KRAS-Driven Combined Pulmonary Adenocarcinoma and Atypical Carcinoid Tumor: A Rare Case with Shared Clonal Origins.

We report a rare case of a combined pulmonary tumor composed of adenocarcinoma and atypical carcinoid, both sharing identical KRAS p.G12S mutations and high PD-L1 expression. A 74-year-old woman with a remote history of stage IV colon cancer presented with acute bilateral pulmonary embolism and was subsequently found to have a right upper lobe (RUL) lung nodule. Histologic examination of the biopsy and subsequent resection material revealed two morphologically distinct tumor components: adenocarcinoma and an atypical carcinoid. Immunohistochemical stains showed divergent marker profiles, while next-generation sequencing (NGS) and PD-L1 immunostaining demonstrated shared KRAS mutation and similar expression patterns, indicating a clonal relationship. To our knowledge, this represents the first reported case of a KRAS-mutated adenocarcinoma and atypical carcinoid tumor with molecular confirmation of clonality. This case underscores the critical role of integrated histologic and molecular analysis in the diagnosis and classification of combined lung tumors.