Large bump on dogs bAck. ?

My ShihTzu has had a lump on her back for over 2 yrs now and the vet "thought" it was just a cyst and worth watching. She has also been getting shots for allergies. Well about a month ago I noticed the lump was getting large fast so I had the vet remove and biopsy it. It turned out to be a Mast Cell Tumor (not cancer). However these tumors give off histimines and the dogs little body can't make antihistimine fast enough to counteract. The tumors can show up anywhere on the body. Moral of the story, my dog didn't have allergies. Her scratching and biting at her paws was the result of the histimines being given off by the tumor. Watch your dog to see if he is biting or scratching. Could be an indication that the lump is a Mast Cell Tumor. Your vet can inject an needle in the lump to draw a sample of it's contents and hopefully identify what is causing it. Good luck.