Background  Extragonadal germ cell tumors (EGCT) are relatively rare. Accurate diagnosis of tumors based on clinical features, pathological morphology and immunohistochemical characteristics is of great significance for the treatment and prognosis of patients.

  Objective  To investigate the clinicopathological features, morphological features and immunophenotypes of EGCT.

  Methods  A retrospective study was performed on clinical pathological data and morphological features of 240 patients with EGCT admitted to the First Medical Center of Chinese PLA General Hospital from December 2005 to December 2016.

  Results  Of the 240 cases, there were 108 males and 132 females. The median age of the patients was 26 years old. Among them, there were 57 cases with age younger than 18 years old (23.7%), 183 cases were older than 18 years (76.3%). Tumors were located in mediastinum (86 cases, 35.8%), central nervous system (39 cases, 16.3%), sacrococcygeal area (37 cases, 15.4%), abdomen (47 cases, 19.6%), and other sites (31 cases, 12.9%). Histological types included 172 mature teratomas (71.7%) , 33 germ cell tumors (13.7%) , 14 yolk sac tumors (5.8%) , 12 mixed germ cell tumors (5.0%), 5 cases of immature teratoma (2.1%) , 3 cases of embryonic carcinoma (1.3%) and 1 case of choriocarcinoma (0.4%) . The positive rate of AFP in yolk sac tumors was 92.8% (13/14). The positive rates of PLAP, CD117 and OCT-4 in germ cell tumors were 96.7%, 90.0%, 100% (29/30, 18/20, 6/6), respectively. CK was positive in embryonal carcinoma and β-HCG was positive in choriocarcinoma.

  Conclusion  Extragonadal germ cell tumors are rare and mainly occur in the mediastinum. EGCT are more common in young women. Its histological and immunophenotypic features are similar to those of intragonadal germ cell tumors, and germinoma is the most common extragonadal malignant germ cell tumor.