Tetracycline seems a little mild for something like that. It can treat respiratory infections, but with a possible abscess or cellulitis, I would think the vet might have prescribed something like clavamox or others. But I am not a vet. Some vets, when not that familiar with chickens are reluctant to prescribe strong antibiotics. Tetracycline is about the only one some feel comfortable using, but it may not treat something that serious. I again wonder about it being cancer, but I hope not.