Mediastinal seminoma: a case report with special emphasis on SALL4 as a new immunocytochemical marker

Cytological diagnosis is a valuable method for detection of mediastinal tumors, and recent reports have shown the usefulness of fine-needle aspiration cytology for diagnosis of mediastinal tumors, including germ cell tumors. We report a case of mediastinal seminoma diagnosed intraoperatively by cytological examination of cystic fluid containing tumor cells. An anterior mediastinal tumor with cystic component was incidentally found in a 28-year-old Japanese male. Cytological examination of the cystic fluid at the time of tumor resection showed single and loose aggregates of large round to polygonal cells with large round nuclei, vesicular chromatin and nucleoli, and mild to moderate amounts of PAS-positive cytoplasm admixed with mature lymphocytes. A diagnosis of seminoma was made intraoperatively. On subsequent immunostaining, the tumor cells showed nuclear positivity for SALL4, a recently recognized germ cell marker that, in the appropriate setting, can be helpful to distinguish germ cell tumors from other mediastinal neoplasms, such as malignant lymphoma, thymoma, and thymic carcinoma.