Association of Programmed Cell Death 1 Inhibitor with Circumorificial Plasmacytosis.

The development of immune checkpoint inhibitors is considered one of the most important advances in cancer treatment. Pembrolizumab is an immune checkpoint inhibitor against programmed death-1 (PD-1) receptors that has demonstrated antineoplastic activity against various malignancies, including non-small cell lung cancer, melanoma, and triple-negative breast cancer. Pembrolizumab is associated with numerous adverse reactions including mucosal and cutaneous reactions referred to as immune-related adverse events. These events can impact patient quality of life and lead to dose reduction or discontinuation of the medication. A comprehensive understanding of pembrolizumab's toxicities is crucial for the initiation of treatment. We present the case of a 27-year-old man with stage IVB thymic carcinoma with a bulky anterior mediastinal mass, bilateral jugular, bilateral peritracheal, and bilateral cardiophrenic lymphadenopathies, and a small pericardial effusion. He received pembrolizumab IV every 3 weeks for 53 cycles over 39 months. The patient developed bleeding oral lesions approximately 38 months after treatment with pembrolizumab. The patient's pembrolizumab treatment was not interrupted and the perioral rash ultimately improved after treatment with steroids.