Sacral parasite with histopathological features of an unequally conjoined twin.

A sacral parasite is a rare congenital anomaly classified etiologically as duplicitas asymmetros, where the parasitic mass is regarded as the tissue remnants of an unequally conjoined twin. We, herein, report a case of a sacral parasitic mass found in a female neonate. A huge sacral mass was found at 30 weeks of gestation by fetal ultrasonography. The fetus was delivered by a Caesarean operation at 37 weeks and 6 days of gestation. The alpha-fetoprotein (AFP) level was 174,640 ng/ml. The mass separated by an operation on the day after her birth, contained structures similar to imperfect limbs and backbone. A variety of tissue and organs having derived from three germ layers were identified within the mass and showed a tendency to be highly differentiated. Even though a large proportion of the central nervous system tissue showed immature and dysplastic features, there were no evidences suggesting the presence of neoplasm. The infant has shown normal growth and is doing well without a recurrence of the sacral mass or any elevation in the serum AFP level during the12-month follow-up. We therefore believe that the present case should be considered a sacral parasite, rather than a sacrococcygeal teratoma. Although it should be recognized that such sacral parasite is very rare, this case provides further information that can be useful for diagnosis.