive statistics, Kaplan-Meier estimation for time-to-event endpoints, and regression modeling (univariable and multivariable, as appropriate) for exploratory association testing. inclusion criteria: Adult patients treated at a participating oncology center in Poland, the Czech Republic, or Slovakia. Histologically confirmed breast carcinoma with triple-negative phenotype, defined as estrogen receptor negative and progesterone receptor negative and HER2 negative according to local pathology standards in routine care. Advanced unresectable locally advanced or metastatic disease at the time of pembrolizumab initiation, treated with palliative intent. PD-L1-positive disease defined as CPS at least 10, assessed in routine practice on primary tumor or metastatic tissue (documentation of tissue source and assay/vendor where available). Receipt of pembrolizumab plus chemotherapy as first-line systemic treatment for advanced or metastatic disease outside a clinical trial. Pembrolizumab plus chemotherapy initiated on or before 15 June 2025. Availability of minimum core dates required for time-to-event analyses, at least treatment start date and at least one follow-up assessment or outcome date (progression date and/or death date and/or last contact date), enabling determination of censoring. exclusion criteria: PD-L1 CPS below 10, unknown CPS, or inability to verify PD-L1 status from source documentation. Non–triple-negative disease (hormone receptor positive and/or HER2 positive). Early-stage disease treated with curative-intent therapy only, without unresectable locally advanced or metastatic disease at the time of pembrolizumab initiation. Pembrolizumab plus chemotherapy administered as second-line or later systemic treatment for advanced or metastatic disease (i.e., prior systemic therapy for metastatic setting before pembrolizumab). Pembrolizumab initiated after 15 June 2025. Pembrolizumab plus chemotherapy given within an interventional clinical trial (if the project scope is restricted to routine-care treatment outside trials). Duplicate records or inability to de-identify the patient record according to the project’s data-sharing requirements.