Careful examination of the MRI revealed a primarily cystic tumor arising from the subperitoneal space, with most likely a myxoid main component. Uterus and ovaries were normal.
Using a gamuts 1 for pelvic extraperitoneal masses, possible diagnoses for a cystic mass with myxoid stroma include myxoma, aggressive angiomyxoma, schwannoma, plexiform neurofibroma.
Judging by the young age of the female patient, the subperitoneal location with puborectal and levator ani muscles rupture, the myxoid component associated with an enhancing vascular component in layered and "onion-peel"-like appearance, our most likely diagnosis was aggressive angiomyxoma 2.