A case of granulocyte colony-stimulating factor and interleukin 6 receptor-producing mediastinal mature cystic teratoma with somatic-type malignancy.

Mediastinal germ cell tumor with somatic-type malignancy is a rare neoplasm. We describe one such case in a 49-year-old Japanese man who had shown an elevated serum concentration of granulocyte colony-stimulating factor (GCSF) and leukocytosis without a shift to the left. Histologically, the tumor formed a teratomatous cyst whose wall contained benign epithelial components, well-differentiated tubular and mucinous adenocarcinoma, and poorly-differentiated pleomorphic carcinoma. Immunohistochemically, both the well differentiated adenocarcinoma and poorly differentiated pleomorphic carcinoma expressed GCSF. Immunohistochemistry and molecular analysis revealed that both components also produced interleukin 6 receptor (IL6R). We diagnosed this tumor as a GCSF- and IL6R-producing mediastinal mature cystic teratoma with somatic-type malignancy. The tumor showed immunohistochemical expression of activated signal transducer and activator of transcription 3. The patient died 6 months after developing systemic symptoms. For a GCSF-producing tumor, complete resection appears to offer the best outcome at present. For any patient presenting with leukocytosis without a shift to the left, a thorough analysis should be conducted, and the tumor diagnosed as early as possible.