Patients with MPNST, who are not eligible for surgery of curative intent, will be considered for treatment with pembrolizumab. Patients will receive pembrolizumab treatment for up to 10 cycles. The patients must discontinue study treatment earlier (before 10 cycles of pembrolizumab) if they experience symptomatic deterioration attributed to disease progression, intolerable toxicity related to pembrolizumab, any medical condition that may jeopardize the patient's safety, use of other non-protocol anti-cancer therapy or pregnancy.