MON-392 - A Thyroid in Disguise : Case of Metastatic Malignant Struma Ovarii

Background: Struma ovarii is a rare form of ovarian teratoma composed mainly of thyroid tissue. In about 5-7% of cases malignant transformation can occur with papillary thyroid cancer and its variants being the most common histology diagnosis.
Clinical

Case: We report a case of a 33 year old female who presented in 2002 with 10cm right ovarian mass occurring after her second pregnancy, for which she underwent ovarian cystectomy complicated by cyst rupture. Initial pathology was erroneously reported as dermoid cyst. In 2004 she was noted to have bilateral adnexal masses and underwent biopsy which showed follicular variant of papillary thyroid cancer (PTCFV). Review of prior pathology slides showed malignant struma ovarii as corrected initial diagnosis. She underwent hysterectomy with bilateral oophorectomy and during surgery was noted to have multiple peritoneal tumor implants. Subsequently, patient underwent total thyroidectomy, with thyroid biospecimen being negative for PTCFV, to prepare her for radioiodine (RAI) therapy of peritoneal metastatic disease. Given that the first dosage of RAI (253 mCi) did not eradicate the disease, despite post-therapy scan documenting RAI-avid disease in the abdomen and pelvis, in 2008 she was referred to our center for further management. At that time, tumor marker Tg was elevated at 906 ng/dl, and the patient underwent debulking surgery (greater and lesser omentectomy, pelvic peritonectomy, cecal and jejunal lymph nodes resection and diaphragmatic stripping). Pathology confirmed metastatic KRAS p.Q61L-mutant PTCFV and debulking was associated with decrement in tumor marker Tg from 906 to 243 ng/dl. Given persistent disease, she was subsequently treated with dosimetry-based RAI aimed at therapy not exceeding maximum tolerated activity of 2 Gy exposure to the bone marrow. Over the course of subsequent 7 years, she underwent therapy with 4 additional RAI courses, that along with her initial RAI therapy, resulted in a total cumulative dosage of 1025 mCi. All post-therapy scans were consistent with RAI-avid disease in the pelvis (4 foci), posterior abdomen (2 foci) and posterior right liver lobe (1 focus). Over the past 10 years, cross-sectional imaging revealed lack of gross residual disease, negative 123I-Whole Body Scan and negative FDG-PET/CT imaging, along with persistently mildly elevated stable Tg ranging between 4.7-5.6 ng/dl with negative anti-Tg antibodies, consistent with biochemical incomplete response per the American Thyroid Association Guidelines.

Conclusion: This case underscores the challenges in managing complicated malignant struma ovarii, with peritoneal seeding being likely a consequence of malignant cystic lesion rupture. Consistently with metastatic RAS-mutant PTCFVs of thyroid origin, this tumor is characterized by RAI-avidity and RAI-responsiveness to radiation damage, making it a viable option for targeted therapy.

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