My 17 Year Old Grandson Has Had Voiding Problems. Has

my 17 year old grandson has had voiding problems. has had 2 occasions of not being able to void bladder. He had a catheter inserted to empty bladder. He was given dynamic scan, no problems were found. Then the dr. performed an MRI and discovered cystic mass between L2 and L3 on spine. Now he is scheduled for another MRI with some sort of dye in order to get a better picture of mass. What could this mass be?