I'll out myself as the former owner of the ~17cm Thing. Mine was a dermoid, which I think meant largely solid, and came to my attention when it DID twist, and that? Is a special kind of agony.

Holy crap, I didn't realize they'd sent you to an oncologist. Like, "Hey, this isn't frightening ENOUGH, so here's a referral directly to a CANCER DOC but don't worry or anything."