Somatic malignancy arising in ovarian mature cystic teratoma (MCT) is a relatively rare phenomenon with an estimated incidence ranging from 0.17% to 5.5%. Most previous studies have been limited by small sample sizes, hindering more precise estimates of incidence as well as providing limited prognostic information. We aimed to conduct a large-scale, multi-institutional study to better define incidence, discuss prognosis, and report occurrences of unusual malignancies arising in MCT. The pathology archives of the Massachusetts General Hospital and Brigham and Women's Hospital were searched for all cases of MCT arising between 2006 and 2021. The pathology reports were reviewed for the presence of somatic malignancy arising within MCT. Cases harboring somatic malignancy were re-reviewed by a gynecologic pathologist, with documentation of a number of histomorphologic variables, including surface involvement, lymphovascular invasion, and tumor size. Sociodemographic variables, adjuvant chemotherapy, disease recurrence/progression, and survival were extrapolated from the medical record. Among 2416 cases of MCT, 40 cases of somatic malignancy were identified. Tumors included squamous cell carcinoma (SCC, n=21), papillary thyroid carcinoma (PTC, n=7), sebaceous carcinoma (n=2), neuroendocrine carcinoma (n=2), and other rarer types. The mean age of patients was 49 years (range: 17.7 to 69.7 y). Follow-up data was available for 20 patients (range: 3 to 196 mo, mean: 80.5 mo). Eleven were ovarian confined without surface involvement
9 were AJCC stage pT1C or higher at the time of diagnosis. Of ovarian confined tumors without surface involvement, only 1 recurred (a follicular variant of papillary thyroid carcinoma) with bone metastases found 72 months after initial diagnosis. Four additional cases, all of which were stage 1C or higher at initial diagnosis, recurred after initial resection, including 2 cases with SCC, 1 melanoma case, and 1 adenocarcinoma ex-Goblet cell carcinoid case. Tumors that recurred tended to have a large malignant component (range: 4 to 23 cm, mean: 16.8 cm). When cases received in consultation were excluded, the overall incidence of incidental somatic malignancy arising in MCT was 0.54% (13 of 2389 cases). Somatic malignancy in MCT is rare, and outcomes largely depend on the stage at initial diagnosis, and possibly, the size of the malignant portion of the tumor. Poor outcomes were noted across multiple histologies. Patients diagnosed with early-stage disease (stage IA) generally had a favorable prognosis, whereas those with advanced-stage disease (stage IC or higher) faced higher risks of recurrence and mortality. Nevertheless, some low-stage patients experienced recurrence, highlighting the need for long-term follow-up for all patients. More aggressive management strategies should be tailored on a case-by-case basis. The focality of residual MCT, in some cases, underscores the need for a thorough sampling of ovarian somatic tumors without a known primary site. Careful pathologic evaluation, particularly of solid areas, is critical to detect malignancy in MCTs.