Spontaneous regression of a combined large cell neuroendocrine carcinoma following bronchoscopic biopsy: A case of cytohistological discordance.

Spontaneous regression (SR) in lung cancer is extremely rare. We report a unique case of apparent biopsy-associated SR accompanied by cytohistological diagnostic discordance in a 68-year-old man. The patient presented with a 14-mm pulmonary nodule. Bronchoscopic forceps biopsy initially yielded a diagnosis of small cell lung cancer (SCLC), whereas concurrent brush cytology suggested non-small cell lung cancer. Unexpectedly, a preoperative computed tomography scan obtained 41 days after biopsy revealed regression of the solid tumour component from 14 mm to 7 mm. The patient underwent surgical resection, and the final pathological evaluation revealed combined lung cancer comprising predominantly adenocarcinoma (95%) with a minor large-cell neuroendocrine carcinoma component (5%). The initial SCLC diagnosis was based on crush artefact within the biopsy specimen, which obscured the characteristic morphology of LCNEC. This case highlights that physical biopsy can trigger transient SR, particularly in neuroendocrine tumours; however, surgical resection remains essential to address heterogeneous disease progression. Furthermore, cytohistological discordance should alert clinicians to the possibility of combined tumours, intratumoural heterogeneity, and morphological pitfalls, ensuring appropriate multidisciplinary management.