[Invasive Mucinous Adenocarcinoma Diagnosed During the Follow-up of Atelectasis:Report of a Case].

A 72-year-old man underwent bronchoscopy for an abnormal shadow on chest imaging, but no definitive diagnosis was made. The lesion was followed for four years as atelectasis, during which time it gradually increased in size. A computed tomography (CT)-guided biopsy suggested mucinous adenocarcinoma; however, a definitive diagnosis remained elusive. Surgical resection was ultimately performed for both diagnostic and therapeutic purposes, and the patient was diagnosed with invasive mucinous adenocarcinoma. Invasive mucinous adenocarcinoma, characterized by abundant mucus components, may be difficult to diagnose through preoperative biopsy. Even when atelectasis is suspected, the possibility of invasive mucinous adenocarcinoma should be considered. Prompt re-evaluation is warranted when an enlarging shadow is observed. In some cases, surgical resection may be necessary as part of the diagnostic strategy.