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

Collision tumours, defined by the coexistence of two histologically distinct neoplasms within a single organ, are rare and typically arise from a shared cell lineage. 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 illustrates diagnostic challenges in distinguishing synchronous primaries from metastases, where limited sampling may obscure dual pathology. Management required balancing nephrectomy for ccRCC with CDK4/6 inhibitor–based therapy for metastatic breast cancer and consideration of adjuvant immunotherapy. Multidisciplinary evaluation and molecular characterisation are essential for optimal precision-guided care in such complex malignancies.