Excellent prognosis of oncocytic thyroid carcinoma in Japanese patients: a single-institution study

Oncocytic thyroid carcinoma (OCA) is a rare thyroid malignancy that has only recently been recognized as a distinct entity from follicular thyroid carcinoma. Till date, studies regarding the prognosis of OCA remain inadequate. In this retrospective study, we investigated the prognosis of OCA among 137 Japanese patients who underwent surgery at our institution. Two patients had distant metastasis before surgery (M1), and two showed distant metastasis after surgery. The 10-year and 15-year distant recurrence-free survival rates were 100%, and 92.7%, respectively. No deaths occurred owing to thyroid carcinoma. Four patients with distant metastasis/recurrence were aged ≥60 years, and their tumors were >4 cm. Further, two patients with M1 had both vascular invasion (VI) and wide capsular invasion (CI). One patient had a poorly differentiated component (approximately 30%), and one showed high cell proliferating activity (Ki-67 labeling index [LI] ≥5%). Of the two patients who developed distant metastasis after surgery, one had VI and wide CI, and although another had minimal CI and was negative for VI, the Ki-67 LI of his tumor was >5%. These findings suggest that Japanese patients with OCA generally had excellent prognosis after surgery; however, older age, large tumor size, wide CI, VI positivity, high Ki-67 LI, and the presence of a poorly differentiated component may be indicators of aggressive behavior. Further multicenter studies with larger samples and longer follow-up durations are necessary to identify clinicopathological factors accurately predicting poor prognosis in patients with OCA.