Interdigitating dendritic cell sarcoma of the spine: Hitherto undescribed lesion

Abstract

To the Editor, Interdigitating dendritic cell sarcoma (IDCS) is an extremely rare hematolymphoid neoplasm with phenotypic features of interdigitating dendritic cells (DCs). Most tumors are located in the lymph nodes, but extranodal lesions as in the skin, soft tissue, and liver have also been reported. The clinical course of IDCS is generally aggressive, and half of the patients die of the disease. To the best of our knowledge, only one case of IDCS involving the central nervous system (CNS) has been reported; however, no IDCS involving the spinal cord has been reported to date. Here, we describe an exceedingly rare case of IDCS involving the spinal cord, which was particularly difficult to diagnose. A Japanese man in his 70s was referred to the hospital because of hand tremors lasting for 10 months and progressive muscle weakness of the left arm for 8 months. Physical examination on admission revealed muscle weakness and atrophy in the left forearm and bilateral thigh muscles. He also showed hyperreflexia in the bilateral biceps and triceps tendons. Magnetic resonance imaging (MRI) revealed low‐intensity mass lesions in the sacral nerve and cauda equina on T2‐weighted images (Figure 1a) and fluorodeoxyglucose positron emission‐computed tomography (FDG PET‐CT) images revealed multiple accumulations in the cervical cord, sacral nerve, and cauda equina (Figure 1b). During the operation, a poorly circumscribed tumor was identified in the posterior root of the first sacral nerve, and biopsy was performed. Microscopically, the tumor showed diffuse proliferation of atypical ovoid‐to‐short spindle cells with eosinophilic cytoplasm (Figure 1c). The tumor cells had indistinct cell borders and indented nuclei, occasionally with nucleoli (Figure 1d). Necrosis was not observed, but occasional mitoses were observed (Figure 1d). A few lymphocytes were mixed with the tumor. Phagocytosis or emperipolesis were not obvious. We first considered neurogenic tumors, specifically anaplastic glioma, or malignant peripheral nerve sheath tumor (MPNST) because of the tumor location; however, immunohistochemical examination showed that the tumor cells were negative for both GFAP and synaptophysin, but positive for S100 protein (Figure 1e). Further immunohistochemical studies revealed that tumor cells were diffusely positive for CD45, CD163, CD68, CD4, and vimentin; and negative for CD20, CD79a, CD3, CD5, CD10, CD30, CD34, CDla, CD21, CD23, bcl‐2, bcl‐6, MUM1, CD56, lysozyme, desmin, alpha‐smooth muscle actin (aSMA), HMB45, melan A, and pankeratin. The Ki‐67 proliferation index was found to be approximately 30%. Based on these morphological and immunohistochemical findings, the patient was diagnosed with IDCS. Electron microscopy of formalin‐fixed paraffin‐embedded tissue was insufficient to observe precise cell structures. Treatment with high‐dose methotrexate therapy was discontinued in two cycles because of cognitive decline in the patient. The patient s general condition gradually deteriorated with intraventricular tumor dissemination along the cavitary surface of the fourth, third, and lateral ventricles. The patient died 5 months after the diagnosis. Interdigitating dendritic cell sarcoma is an extremely rare histiocytic and DC neoplasm and shows myeloid‐ derived DC differentiation. IDCS occurs in patients belonging to a wide range of generations, but mainly affects middle‐aged adults. Microscopically, tumor cells are ovoid or spindle, occasionally forming fascicles or whorls. Sometimes, the cells are polymorphic or epithelioid with eosinophilic cytoplasm and indistinct cell borders. The nuclei are generally vesicular with eosinophilic nucleoli. Necrosis is observed in about 40% of the cases, although there was no necrosis in this case. Lymphoplasmacytic infiltration is often detected in tumors, as in this case. IDCS consistently expresses S100 protein and vimentin, but is negative for markers of follicular DCs (CD21, CD23, and CD35), lymphocytic, myeloid cells, and Langerhans cells. The expression of CD68, lysozyme, and CD45 is variable. The Ki‐67 labeling index varies between 0 and 70% with a median value of 10%. Differential diagnoses include glioma, MPNST, lymphoma, melanoma, histiocytic sarcoma (HS), follicular DC tumor, and fibroblastic reticular cell tumor. The immunohistochemical