Abstract:
Invasive urothelial carcinoma is the most common malignancy of the renal pelvis, often exhibiting variant histology or divergent differentiation. Mixed small cell and sarcomatoid carcinoma of the renal pelvis is an exceedingly rare and aggressive entity with limited literature describing its presentation and management. We present the case of an 84-year-old male who presented with vomiting, right-sided back pain, and acute kidney injury. Imaging revealed a 58 mm mass in the right renal pelvis causing pelvicalyceal dilatation. The patient underwent a right nephroureterectomy under the suspicion of transitional cell carcinoma.
Histopathological examination of the resected specimen revealed a high-grade biphasic neoplasm with both neuroendocrine and sarcomatous components. The epithelial component exhibited features consistent with small cell carcinoma, while the stromal component showed spindle cell morphology with marked pleomorphism. Immunohistochemistry confirmed the diagnosis: the epithelial cells were positive for CAIX, synaptophysin, CD56, EMA, and PAX8, supporting neuroendocrine differentiation; the stromal component showed vimentin positivity, indicating a sarcomatoid phenotype. The tumor extended into perinephric fat, consistent with a high-grade and locally advanced lesion. Final diagnosis from the specialist center confirmed mixed small cell and sarcomatoid carcinoma.
This case highlights the diagnostic complexity and rare histological presentation of urothelial carcinoma with divergent differentiation. Recognition of such rare variants is critical, as they carry different prognostic and therapeutic implications. Immunohistochemistry played a crucial role in confirming the diagnosis, ruling out other differential diagnoses such as lymphoma, lymphoepithelioma-like carcinoma, and poorly differentiated urothelial carcinoma.
The management of mixed small cell and sarcomatoid carcinoma remains challenging due to the tumor’s high-grade features, aggressive behavior, and lack of standardized treatment protocols. While nephroureterectomy offers local control, the role of systemic chemotherapy, particularly platinum-based regimens, remains extrapolated from small cell lung and bladder carcinoma data. The sarcomatoid component’s resistance to conventional chemotherapy further complicates management. Emerging therapies such as immune checkpoint inhibitors and molecularly targeted treatments are being explored, though their role in such rare mixed histologies remains investigational. Ongoing surveillance is crucial given the high risk of recurrence and progression.
This case underscores the importance of considering rare histologic variants in renal pelvic tumors and highlights the essential role of multidisciplinary evaluation including pathology, oncology, and urology in ensuring accurate diagnosis and optimal patient care.
