Background: A negative FDG-positron emission tomography/computerized tomography (PET) predicts the absence of viable seminoma cells after chemotherapy in men with metastatic seminoma. In this study, we assessed whether patients (pts) with low-volume metastatic seminoma can be treated with 2 cycles of etoposide-cisplatin (EP) followed by only one cycle of carboplatin (CARBO) on the basis of a negative interim PET, thereby limiting the burden of toxicity. Methods: In this non-randomised, multiple-center, phase 2 trial (NCT01887340), we enrolled pts with low-volume metastatic seminoma (with good prognosis according to IGCCCG and the Medical Research Council classifications). All pts with baseline PET-positive received EP for two cycles. After completion of first two cycles, pts underwent a second PET to assess response. Patients with a persistent positive PET (based on local review) proceeded directly to two additional EP cycles (for a total of 4); those who achieved a PET-negative received only one cycle of CARBO (AUC=7). The primary outcome was the proportion of pts who were PET-negative on interim PET and received de-escalating chemotherapy. Secondary endpoints include progression-free survival (PFS) and overall survival (OS). Results: Between June 2013 and July 2017, 102 pts were enrolled in the study. Three pts were deemed ineligible or not evaluable and thus 99 patients received treatment. After 2 first EP cycles, PET was available in 94 pts. Interim PET was negative in 68 pts (72%) and positive in 26 pts (28%). Overall, 63 pts (67.0%; 95% CI 57.5-76.5) were PET-negative and proceeded to one single cycle of CARBO. Overall, 24 (25.5%, 95% CI 17.1-34.9) patients had a PET positive after 2 EP and received 2 additional cycles of EP. After a median follow-up of 34.4 months, only 8 patients relapsed (2 in EP group and 6 in CARBO group). 2-year PFS rates were respectively 93.7% (95% CI 84.9-97.5) in the CARBO group and 92.9% (95% CI 77.4-98.0) in the EP group. Only one patient died during the 2 first cycles. Conclusions: De-escalating treatment based on a negative PET after 2 cycles of chemotherapy appears to be safe and feasible in the majority of patients with low-volume metastatic seminoma. Clinical trial information: NCT01887340