True malignant histiocytosis developed during chemotherapy for mediastinal immature teratoma.

We report an autopsy case of true malignant histiocytosis that developed during chemotherapy for mediastinal immature teratoma. The patient was a 14-year-old boy who exhibited hepatosplenomegaly while receiving chemotherapy for a mediastinal immature teratoma that had been resected 11 months before. The spleen and liver of the excisional biopsy displayed infiltration of multinucleated giant atypical cells with prominent erythrophagia in massive aggregations. These atypical cells expressed CD68, alpha1-antitrypsin, alpha1-antichymotrypsin, lysozyme, and vimentin, suggesting that the tumor cells may have been derived from macrophages. Immunocytochemistry showed p53 expression in the tumor cells of the malignant histiocytosis, as well as in the elements of the immature teratoma. Direct sequence analysis showed the p53 mutation in the tumor cells of the immature teratoma to be a mutation at codon 175 (exon 5), whereas the mutation in the malignant histiocytosis occurred at codon 285 (exon 8), ie, polyclonality was exhibited and these features suggested that the malignant histiocytosis arose independently from the immature teratoma during the chemotherapy.