Cytologic diagnosis of follicular dendritic cell tumor: A diagnostic dilemma

Dear Dr. Bedrossian: Follicular dendritic cells (FDCs), interdigitating dendritic cells (IDCs), and Langerhans cells are nonlymphoid, nonphagocytic accessory cells of the lymphoid system. Their main role is the capture and presentation of antigens and immune complexes.1–3 The existence of primary neoplasm of lymph nodes showing features of FDC differentiation was first demonstrated by Monda et al. in 1986.4 The initial report was followed by several short series and isolated case reports.5 Two relatively large series (13 and 17 cases) have appeared.6,7 Although the histopathologic features of FDC tumor have been dealt with quite extensively, there are few cytologic descriptions of nodal and extranodal FDC tumor.8–12 Herein, we report a case of FDC tumor seen on cytology. The patient, a 50-yr-old Indian male, presented to the cytology outpatient department with left inguinal lymphadenopathy. On examination, there was a 5 cm diameter discrete and mobile left inguinal lymph node. Systemic examination was noncontributory. A fine needle aspiration (FNA) was performed. The smears were cellular and showed a polymorphic population of cells comprised of small mature lymphocytes, follicular center cells, plasma cells, eosinophils, and mono-, bi-, and multinucleated large immature cells. These immature cells showed open nuclear chromatin and prominent nucleoli resembling Reed-Sternberg cells (Fig. 1). A diagnosis suggestive of Hodgkin’s disease was offered. The patient presented to us a month later with bilateral inguinal lymphodenopathoy (6 cm in diameter). There was a history of irregular intermittent fever and weight loss. FNA cytology showed almost similar features. The possibility of metastatic malignant melanoma was considered; however, immunocytochemistry for HMB45 and cytokeratin was also negative. Bone marrow aspiration and bilateral trephine biopsies showed infiltration by similar cells. Subsequently, the lymph nodes were excised and sent for histopathologic examination. The sections showed complete effacement of nodal architecture by oval to spindle cells arranged in sheets and interlacing fascicles, at many places producing a nodular pattern of growth. The residual normal germinal centers and paracortex were seen between the fascicles and nodules of neoplastic cells. The tumor cells were round to oval with an ill-defined cytoplasmic border, in places showing a syncytial appearance (Fig. 2). These cells showed vesicular nuclei, prominent nucleoli, and occasional pseudoinclusion. Some bi-, tri-, and multinucleated cells were seen. In foci, there was an intimate admixture of tumor cells and small lymphocytes with perivascular cuffing of mature lymphocytes, almost resembling the picture of a metastatic lesion. The neoplastic cells showed immunoreactivity for S-100 protein and vimentin, but were negative for CK, CD1a, CD20, and CD3. Based on the histomorphologic features and limited immunohistochemistry available, FDC tumor was suggested. FDC tumor typically presents as an intranodal process.4–7 The reported age of presentation ranges from 17–63 yr; however, it appears to be a neoplasm primarily of young adults under 50 yr, with equal sex distribution. The usual presentation is that of a single mass. In 50% of cases, the lesion manifests as a solitary, slow-growing, and painless cervical lymphadenopathy.4,6,7 Associated findings are rare and include EBV genome within the tumor cells and Castleman’s disease of hyaline-vascular type. FDC tumors can pose considerable diagnostic difficulties on both cytomorphology and histopathology. The diagnosis depends on the recognition of morphologic patterns, appropriate immunophenotyping, and knowledge of the diverse presentations of this tumor. Various other entities that enter into differential diagnoses of FDC tumors include metastatic carcinoma, malignant melanoma, large-cell lymphoma, Hodgkin’s disease, malignant fibrous histiocytoma, ectopic or orthotopic thymoma, and, rarely, ectopic meningioma.5,7 The pathognomonic features on histology to distinguish it from other entities include: 1) admixture of malignant cells and benign-appearing mature lymphocytes, *Correspondence to: Dr. Pranab Dey, M.D., MIAC, Specialist Cytologist, Histopathology Laboratory, Kuwait Cancer Control Centre, Post Box No 42262, 70653 Shuwaikh, Kuwait. E-mail: deypranab@hotmail.com Received 7 February 2003; Accepted 2 July 2003 DOI 10.1002/dc.10392 Published online in Wiley InterScience (www.interscience.wiley.com).