Mucinous Tubular and Spindle Cell Carcinoma of the Kidney: A Rare Renal Neoplasm—Case Report and Literature Review

<b>Background and Clinical Significance:</b> Mucinous tubular and spindle cell carcinoma (MTSCC) is an uncommon subtype of renal cell carcinoma, representing 1-4% of epithelial renal tumors. It usually shows a low-grade morphology and indolent behavior, although sarcomatoid variants with an aggressive course have been described. Because of its overlap with papillary renal cell carcinoma (papRCC), sarcomatoid RCC, mesenchymal tumors, and oncocytic neoplasms, diagnosis requires the integration of imaging, histopathology, and immunohistochemistry. <b>Case Presentation:</b> We report a 71-year-old female who presented with a three-month history of right-sided lumbar pain and intermittent hematuria. Her laboratory tests were unremarkable. Contrast-enhanced CT revealed a well-circumscribed nodular lesion in the mid-portion of the right kidney, measuring 50 × 47 × 52 mm. The patient underwent right nephrectomy. Macroscopic findings revealed an encapsulated, yellowish-gray nodule (5.2 × 5 × 4 cm) without renal pelvis invasion. Microscopically, the tumor consisted of cuboidal- to spindle-shaped cells arranged in cords and tubular structures within a mucinous stroma, with focal necrosis and foamy macrophages. Immunohistochemistry showed positivity for CK19, CK7, EMA, PAX8, and AMACR, with a Ki-67 index <10%, while CD117, RCC, CD10, and chromogranin were negative. Together, the low Ki-67 proliferation index, absence of invasion, and low-grade histological architecture confirmed MTSCC of low malignant potential. At a five-year follow-up, the patient remained disease-free. <b>Conclusions:</b> MTSCC is a rare renal neoplasm that can be diagnosed by integrating clinico-radiological, histopathological, and immunophenotypic features. Molecular profiling may further distinguish MTSCC from papRCC and identify aggressive variants. Surgical excision remains the cornerstone of management, supported by vigilant long-term follow-up.