RESUMEN
Interdigitating reticulum cell sarcoma is a neoplasm that shows differentiation similar to that of non-neoplastic interdigitating reticulum cells, which normally reside in the paracortical region of the lymph node. Interdigitating reticulum cell sarcoma is extremely rare and its definite identification requires a multiparameter study, including microscopic, enzymatic histochemical, immunophenotypic and molecular findings of the tumor cells. We report a case of interdigitating reticulum cell sarcoma in the right upper jugulodigastric lymph node of a 42-year-old male. The patient wad initially diagnosed as metastatic undifferentiated carcinoma, so we tried to find primary site but failed. The patient was treated by radiation therapy under the diagnosis of metastatic neck node. Two years later, the patient was finally diagnosed to interdigitating reticulum cell sarcoma by pathological review and immunohistochemical studies. The patient has been alive well at 55 month of follow-up.