What Causes Painful Hard Lump On Wrist?

My daughter, age 12, has a lump on top of her wrist that is now the size of a golfball. Where it was originally smaller and not painful, it is now constantly a source of pain for her. We assumed this was a ganglion cyst and her pediatrician said there was really no treatment for this. We are concerned there could be more to it. What was soft is now hard. On her left hand and she is right handed. So there really isn t a time she is overusing it during any activity. Can you offer any suggestions? We have tried heat, cold and resting it. So far it is no better. Thank you in advance for your reply!