It is often difficult to differentiate metachronous primary lung cancers from local pulmonary recurrences when the histopathological findings are similar. A 43-year-old man underwent right upper lobectomy with lymph node dissection for primary lung adenocarcinoma (p-T2aN0M0, stage IB). Fifteen years later, he developed a lung nodule in his right middle lobe. The tumor was preoperatively thought to be a metachronous second primary lung adenocarcinoma, and was surgically resected. Histopathological findings for both tumors were of poorly differentiated adenocarcinoma with mucus production. Both tumors also harbored the EML4 (echinoderm microtubule-associated protein-like 4)-ALK (anaplastic lymphoma kinase) fusion gene (variant 3a+b). Based on this molecular finding, the pulmonary nodule was considered to be a recurrence after very long latent period.
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