Yolk sac tumor initially diagnosed by ascitic fluid cytology in a postmenopausal woman: a rare case

Yolk sac tumor (YST) in postmenopausal women is rare and typically presents at an advanced stage including peritoneal involvement and ascites. However, detection of malignant YST cells in ascitic fluid remains exceedingly uncommon. We report a very rare case of metastatic YST initially diagnosed by peritoneal fluid cytology. A 75-year-old woman presented with a large left adnexal mass, thickened endometrium, and large-volume ascites. Cytologic examination of ascites demonstrated abundant malignant cells that were strongly and diffusely positive for SALL4 and Glypican-3, focally positive for AFP, and negative for PAX8, ER, WT-1, p16 and CK7. These findings supported a diagnosis of YST, which was further confirmed by markedly elevated serum AFP and subsequent endometrial curettage. In elderly women with advanced genital tract malignancy and malignant peritoneal fluid, YST is rarely included in the initial differential diagnosis. This case underscores an important clinical implication in the evaluation of diagnostically challenging malignant effusions: when routine morphologic and immunophenotypic findings do not support a common gynecologic or Müllerian neoplasm, germ cell markers—including glypican-3, AFP, and SALL4—as well as measurement of serum AFP should be incorporated into the diagnostic workup. Recognition of this rare possibility is essential for accurate diagnosis and appropriate management, including monitoring serum AFP levels if elevated.