A rare case of isolated pulmonary metastasis from a colorectal tubulovillous adenoma with focal carcinoma.

BACKGROUND: Isolated pulmonary metastasis originating from an early-stage colorectal adenoma with only focal carcinomatous transformation is exceedingly rare and poses significant diagnostic challenges. CASE PRESENTATION: A 68-year-old man was found to have a solitary pulmonary nodule on routine chest computed tomography, radiologically highly suggestive of primary lung cancer. The patient underwent video-assisted thoracoscopic surgery, during which frozen-section analysis indicated invasive adenocarcinoma, prompting a lobectomy with systematic lymph node dissection. However, definitive histopathology, combined with immunohistochemistry, demonstrated that the lung lesion was metastatic adenocarcinoma of colorectal origin. Subsequent colonoscopy identified tubulovillous adenomas in the ascending colon and a tubulovillous adenoma with high-grade intraepithelial neoplasia and focal adenocarcinoma in the sigmoid colon. CONCLUSION: This case demonstrates that even a colorectal tubulovillous adenoma with only focal carcinomatous transformation can give rise to isolated pulmonary metastasis - a phenomenon that is underrecognized. When a solitary pulmonary nodule exhibits classic primary lung cancer features on imaging, the possibility of metastasis from an occult colorectal primary must still be considered. Integrated morphological and immunohistochemical evaluation, along with appropriate gastrointestinal follow-up, is essential to avoid misdiagnosis and to guide correct staging and management.