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B4155/11 Consultation case 20 year old female with a left ovarian mass. The biopsy material was originally submitted as tissue fragments, the largest of which measured 100x55x45mm. We received 6 formalin-fixed paraffin embedded tissue blocks with a diagnosis of ‘ovarian strumal carcinoid’ Histology Microscopically, we described a regressively changed tumor with variable architectural patterns (solid alveolar areas, areas of trabecular growth and formation of microfollicles containing homogenous eosinophilic material). The tumor cells were relatively uniform with focal nuclear anisomorphism, frequently showing nuclear clearing, grooves and rarely, pseudoinclusions. Immunohistochemically, the tumor cells were strongly diffusely TTF-1, thyroglobulin and HBME-1 positive. CD56 and CK19 were focally weakly positive and synaptophysin and chromogranin negative. Mitotic activity was low. Diagnosis Follicular variant of papillary carcinoma arising in a monodermal ovarian teratoma (struma ovarii). Differential diagnosis: - Metastasis of thyroid carcinoma to the ovary - Strumal carcinoid - Sertoli-Leydig tumor Discussion Struma ovarii (ovarian teratomas comprising over 50% thyroid tissue) account for less than 5% of all ovarian teratomas. They are usually unilateral and most patients are in their 5th decade. Malignant transformation is observed in only about 5% of cases. Criteria for diagnosis of malignancies in struma ovarii are the same as those for diagnosis of malignancy in the thyroid gland. Papillary carcinomas are diagnosed based on nuclear features (optical clearing, elongation, overlapping, grooves and pseudoinclusions) while follicular carcinomas are more diagnostically challenging as struma ovarii lacks capsules and tends to have irregular margins. Metastatic spread of papillary carcinomas arising in struma ovarii is usually to the peritoneum, mesenterium and omentum. HBME-1 positivity is indicative of malignancy but does not imply papillary differentiation. CK19 is expressed in papillary carcinomas and though it is also focally expressed in reactive follicular cells it is not seen in follicular proliferations. It is necessary to distinguish primary malignancy in struma ovarii from thyroid carcinoma metastatic to the ovary. In cases of metastasis, ovarian involvement is usually bilateral and the ovaries have no teratomatous features. The prognosis of thyroid-type malignancy arising in struma ovarii is difficult to estimate due to its rarity and the lack of a standard approach to treatment. Reference Cheung C, Ezzat S, Freeman J, Rosen I, Asa S. Immunohistochemical diagnosis of papillary thyroid carcinoma. Mod Pathol 2001;14:338-342 Salman W, Singh M, Twaij Z, A case report of papillary thyroid carcinoma in struma ovarii and review of the literature. Pathology Research International 2010;352476 Tavassoli F.A., Devilee P (Eds.): World Health Organization Classification of Tumors. Pathology and Genetics of Tumors of the Breast and Female Genital Organs. IARC Press: Lyon 2003 Wolff E, Hughes M, Merino M, Reynolds J, Davis J, Cochran C, Celi F. Expression of benign and malignant thyroid tissues and the importance of multimodal management as illustrated by a BRAF-positive follicular variant of papillary thyroid cancer. Thyroid 2010;20:981-987 Yucesoy G, Cakiroglu Y, Mueyyinoglu B, Besnili B, Yucesoy I. Malignant struma ovarii: A case report. J Korean Med Sci 2010;25:327-329