Answer: Hamartoma (tailgut cyst)

Histology: On gross inspection, the tumor was seen as a multiloculated cyst. There was no anatomic connection to the rectum. The cysts are lined by an admixture of stratified squamous, ciliated columnar, and mucin-secreting columnar cells. The wall of the cysts contains disorganized fascicles of smooth muscle, but there is a notable absence of adnexal structures, neural elements, or other heterologous mesenchymal tissues.

Discussion: Retrorectal cystic hamartomas, or tailgut cysts, are rare congenital lesions that typically present as presacral masses. These cysts are most likely derived from remnants of the embryonic tailgut. Clinically, they are frequently unrecognized and misdiagnosed. Malignant change is extremely rare, but the published literature is peppered with case reports of adenocarcinomas and neuroendocrine tumors arising from tailgut cysts.

The clinical diagnoses in our cases were often delayed, which in part may be due to unfamiliarity with this entity. The main histopathologic diagnostic difficulty is distinction from presacral mature cystic teratomas. Unlike cystic teratomas, the histopathologic findings of tailgut cysts do not include adnexal structures, neural tissues and mesenchymal tissues showing heterologous differentiation. Given the limited risk of malignant transformation, these lesions should be meticulously inspected and adequately sampled to rule out a coexisting carcinoma, which may be a focal finding. Complete excision of the multicystic process is advised to prevent recurrent draining sinuses and eliminate the possibility of malignant change.