Abstract
RATIONALE: Novel treatment strategies such as immunotherapy are being evaluated to further improve the outcomes of colorectal cancer patients. To our knowledge, this is the first report to show both the successful treatment of pulmonary squamous cell carcinoma (SCC) with pembrolizumab alongside histological and immunohistochemical findings of resected colon cancer under immunotherapy for lung cancer. PATIENT CONCERNS: This patient was a 70-year-old man who presented with a right lung tumor and simultaneous adenocarcinoma of the sigmoid colon. DIAGNOSES: Biopsy examination revealed squamous cell carcinoma in the right lung and adenocarcinoma of the sigmoid colon. INTERVENTIONS: The patient underwent successful pembrolizumab treatment as first-line immunotherapy for lung cancer, as demonstrated by computed tomography, and the sigmoid colon tumor was excised during an immunotherapy-free window. OUTCOMES: No unusual tumor growth in the right lung or abnormal abdominal signs was observed during the 9-month follow-up. LESSONS: Microscopically, the resected colon cancer specimen was characterized by numerous lymphoid cells in the partial stroma, with a large number of infiltrating lymphocytes consisting of CD3+, CD8+ T cells. In summary, this case demonstrates how immunotherapy affects PD-L1-negative colon cancer and indicates future treatment prospects.