Cytokeratin-positive malignant tumors with reticulum cell morphology - A subtype of fibroblastic reticulum cell neoplasm?

Cytokeratin-positive interstitial reticulum cells (CIRCs) have been describ ed as a subset of fibroblastic reticulum cells (FBRCs) normally found in ly mph nodes, the spleen, and tonsils. Although tumors derived form other reti culum (dendritic) cells, specifically follicular dendritic cells, interdigi tating dendritic cells, and cytokeratin-negative FBRCs, have been well docu mented and are now accepted, this is not the case for tumors of CIRCs. A po ssible reason for this failure is the difficulty in distinguishing them fro m other tumors, particularly carcinoma. We report three cases of cytokerati n-positive malignant tumors with a reticulum cell morphology: two located i n the mediastinum and one in the soft tissue in the proximal forearm. All c ases coexpressed vimentin, and one case coexpressed smooth muscle actin and desmin, resulting in a phenotype similar to that of some normal CIRCs. Alt hough metastatic carcinoma from an occult or regressed primary tumor cannot be excluded completely, we raise the possibility of a CIRC origin for thes e cases.