Sarcoid‐like reaction of the extrathoracic lymph node in a patient with lung adenocarcinoma treated with pembrolizumab

Immune checkpoint inhibitors (ICIs) have become the standard of care for the treatment of non‐small cell lung cancer (NSCLC). With the increasing use of ICIs, clinicians should be familiar with their immune‐related adverse events, including sarcoid‐like reactions, which have been associated with the use of ICIs in patients with cancer. Sarcoid‐like reactions are caused by uncontrolled T helper 1‐mediated immune responses resulting from ICIs, but their pathophysiology is not fully understood. Sarcoid‐like reactions are often clinically important because they mimic metastases from treated cancer. According to previous reports, sarcoid‐like reactions are typically observed in intrathoracic locations (lung and/or mediastinal lymph nodes) and the skin. In this study, we report an extremely rare case of extrathoracic sarcoid‐like reaction in the right external iliac lymph node following two cycles of pembrolizumab therapy in a patient with lung adenocarcinoma. The laboratory data and computed tomography images suggested that infectious and autoimmune diseases were not considered to be the causative agents. Residual bone metastasis might have caused T helper 1‐mediated immune responses by pembrolizumab, and contributed to sarcoid‐like reactions in the right external iliac lymph node. Sarcoid‐like reactions should be considered in the differential diagnosis of patients with lung cancer treated with ICIs who exhibit worsening extrathoracic lymph node swelling. Clinicians should be cautious not to mistake extrathoracic sarcoid‐like reactions of the lymph nodes for progression of the treated disease.