Struma ovarii is a rare monodermal ovarian teratoma in which thyroid tissue is the predominant or exclusive element. Malignant transformation is known to occur, usually as classical papillary thyroid carcinoma (PTC). However, the recognition of follicular variant PTC raises the possibility of similar malignancy arising in struma ovarii. We examined 13 cases of struma ovarii to determine if these lesions can exhibit histological, immunohistochemical, and/or molecular features of follicular variant PTC. Ten of these had atypical histology, cytologic features of PTC, and all ten showed diffuse positivity for CK19; eight of these were also positive for HMBE-1. Mutational analysis for BRAF identified no V600E mutations. However, seven of the ten cases with features of follicular variant PTC exhibited a rearranged in transformation (RET)/PTC rearrangement by reverse transcriptase polymerase chain reaction (RT-PCR). The three cases considered to be benign based on histologic and cytologic criteria were negative for CK19 and HBME-1 by immunohistochemistry, and had no evidence of BRAF mutation or ret/PTC-1 and ret/PTC-3 rearrangements. These results indicate that follicular variant PTC can occur in struma ovarii and that such lesions exhibit the same morphologic and immunohistochemical profile as follicular variant PTC in thyroid. The application of molecular testing to verify the diagnosis can be valuable, as these lesions may harbor ret/PTC gene rearrangements.
