Figure 13b. Yolk sac tumor of the sacrococcygeal region in a 7-month-old girl. A cystic teratoma of the sacrococcygeal region was found at birth. After surgical resection, the tumor grew again. (a, b) Sagittal T1-weighted (a) and T2-weighted (b) MR images show a heterogeneous sacrococcygeal mass with high signal intensity (white arrows). The dorsal part of the tumor appears cystic, and fatty tissue is not evident. The tumor margin is ill defined with infiltration of adjacent tissue (black arrow). (c) Photomicrograph (original magnification, x200; H-E stain) shows a reticular pattern, which is characterized by a loose meshwork of communicating spaces lined by primitive tumor cells. The neoplastic cells have hyperchromatic, irregular nuclei with unusual shapes. A glomeruloid body is seen (arrows), which is a characteristic feature of yolk sac tumors. (d) Photomicrograph (original magnification, x400; 
-fetoprotein immunohistochemical stain) shows that the tumor contains yolk sac epithelium.