Background: At diagnosis, 10 to 15% of testicular pure seminomas have a stage II defined by the presence of retroperitoneal lymph node metastases. The optimal choice of treatment modalities are associated with excellent efficacy but also acute and late toxicities. De-escalating treatment for seminoma patients with stage IIb, IIc and III and good prognosis according to IGCCCG (International Germ Cell Cancer Collaborative Group) based on negative FDG-PET (Fluorodeoxyglucose Positron Emission Tomography) after 2 cycles of EP (etoposide, cisplatine) chemotherapy seems feasible and safe according to SEMITEP cohort 2 results (Loriot Y, and al GETUG SEMITEP Trial: Eur Urol. 2022 Aug;82(2):172-179). Serum levels of microRNA (miR)-371a-3p (miRNA-M371) have been significantly associated with clinical stage and response to treatment in testicular germ cell tumors, with sensitivity and specificity higher than those of classic markers. The aim of the study NCT05529251 is to propose a new therapeutic approach for the stages IIa/IIb. Methods: This phase II, multicenter, prospective, randomized, non-comparative, de-escalation study will include patient with primary testicular seminomatous germ cell tumor with stage IIa/IIb < 3 cm in largest diameter seminoma, histologically proved after orchiectomy and good prognosis according to IGCCCG and LDH (Lactate DesHydogenase) < 2.5 x Upper Limit of Normal (ULN). They must have progressive disease and no prior treatment with radiotherapy or chemotherapy. In case of negative week-3 (after 1 EP cycle) PET-scan, patients will be randomized according to 2 arms ARM A: Boost of radiotherapy 20 to 30 Gray (Gy); ARM B Carboplatin AUC7 chemotherapy. In case of positive week-3 PET-scan: 3 courses of EP chemotherapy (ARM C). Primary outcome will include progression-free rate at 36 months. Secondary Outcome Measures will be serum level of miRNA M371, correlation with response to treatments and PET scan results, overall survival (OS), quality of life and tolerance to treatment. Blood samples (miRNA-M371) will be collected at screening, at the time of randomization before second cycle of chemotherapy or radiotherapy, at the end of treatment and at relapse. Enrollment has started in October 2022. A total of 90 patients will be included in the interventional study, leading to approximately 60 patients with negative FDG-PET randomized. Clinical trial information: NCT05529251.