Hi, Christine,
You MUST discuss this with your vet. The thirst may be associated with the level of the dose of prednisolone, which it may be possible to reduce.
A 10 year old boxer, with an active carcinoma, cannot have a very long live expectation, and I fear that you will shortly have to make a decision as to what is kindest for her. A sad and difficult decision, but it is your responsibilty.
Walter