Highly differentiated follicular thyroid-type carcinoma of the ovary reconsidered.

More than a decade ago, Roth and Karseladze1 first described the entity of highly differentiated follicular thyroid carcinoma of ovarian origin (HDFCO) and reassessed the previously applied term peritoneal strumosis. Subsequently, similar ovarian cases were reported as ordinary struma or follicular adenoma-like lesions lacking a capsule.2 3 In 2018, the alternate term extraovarian struma was proposed based strictly on the molecular findings of their cases.4 In 2019, the term benign deportation was suggested for those HDFCOs in which the dissemination is confined to the peritoneum and peritoneal lymph nodes.5 Histologically, HDFCO can resemble ordinary thyroid tissue, colloid or adenomatous nodule, or an adenoma-like lesion; however, a capsule is never identified in ovarian thyroid-type lesions. Because of the latter findings, the terminology of neoplasms arising in ovarian struma differs from that applicable to the cervical thyroid gland. A perplexing feature of HDFCO is that the diagnosis cannot be established until extraovarian dissemination occurs. Thus, the diagnosis of HDFCO is often not ascertained until several decades after the excision of the original ovarian struma. HDFCO has been accepted as an unusual form of struma-derived thyroid-type carcinoma by authorities in the field.5 6 The term struma-derived thyroid-type carcinoma is applicable to all malignancies that arise in struma ovarii. To distinguish HDFCO from the more common forms, we apply the term typical struma-derived carcinoma for the latter. Cases of ovarian struma-derived carcinoma that resemble papillary or follicular cervical thyroid gland carcinoma and especially HDFCO have a favourable prognosis.1 Herein, we report three cases of HDFCO including the molecular findings of these neoplasms that previously were reported in abstract form and later in full text.7 8 ### Case series and histological validation Formalin fixed, paraffin-embedded sections of the neoplasms in this investigation were stained with H&E. In case 1, the index case, samples for …