Fine-needle aspiration cytology: true histiocytic lymphoma/histiocytic sarcoma

True histiocytic lymphoma/histiocytic sarcoma is an extremely rare, malignant histiocytic tumor. This report is of such a case in a 52-yr-old woman. Fine-needle aspiration (FNA) smears showed large pleomorphic nuclei, prominent nucleoli, moderately dense abundant cytoplasm, and scattered cells with cytoplasmic vacuoles and some with reniform nuclei. Small numbers of background lymphocytes and benign histiocytes were present as well as many multinucleated tumor cells. The immunophenotype was CD45, Lysozyme, CD68 (PGMI), CD43, and S-100 positive. Genotypic analysis revealed a germline configuration. This type of tumor has a large cytological differential diagnosis and immunophenotyping is essential for diagnosis.