Figure 1. Contrast computed tomography reveals a hypodense cystic lesion measuring about 99*93 mm in the left retro-rectal region (arrowhead) without contrast enhancement, causing a rectal deviation to the right side.

She underwent surgical cystectomy drainage and incomplete resection of the cyst. Histopathologic examination showed a cyst wall containing smooth muscle bundles lined by squamous or columnar epithelium. The stroma was infiltrated by nests of neoplastic cells with vesicular nuclei and abundant eosinophilic cytoplasm with foci of keratin-like material (Figure 2). In immunohistochemical staining, CK7, HMWCK, and P63 were positive, but CK20 was negative. These findings were compatible with squamous cell carcinoma arising on tailgut cyst.