I also have a GSD, she is 12 yrs old. 3 yrs ago she had a mast cell tumour in her nose and I was lucky enough to be insured so I put her through chemo and radiotherapy.
I'm sorry but the truth, from my experience, is I would never have put Livvy through it if I had have known the side effects of her wetting herself, being unable to lift herself up for days and what has now been a 3 year ongoing unsucessful battle to try cure what the vets said is a cronic nose infection which, due to the treatment, she is unable to fight off completely.
I was told 2 weeks ago that it is likely the tumour is back and she is now having frequent nose bleeds.
Saying that, I am not going to put her through chemo or radiotherapy again, I refuse to put her through all the stress again. She is currently a happy dog who is my ray of sunshine and whilst she is not in any pain I will maintain her as best I can so that she has a happy life, no matter how long or short that life may be.
I'm sorry I am not the bearer of good news. I wish you and your dog all the best.