Yolk sac tumors (YST), formerly known as endodermal sinus tumors, are uncommon ovarian neoplasms. They are most common in the second and third decades of life and are rare in women older than 39 years of age. YST exhibit various histologic patterns. Microfollicular pattern resembling thyroid tissue has not been reported. We introduce here a case of yolk sac tumor in a 39 y/o female, presented with pelvic masses and high level of serum Alpha-fetoprotein (Alpha Fetoprotein) (> 1000 IU/ml). The patient underwent left salpingo-oophorectomy. On microscopic study, morphologic findings demonstrated a neoplasm composed of follicular structures with eosinophilic intraluminal content, resembling struma ovarii on low-power field. On Immunohistochemistry study, the tumor cells were positive for Alpha-fetoprotein and Glypican-3. Epithelial Membrane Antigen (EMA), Thyroglobin (TG), Thyroid transcription Factor-1 (TTF1) and sex-cord stromal markers were negative. This case introduces a new histomorphologic pattern of yolk sac tumor in a middle-aged woman. It is important to know that yolk sac tumor can exhibit various histological appearances. Regarding patient’s clinical and laboratory findings, this differential diagnosis should be kept in mind for proper diagnosis of similar cases.
