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We describe two elderly patients with follicular lymphoma (FL) involving the skin and superficial soft tissues, with a striking proliferation of follicular dendritic cells (FDC). In addition, one patient had bone marrow involvement by FL. Histopathologically, the most remarkable feature in both cases seen at low magnification was a striking pallor of the constituent cells, which were arranged in fascicles, whorls, and round islands. The majority of the cells had the typical cytologic features of FDCs. They were intimately intermingled with centroblasts and centrocytes. A large amount of the clear cytoplasm and the pale nuclei of FDCs, which predominated in the tumors, caused the striking overall pallor of the lesions. Small reactive lymphocytes were scattered between the fascicles. A vague follicular growth pattern was seen only focally. The mantle zones were markedly reduced or absent so that the follicles were seen lying unseparated. The close intermixture of the FDCs and the germinal center cells was responsible for the FDCs appearing to be decorated with B-associated marker, and the germinal center cells seemed to be stained to some degree with FDC-markers. The tumor bulk demonstrated a diffuse and strong reaction with CD10, CD20, CD21, CD35, and stained weakly with CD79a. Fascin and CD23 showed only a weak and focal staining pattern. Bcl-2 decorated large centroblasts and small reactive T-cells. The tumor bulk was negative for actin, EMA, cytokeratins, vimentin, desmin, and factor XIIIa. The proliferative index was rather low; MIB-1 mainly decorated large centroblasts. No monoclonal rearrangement of IgH genes was detected. Epstein-Barr virus was not identified. Electron microscopy revealed typical features of FDCs intermingled with germinal center cells. Such cases may represent a diagnostic pitfall, as FDC overgrowth can mask FL and give the neoplasm the appearance of FDC sarcoma/tumor. We believe that, in both cases, the FDC proliferation had a reactive character.

We present two cases of benign pulmonary epithelial tumors located in the middle lobes of the lungs. The patients were women of 56 and 36 years of age. The tumors were morphologically similar to plasma cell granuloma. At the tumor periphery, the lesion cells formed thin organoid rows of cells and vague trabeculae, disclosing morphologically their epithelial nature. In addition, the tumors strongly stained immunohistochemically with antibodies to cytokeratins, TTF-1 and EMA, and they were negative for immunoglobulin kappa and lambda light chains. We are not aware of similar tumors described in the literature, and we suggest the name "epithelial plasma cell granuloma-like tumors" for these lesions.

We report on a 36-year-old man with a recurrent mass on the thenar side of the right hand. Against the background of dense inflammation, inflammatory malignant fibrous histiocytoma was first diagnosed, but typical morphologic features in the recurrent tumor led to a correction of the diagnosis--inflammatory myxohyaline tumor. Massive emperipolesis of the neoplastic cells, forming the so-called "cell microabscesses", dominated the histologic picture in both primary and recurrent lesions. Awareness of this unique feature helps in differentiating this entity from both benign inflammatory processes and clearly malignant tumors.

A case of dermatofibrosarcoma protuberans (DFSP) with epithelial membrane antigen (EMA)-positive cells is described. The tumor was excised from the left groin of a 28-year-old woman. It showed characteristic histologic features of DFSP with typical diffuse immunohistochemical positivity for CD34. Moreover, scattered neoplastic cells expressed EMA, suggesting perineural cell differentiation. Ultrastructural study found perineurial cell features, such as thin long bipolar cytoplasmic processes, pinocytotic vesicles, fragments of external lamina and/or external lamina-like material, attachment plaques, and desmosome-like junctions. This observation, together with previous immunohistochemical findings of EMA-positive cells in a subset of DFSPs, strongly suggests perineurial cell differentiation in these tumors. DFSP should be included in the differential diagnosis of EMA-positive spindle cell lesions of superficial soft tissue and skin.

We describe three cases of benign signet-ring cell aggregates in the colon associated with pseudomembranous colitis, adenomatous polyp of the colon and ulcerated mucosa of the gallbladder excised for gallstones. In all cases, we found loose, benign signet-ring cell aggregates overlying the ulcerated mucosa surface, simulating signet ring-cell carcinoma. The most important sign of the benign signet-ring cell aggregates is that they are always confined to the surface of the mucosa of the intestine or gallbladder mucosa or crypts of the intestinal epithelium. In no case did we see an invasion of these cells into the lamina propria of the mucosa. In all cases, the benign signet-ring cell aggregates were immunohistochemically positive with antibodies to cytokeratins. The occurrence of benign signet-ring cell aggregates is a rare and very misleading diagnostic pitfall which must be differentiated from signet-ring cell carcinoma of the colon and gallbladder.