Emergence of mature teratoma following treatment of sacrococcygeal endodermal sinus tumor: CT and MR imaging with pathological correlation.

We report a case of pseudoretroconversion in which, clinically and pathologically, it appeared that an endodermal sinus tumor retroconverted to mature teratoma. An 8-month-old white female infant with a sacrococcygeal tumor, diagnosed by biopsy as endodermal sinus tumor (yolk sac tumor), was treated with chemotherapy. The residual tumor after treatment was mature teratoma confirmed by imaging and by pathological examination. We present the computerized tomography (CT) and magnetic resonance imaging (MRI) findings with pathological correlation.