MR imaging of complex tail-gut cysts.

Retrorectal-cyst hamartomas (RCH) are rare developmental tail-gut cystic tumours of the retrorectal space, which occasionally undergo malignant transformation. We describe the magnetic resonance imaging (MRI) findings in two patients with RCH and in a third patient with unclassified sarcoma arising from a RCH. The RCH were hypointense or hyperintense on T1-weighted images and hyperintense on T2-weighted images; they did not enhance and they contained multiple septations. A solid component in the periphery of one cyst was markedly hypointense on T2-weighted images in keeping with fibrous material. The sarcoma arising from the wall of the RCH enhanced and was of intermediate signal intensity on all sequences. MR may help establish the diagnosis of RCH if an unenhanced cystic tumour is discovered in the retrorectal space and it can help detect those rare cases of malignant transformation of these developmental tumours.