Histologically bland "extremely well differentiated" thyroid carcinomas arising in struma ovarii can recur and metastasize.

Struma ovarii (SO) infrequently harbor carcinomas that are histologically similar to those arising in the eutopic thyroid. We identified 10 such cases in our files. Eight patients presented with pelvic-related symptoms whereas 2 were incidentally discovered during pregnancy, all with disease confined to the ovary. There were 8 papillary thyroid carcinomas (PTCs) (2 classic and 6 follicular variant) and 2 poorly differentiated thyroid carcinomas. Two of the 10 thyroid carcinomas relapsed after an initial diagnosis of "benign" struma. Both occurred in young women with ovarian cysts discovered during pregnancy. The cystectomy from 1 patient showed thyroid follicles with nuclear features of the follicular variant of PTC whereas the cyst from the second patient showed thyroid follicles with subtle nuclear features, suggestive but not diagnostic of PTC. Both patients presented with disseminated PTC 3 and 4 years after the initial diagnosis, involving the pelvis in both cases and also the liver parenchyma in 1 case. The 2 patients received radioactive iodine therapy after thyroidectomy and are both alive with disease 6 years after diagnosis. The criteria separating hyperplastic nodules from well-differentiated follicular variant of PTC in the thyroid gland seem to be applicable to thyroid-type carcinomas arising in SO. The propensity for adverse clinical behavior does not seem to be related to the grade or histologic type of carcinoma in this small series. The hormonal milieu during pregnancy may lead to progression of malignant SO and such patients should be closely followed, particularly if their treatment consists of cystectomy alone.