Two Cases of Metastatic and Recurrent Non-Clear Cell Renal Cell Carcinoma Re-Diagnosed as Renal Mucinous Tubular and Spindle Cell Carcinoma during Long-Term Follow-Up

Renal mucinous tubular and spindle cell carcinoma (MTSCC) is a rare, low-grade renal epithelial neoplasm. MTSCC has a lower malignant potential and shows relatively good prognosis, but can be difficult to distinguish from other renal cell carcinoma (RCC) subtypes. Here, we report two cases of metastatic and recurrent renal MTSCC diagnosed after long-term follow-up. Case 1 was a 79-year-old man with a history of macroscopic hematuria in whom a right kidney mass was detected and diagnosed as RCC (cT3bN0M0). After a radical nephrectomy, microscopic findings showed that the tumor consisted of spindle cells arranged in tubular patterns embedded in sarcomatoid tissues ; we diagnosed it as unclassified RCC with sarcomatoid differentiations (pT3aN0M0). Thereafter, metastases were twice detected and resected completely. The patient had no evidence of disease at his most recent follow-up, 10 years 1 month after the initial surgery. Case 2 was in a 72-year-old man in whom a right kidney mass, swollen lymph nodes, and a lung node were incidentally detected. This tumor was diagnosed as RCC (cT4N2M1), and radical nephrectomy and lymph node dissections were carried out. From the microscopic findings, we diagnosed papillary RCC type-2 (pT3aN2M1). After the surgery, pleural and bone metastases was detected. Despite sequential treatments with IFN-α and sunitinib, the patient suffered indolent-growing metastases and died at 5 years 6 months after operation. As these patients had relatively good prognoses despite assumed aggressive RCC subtypes, we reviewed their pathological findings. In both cases, tumor samples showed tubules lined by short cuboidal cells that were set within myxoid stromata and spindle cells ; we finally diagnosed these cases as renal MTSCC