- Participants in cohort C must have completed systemic therapy (4-6 cycles cisplatin or carboplatin + etoposides) and NOT be a candidate for consolidation thoracic radiotherapy or PCI. Participants in cohort C must initiate therapy with pembrolizumab within 6 weeks of the last dose of chemotherapy (therapy must not start within 2 weeks from the last dose). Participants in cohort C must not have had progression of disease prior to the start of therapy.