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The development of a classification system which reflects, the underlying genetic
abnormalities which lead to the phenotype of the neoplasm, requires a diagnostic
category to which can be assigned tumors which do not fit into any of the
established categories. This is "renal cell carcinoma, unclassified."Any primary
renal epithelial malignancy which does not fit into the recognized diagnostic
categories belongs in this one. Features which might prompt this diagnosis are
shown in the box.
Renal Cell Carcinoma, Unclassified
Contains morphology not accepted in recognized categories
Apparent composites of recognized types
Sarcomatoid without recognizable epithelial elements
Mixtures of epithelial and stromal elements
Unrecognizable cell types
Differential diagnosis: The differential diagnostic possibilities are broad.
Metastasis should always be considered when a peculiar-looking carcinoma is found
in the kidney. Since a variety of appearances have been ascribed to collecting
duct carcinoma, some tumors which we would consider to be renal cell carcinoma,
unclassified, would be considered by others to be collecting duct carcinoma.
Epithelioid angiomyolipoma (see below) is another differential diagnostic
consideration and immunohistochemistry for HMB-45 and actin can be helpful in
making this distinction.