Validation of a Treatment Algorithm for Poor-Risk NSGCTnon Seminomatous Germ-cell Tumors

This is a prospective multicenter, non-randomized research program that includes: * a phase IV study (for all patients) with a collection of tissue specimens of tumor, * a phase II study (for patients with primary mediastinal tumors and an unfavorable decline in tumor markers), * and a diagnostic study (for all patients, except patients with brain metastases at baseline or patients for whom any brain MRI is contra-indicated). The main question it aims to answer is improving outcome for young adults with poor-prognosis Non Seminomatous Germ Cell Tumor (NSGCT) is to validate prospectively the efficacy and safety of a personalized treatment based on early tumor marker kinetic assessment in real life for patients with poor-prognosis NSGCT. Participants will be followed-up according to the assessment of decline kinetics of the tumor markers at the end of a first chemotherapy cycle and according to the localisation of the primary lesion if unfavorable. * In the case of a patient with a favorable decline of the tumor markers, he will be treated by 3 additional standard chemotherapy cycles. * In the case of a patient with a testicular or peritoneal primary tumor and an unfavorable decline of the tumor markers, the patient will be treated by a dose-dense standard therapy. * The patient with a mediastinal primary tumor and an unfavorable decline of the tumor markers will be proposed to enter the phase II part of the study or to enter the dose-dense regimen like the other primary localisations. If the patient consents and is eligible for phase II part, he will undergo either an early surgery if feasible or a high-dose chemotherapy if the early surgery is not possible.