Pembrolizumab in Treating Participants With Unresectable Thymoma or Thymic Cancer

This study is testing pembrolizumab — an immunotherapy drug that helps the immune system recognize and attack cancer — in patients with thymoma or thymic carcinoma (cancers of the thymus gland in the chest) that cannot be surgically removed. **You may be eligible if...** - You have unresectable thymoma or thymic carcinoma (the tumor cannot be completely removed by surgery) - You have measurable disease on imaging - You are in good physical condition (ECOG 0 or 1) - You have received any number of prior treatments - Your blood counts, kidney, liver, and heart function meet the required levels **You may NOT be eligible if...** - You have a significant autoimmune disease such as myasthenia gravis, lupus, or rheumatoid arthritis (minor autoimmune conditions may be acceptable) - You have active HIV, hepatitis B, or hepatitis C - You have had another cancer within the past 2 years (with limited exceptions) - You have previously received a PD-1 or PD-L1 immunotherapy drug - You are pregnant or breastfeeding - You are currently on immunosuppressive medications - You have active tuberculosis Talk to your doctor to see if this trial is right for you.