Papillary renal cell carcinoma with low-grade spindle cell foci: A mimic of mucinous tubular and spindle cell carcinoma

The solid variant of papillary renal cell carcinoma (PRCC) is distinguishable genetically from mucinous tubular and spindle cell carcinoma (MTSC) of the kidney by the presence of trisomy for chromosomes 7 and 17; however, at the morphologic and immunohistochemical levels, these neoplasms overlap significantly. The key morphologic feature distinguishing these two is thought to be the low grade of the spindle cell areas of MTSC; spindle cell areas in PRCC generally signify sarcomatoid change and are high grade. We report 5 cases of PRCC with low-grade spindle cell foci, closely mimicking MTSC. All patients were male, and ranged in age from 17 to 68 years. All tumors were predominantly solid, featuring compact areas of low-grade spindle cells lining thin, angulated tubules. Mucinous stroma was not appreciated in any case. All cases were diffusely immunoreactive for cytokeratin 7, and focally CD10 positive. All 5 cases showed trisomy of chromosome 7, and 3 of 5 showed trisomy of chromosome 17 by fluorescence in situ hybridization, supporting classification as PRCC. These cases further reported morphologic overlap between MTSC and PRCC. Before a diagnosis of metastatic MTSC or MTSC with unusual morphology is rendered, the possibility of PRCC with low-grade spindle cell foci should be considered. Fluorescence in situ hybridization analysis effectively separates these morphologically very similar yet genetically distinctive entities.