Synchronous Duodenal Cancer and Lung Cancer Harboring an Epidermal Growth Factor Receptor Mutation Treated with Erlotinib and Oral Fluoropyrimidine

Chemotherapy for multiple primary cancers is challenging. We describe a case of synchronous duodenal cancer with lung cancer harboring an epidermal growth factor receptor (EGFR) mutation treated with erlotinib and S-1, an oral fluoropyrimidine agent. A 78-year-old woman with advanced EGFR-mutated lung adenocarcinoma was simultaneously diagnosed with duodenal adenocarcinoma. After the treatment with erlotinib, the lung cancer responded well, but her duodenal cancer showed no response. S-1 was added to erlotinib, and the duodenal cancer demonstrated a good response with tolerable toxicities. The concurrent use of erlotinib and S-1 was safe and efficacious for synchronous lung cancer harboring an EGFR mutation and duodenal cancer.