Aim: Pathohistological diagnostics (PD) of the Yolc Sac tumor (YST) of primary mediastinal localization. Introduction: YST are malignant germ cell tumors of primitive endodermal gonadal differentiation. In 1% -5% of cases their localization is extragonadal, including mediastinal in 50% -70% published cases. Case report: A 30-year-old man was admitted to the Thoracic Surgery Clinic for diagnosis and treatment of mediastinal tumor with lungs and liver metastases. Laboratory analyzes revealed high elevations in serum alpha-fetoprotein (AFP), in addition moderate elevation of beta-chorionic gonadotropin (ß-HCG). No pathological changes in testis and retroperitoneum have been found. An open biopsy of the mediastinal change was made for PD. Histologically, tumor tissue showed a significant degree of necrosis. There were reticular, microcystic and pseudopapillary forms of neoplastic cell growth, surrounded by myxomatosis stroma. Tumor cells cytoplasm was scarce and vacuolated with high nuclear polymorphism and hyperchromasia. Positive immunohistochemical (IMH) reaction was obtained for panCK, AFP, PLAP. Immunoreactivity for TTF-1 was found in about 50% of tumor cells, as well as the focal reaction for ß-HCG in rare multinuclear cells. Reactions for D2-40, CD5, CK-7, CK-20, p63, CD117, CD30, napsin A and calretinin were negative. Conclusion: The results of our analysis showed the expression of divergent endodermal linear markers in the mediastinal YST, especially TTF-1 expression. The diagnosis of YST in small biopsies can be difficult and requires using of a wide range of IMH markers in order to closer the differentiation of primary tumor localization and the application of appropriate chemotherapy

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