Table 1. Summary of Metastatic Mature Teratoma and Growing Teratoma Syndrome.

	Definition	Clinical Features	Histologic Features	Imaging Features
Metastatic mature teratoma	- Residual tumor at sites of metastatic NSGCT following chemotherapy	- Usually asymptomatic - Patients have an overall good prognosis after complete resection	- Well-differentiated tumor with at least one germinal layer - No embryonal malignant cells - Frequent association with necrosis and fibrosis	- CT: partially cystic, often multiloculated with enhancing septations - Fat and calcification more common in female - MR: variable; T2-hyperintensity due to cystic or necrotic content; T1-hyperintensity due to proteinaceous cyst content; enhancing septations - Ultrasound: partially cystic mass with anechoic fluid spaces - PET-CT: not recommended for post-chemotherapy surveillance of NSGCT; teratoma is non-avid or mildly avid; intense uptake may be indicative of viable malignancy or malignant transformation
Growing teratoma syndrome	- Enlarging masses histologically confirmed as metastatic mature teratoma during or after chemotherapy with normalized serum tumor markers	- Potential for local mass effect on or invasion of surrounding structures may result in medical or surgical complications	- Metastatic mature teratoma with enlarged cystic spaces	- Imaging characteristics of metastatic mature teratoma with increasing size and expanding cystic spaces - Potential imaging findings related to urinary, biliary, bowel, or vascular complications

NSGCT = non-seminomatous germ cell tumor