Metastatic mucinous tubular and spindle cell carcinoma of the kidney: Case report and literature review.

Mucinous tubular and spindle cell carcinoma is a rare renal neoplasm first classified by the WHO in 2004. It typically consists of low-grade cuboidal cells in tightly packed tubules, merging with spindle cells in a myxoid stroma with basophilic mucin. High-grade features, including sarcomatoid transformation, necrosis, and vascular invasion, are associated with poor prognosis and metastasis. Here, we present the second reported case of mucinous tubular and spindle cell carcinoma metastasizing to bone despite lacking high-grade features.