All patients will receive 1 dose of 200 mg pembrolizumab IV as single agent upfront, followed by 2 cycles of IV P-ICE (pembrolizumab, ifosfamide, carboplatin, etoposide) and a PET/CT-based restaging. Following a PET-guided treatment strategy, patients will then receive either another 2 cycles of IV P-ICE in case of a negative PET (i.e., Deauville score 1-3), or 2 to 4 cycles of IV P-DHAP (pembrolizumab, dexamethasone, cytarabine, cisplatin) in case of a positive PET (i.e., Deauville score > 3). After completion of treatment with P-ICE or P-DHAP, respectively, patients will receive a consolidation therapy with pembrolizumab for another 6 cycles. In case of non-CR after at least 4 cycles of combination therapy (4x P-ICE or 2x P-ICE + 2x P-DHAP), physicians may decide to go for HDCT or an alternative standard of care treatment option.

Pembrolizumab (200mg) alone or in combination with ICE (or DHAP in case of PET positive findings after P-ICE) will be admistered. 6 additional courses of pembrolizumab will be administered after the completion P-ICE or P-DHAP

The proportion of patients with a Deauville score of 1-3 in the centrally reviewed restaging after treatment with 1 x pembrolizumab + 4 cycles of pembrolizumab and chemotherapy combined (4x P-ICE or 2x P-ICE + 2x P-DHAP)