Solid variants of papillary (chromophil) renal cell carcinoma: clinicopathologic and genetic features

Papillary renal cell carcinomas (RCCs) traditionally have been defined histologically as tumors with at least 50% true papillae. However, these tumors also have characteristic immunohistochemical and genetic features that separate them from other RCCs. We identified six tumors composed of solid sheets of cells without true papillae but that otherwise resembled papillary RCCs, which we feel represent solid variants of papillary RCCs. All six tumors were primary lesions of the kidney, all were strongly reactive for epithelial membrane antigen, cytokeratin 7, and callus keratin, and all were negative for the high molecular weight keratin antibody 34BE12. Four of four tumors tested showed trisomies for chromosome 7, chromosome 17, or both by either cytogenetic analysis or fluorescence in situ hybridization. Four cases were composed of solid sheets of cells containing distinct micronodules that in some cases resembled abortive papillae. The cells composing the micronodules had abundant eosinophilic cytoplasm, open chromatin, and in some cases prominent nucleoli. The intervening cells had similar nuclei, but the amount of cytoplasm was variable. In three of these micronodular cases, multiple tumors diffusely replaced the kidney; in the fourth case two typical clear cell RCCs and a typical papillary RCC were also present in the same kidney, but the micronodular tumor was unifocal. The two remaining cases were solitary tumors consisting of solid sheets of cells forming ill-defined tubules. These cells had scant clear cytoplasm and small round to elongate nuclei with occasional nuclear grooves but only rare small nucleoli. Limited follow-up has shown no evidence of disease in any patient thus far. The differential diagnosis includes "renal adenoma," renal adenomatosis, metanephric adenoma, and clear/granular cell RCC. We conclude that solid variants of papillary RCCs lack true papillae but have characteristic histologic, immunohistochemical, and genetic features.