A retroperitoneal cyst arising from an appendix fragment.

A 52-year-old man was first referred to the clinic for a review of an incidentally detected 9 × 5 cm simple right-sided retroperitoneal cystic lesion (Fig. 1a,b) from computed tomography (CT) of the spine for investigation of lower back pain in 2014. A follow-up CT of the abdomen/pelvis 12 months later showed no change in the size and characteristics of the lesion (Fig. [Extracted from the article]