A thorough literature search revealed no previous reports of this entity, and we 
are the first to describe a case of a high-grade sarcoma arising from a 
recurrent immature teratoma misdiagnosed as growing teratoma syndrome. The 
patient was a 23-yr-old female, diagnosed at the age of 20 with a Stage IIIB 
immature ovarian teratoma. After surgery and chemotherapy, the patient developed 
multiple liver and pelvic masses that were diagnosed as mature teratomas based 
on small samples obtained by computed tomography-guided core biopsy. Three years 
after diagnosis the patient presented with severe respiratory difficulty and 
following resection, the final pathology revealed multiple tumors with foci of 
high grade sarcoma compatible with primitive neuroectodermal tumor/extraskeletal 
Ewing sarcoma based on morphology and immunohistochemistry (CD99, CD56). 
However, on the basis of further immunostaining and fluorescent in situ 
hybridization studies negative for rearrangement of EWSR1, the final pathologic 
diagnosis was high-grade unspecified (undifferentiated) sarcoma. This case 
illustrates the pitfalls of biopsying 1 site in a patient with recurrence of a 
heterogeneous tumor such as immature ovarian teratoma, especially when rendering 
a benign diagnosis such as growing teratoma syndrome. It is of utmost importance 
to adequately sample large-volume recurrent teratomas, and we suggest biopsying 
several different sites, to increase the likelihood of detecting a malignant 
component.