Cytology of a sarcomatoid renal cell carcinoma with unusual coexpression of S-100 protein: a case report, review of the literature and cytologic-histologic correlation.

Sarcomatoid renal cell carcinoma (SRCC) is an aggressive variant of renal cell carcinoma; it is an uncommon, malignant neoplasm, and its diagnosis is usually based on the histologic evaluation of a nephrectomy specimen. Patients are treated with systemic therapy and, generally, a nephrectomy is not performed. Therefore, arriving at an accurate diagnosis is critical for planning further management. Cytomorphological findings of SRCC have rarely been reported. This article reports the cytological findings in a case of SRCC composed predominantly of spindle cells and discusses its differential diagnosis in a 50-year-old female who presented with sharp right groin pain. A CT scan showed an enlargement of the right kidney. Cytology smears showed a malignant neoplasm composed predominantly of spindle cells. A panel of immunohistochemical stains performed on a core biopsy confirmed the epithelial nature of the spindle cells. A diffuse positive staining of the neoplastic cells for S-100 protein was also observed. It has been reported in the medical literature that almost all SRCC cases demonstrate negative staining for S-100 protein, with only a single case having been reported as focally positive. The cytologic differential diagnosis of spindle cell neoplasm with expression of S-100 protein should be broadened to include SRCC. Furthermore, the S-100 protein expression in SRCC that was observed in this case merits further investigation.