Secured Access to Pembrolizumab for Patients With Selected Rare Cancer Types

The study plans to enrol up to 350 patients in total. Eligible patients who have provided their written informed consent for study participation will be assigned to one of 7 cohorts determined by indication: * Cohort 1: Rare sarcoma * Cohort 2: Rare ovarian cancer * Cohort 3: Primary central nervous system lymphomas * Cohort 4: Rare thyroid cancer * Cohort 5: Rare malignant neuroendocrine cancer * Cohort 6: Germ-cell cancer * Cohort 7: NK/T-cell lymphoma Between 20 and 50 patients will be enrolled in each cohort with the exception of the cohort 1 for which up to 80 patients may be enrolled (extension introduced under amendment 5 of this protocol). Following the amendment 6, up to a maximum of 50 additional patients may be included in the Sarcoma (cohort 1), Rare ovarian cancer (cohort 2) or Primary central nervous system lymphoma (cohort 3) cohorts, within the limit of 350 patients to be included in total. The study will use a two-stage Bayesian enrichment design. The first stage treats all patients from the different cohorts with the investigational product and identifies possibly sensitive indications. The second stage will compare outcomes among subsets of patients in the identified cohorts to distinguish between subpopulations of patients who may benefit from the treatment and patients for whom there is no evidence of efficacy. All participants who reach the maximum treatment duration per protocol and stop the pembrolizumab with a clinical benefit (prolonged stable disease, partial or complete response), may be eligible for up to an additional 1 year (approximately 17 cycles) of pembrolizumab treatment if they progress during the follow-up period. This retreatment is termed the Second course or Re-challenge. The second pembrolizumab course will continue until 12 months of retreatment, progression or until the end of the study, whichever occurs first.