Therapeutic Challenges of a Renal Collision Tumour Comprising Clear Cell Renal Cell Carcinoma and Metastatic Breast Carcinoma: A Case Report

Abstract: Collision tumours, defined by the coexistence of two histologically distinct neoplasms within a single organ, are rare and typically arise from different cell lineages. We report a unique case of a 64-year-old woman with a history of estrogen receptor (ER)–positive, HER2-negative breast carcinoma who developed a renal mass later identified as a collision tumour comprising clear cell renal cell carcinoma (ccRCC) and metastatic breast carcinoma. Initial imaging and biopsy suggested ccRCC alone; however, post-nephrectomy histopathology revealed metastatic breast carcinoma foci within the ccRCC and renal sinus fat, confirmed by ER, GATA3, and mammaglobin positivity. This case highlights diagnostic challenges in distinguishing synchronous primaries from metastases, particularly where biopsy sampling may obscure dual pathology. Management required balancing treatment of localised ccRCC with systemic therapy for metastatic breast cancer. Multidisciplinary evaluation is essential to guide optimal care.