Objective The aim of this study was to analyze the oncological outcome of stage I malignant ovarian germ cell tumors patients included in the MITO-9 study to identify those who might be recommended routine surveillance alone after complete surgical staging.

Methods MITO-9 was a prospective observational study analyzing data collected between January 2013 and December 2019. Three groups were identified: group A included 13 patients stage IA dysgerminoma and IAG1 immature teratoma; group B included 29 patients with stage IB–C dysgerminomas, IA–C G2–G3 immature teratomas and stage IA mixed malignant ovarian germ cell tumors and yolk sac tumors; and group C included five patients (two patients with stage IC1 and one patient with stage IC2 yolk sac tumors and two patients with mixed-stage IC2 malignant ovarian germ cell tumors).

Results A total of 47 patients with stage I conservatively treated malignant ovarian germ cell tumors were analyzed. Two patients in group B were excluded from the routine surveillance alone group due to positive surgical restaging. Therefore, a total of 45 patients were included in the study. Median follow-up was 46.2 months (range; 6–83). In total, 14 of 45 patients (31.1%) received chemotherapy, while 31 (68.9%%) underwent surveillance alone. One patient in group A, with stage IA dysgerminoma had a relapse, successfully managed with conservative surgery and chemotherapy. None of the patients in group B and C relapsed. All patients were alive at completion of the study. Overall, among 31 patients (68.9%) who underwent surveillance alone, only one patient relapsed but was treated successfully.

Conclusions Our data showed that close surveillance alone could be an alternative option to avoid adjuvant chemotherapy in properly staged IB–C dysgerminomas, IA–IC G2–G3 immature teratomas, and IA mixed malignant ovarian germ cell tumors with yolk sac tumor component.