A Sui Generis Report: Synchronous Mucinous Tubular and Spindle Cell Carcinoma of the Kidney and Invasive Breast Carcinoma

Multiple primary malignant neoplasms (MPMNs) are rare entities characterized by the presence of two or more distinct primary malignancies in a single patient. Synchronous occurrence of invasive breast carcinoma and mucinous tubular and spindle cell carcinoma (MTSCC) of the kidney is a “sui generis” phenomenon, and besides, MTSCC is an exceedingly rare disease, constituting < 1% of renal tumors. This case report details a 49‐year‐old hypertensive postmenopausal female who presented with a right breast lump and episodic hematuria. Imaging studies revealed a neoplastic right breast mass with axillary metastases and an incidental renal mass suspicious for malignancy. Histopathological examination on biopsy confirmed invasive breast carcinoma of no special type (Grade 3, HER2‐enriched subtype) and MTSCC (WHO‐ISUP Grade 2). The patient underwent neoadjuvant chemotherapy followed by right mastectomy with axillary clearance and subsequent left radical nephrectomy. Postsurgical histopathology confirmed the dual malignancy, and the patient received locoregional radiotherapy for breast cancer. At 18 months follow‐up, there was no evidence of recurrent renal disease, and the patient remains on maintenance trastuzumab. This report underscores the significance of thorough diagnostic evaluation in patients presenting with MPMNs to identify synchronous neoplasms and provides a comprehensive summary of the current understanding of the clinicopathologic and immunohistochemical features of MTSCC, along with its differential diagnoses. Our case, distinguished by cytoarchitectural variations and a paucity of mucin, provides a rare perspective on this infrequently encountered neoplasm and enriches the limited global body of literature concerning this specific MTSCC subtype.