Rare case of rectal carcinoid with synchronous primary carcinoid tumors of the lung misdiagnosed as lung metastases.

A 68-year-old woman was referred to our hospital for rectal surgery after a pathological diagnosis of rectal carcinoid with venous invasion following endoscopic submucosal dissection of a 5 mm-sized submucosal tumor in the lower rectum. Chest CT showed nodules in the left upper lobe and right lower lobe, but positron emission tomography and somatostatin receptor scintigraphy showed no hyperaccumulation in the lung nodules. CT-guided needle biopsy was performed on the nodular lesion in the left upper lobe, which showed focal growth of tumor cells with a high N/C ratio and positive synaptophysin, leading to a diagnosis of pulmonary metastasis of rectal carcinoid. Since the patient was asymptomatic and did not wish to undergo surgery or chemotherapy, she was followed up strictly with sufficient informed consent. Three years have passed since the diagnosis, and there is no tendency for the lung metastasis to increase, and no other new lesions have been observed. The disease had not progressed and remained stable. Therefore, immunohistological analysis of the lung biopsy specimen was performed again, which was positive for TTF-1 and negative for CDX2. Consequently, the diagnosis was changed to primary lung carcinoid tumors, and the patient remains under follow-up with no disease progression.