甲狀腺濾泡癌(thyroid follicular carcinoma)從細胞學的表現無法與濾泡腺瘤(follicular adenoma)區分，因此兩者被合稱為濾泡性腫瘤(follicular neoplasm)，需在組織學檢查時發現有腫瘤侵犯包膜血管或包膜，方可確定為濾泡癌。濾泡癌傾向侵犯血管，以血行性轉移為主。本文介紹一個66歲男性病例，曾因結節性甲狀腺腫(nodular goiter)接受次全甲狀腺切除術(subtotal thyroidectomy)，病理診斷為濾泡腺瘤。於9年後意外發現右下後肺部有一腫瘤，電腦斷層掃描導引肺部切片檢查發現為甲狀腺組織。此時重新檢視9年前切除之甲狀腺結節病理切片，發現已有甲狀腺濾泡癌細胞侵犯包膜血管，所以當時之診斷應修正為甲狀腺濾泡癌，現今情形為濾泡癌的遠端轉移。病人使用放射性碘治療後於門診追蹤，並持續服用甲狀腺素。此病例在手術九年後方發現肺部轉移，且其肺部轉移病灶為單一結節，皆為濾泡癌之罕見表現。針對濾泡性腫瘤，詳細的病理檢查以確定有無包膜或包膜血管侵犯至為重要，以避免濾泡癌被誤診為濾泡腺瘤的可能性。濾泡癌常以遠端轉移的型式復發，在有濾泡癌或濾泡腺瘤病史之病人出現腫瘤時，濾泡癌轉移應為優先考慮。

Thyroid follicular carcinoma and follicular adenoma are indistinguishable cytologically, thus they are grouped together as follicular neoplasms. The diagnosis of follicular carcinoma depends on either vascular or capsular invasion histologically. Follicular carcinoma often invades blood vessels, and hematological spreading is the usual metastatic route. We presented a 66-year-old man receiving subtotal thyroidectomy for nodular goiter, which was diagnosed as follicular adenoma pathologically. One tumor over right lower posterior lung field was noted incidentally 9 years after the operation. Computerized tomography guided lung biopsy for the tumor revealed thyroid tissue. We reviewed the previous pathological specimen of thyroid nodule and found vascular invasion. The previous diagnosis should be modified as thyroid follicular carcinoma, and the new pulmonary lesion was a distant metastasis. The patient received radioactive iodine and long-term thyroxine treatment. In this case, both the lung metastasis found 9 years after surgery for a thyroid nodule and the lung metastasis presenting as a solitary tumor are rare presentations of follicular carcinoma. For a follicular neoplasm, detailed pathological examination to detect the possible vascular or capsular invasion is important to avoid the possibility of follicular carcinoma misdiagnosed as follicular adenoma. Follicular carcinoma often recurs with distant metastasis. Metastasis of follicular carcinoma should be the first impression when a tumor is found in a patient with history of follicular carcinoma or follicular adenoma.