Not all masses are metastases: a diagnostic dilemma resolved – synchronous HER2-positive breast cancer and renal oncocytoma

Introduction: In oncology, the discovery of a synchronous renal mass during breast cancer staging often signifies metastatic disease, drastically altering prognosis and management. We present a critical case where this assumption was challenged by a rare benign entity, renal oncocytoma, highlighting a pivotal diagnostic crossroads. Case presentation: A 48-year-old woman presented with a right breast lump. Biopsy confirmed HER2-positive invasive carcinoma. Staging positron emission tomography computed tomography revealed an 18F- fluorodeoxyglucose (FDG)-avid left renal mass, highly suspicious for metastasis. A subsequent renal biopsy revealed an oncocytic neoplasm. This finding prompted a curative-intent strategy: the patient underwent simultaneous right breast-conserving surgery and left laparoscopic partial nephrectomy. Final histopathology confirmed stage pT1N0 breast cancer and renal oncocytoma. The patient recovered well and received adjuvant paclitaxel-based chemotherapy with trastuzumab, followed by radiation to the breast, and is planned for continuation of trastuzumab to complete 1 year of anti-HER2 therapy. At 6 months of follow-up, the patient remains asymptomatic with no clinical or radiological evidence of disease recurrence. Discussion: This case highlights a critical clinical caveat: benign lesions can masquerade as metastases. Our report describes the management of synchronous HER2-positive breast cancer and renal oncocytoma. It exemplifies how a biopsy-driven approach prevented unnecessary systemic therapy and radical nephrectomy, preserving renal function and upholding curative intent. Conclusion: This experience mandates that tissue confirmation of synchronous lesions be integral to staging protocols. Multidisciplinary review is indispensable for navigating such complex presentations and avoiding therapeutic misadventures.