My Daughter Has A Ganglion On The Tendon In Her

My daughter has a ganglion on the tendon in her rt. leg near the knee that showed on her MRI of the knee, which was hurt 11 years go. She has had pain on & off in the front & back of the knee. The first MRI (11 years ago) did not show a ganglion and was normal & the recent MRI was unremarkable, except for the ganglion. Her orthopedic surgeon says the ganglion would not cause her pain & did not advise to go in to take it out, but was thinking of doing arthroscopic surgery to look around the area. What caused the ganglion? Thank you