A case of napsin A-positive metastatic lung cancer originating from the colon

BACKGROUND We report a case of napsin A-positive metastatic lung cancer originating from the colon. No cases of napsin A-positive metastatic lung tumors originating from colorectal cancer have been reported previously. CASE PRESENTATION Computed tomography identified a small lung nodule in a 70-year-old male patient, 18 months after resection for rectal cancer. The size of the lung tumor increased from 1.8 to 2.1 cm in 6 months and metastasis from the rectal cancer was suspected. Resection of the lung tumor was performed, and the histological features of the lung tumor revealed findings typical of colorectal adenocarcinoma and resembled those of the original rectal cancer. Furthermore, the metastasis stained positive for napsin A and thyroid transcription factor-1 (TTF-1) on immunohistochemical evaluation, and immunohistochemical analysis identified the same results in the rectal specimen. CONCLUSIONS These findings led us to believe that this was a rare case of napsin A-positive metastatic lung cancer originating in the colon. The patient was treated with chemotherapy for recurrent rectal cancer, and no other metastases were found after the lung resection. This is the first report of napsin A-positive colorectal cancer metastasizing to the lung.