What is the normal symptoms that can happen if the large growth on my cats neck is cancer.?

I have a cat who I have had all my life (21years old). Well last year around June 2009 I had a tumer removed from my cats toe. So they took the whole toe, and they said the tumar was benign. Well then about 6 months later a growth started growing on her neck. Well when I took her to the vet in 2009 she weighed 10lbs and then I recently took her to the vet maybe 3 weeks ago. She weighed 6.15lbs. Also the vet said that basically there is no way to tell without doing a biopsy and at her age if it were a tumar its not like she could do chemotherapy. So recently it has gotten really big and then now there is a spot on it that has scabbed. My cat still acts loving and purrs. She does have a limp though when she walks, and the lump feels like it could be filled with fluid. There are hard spots to it also. So does anyone have any ideas on what I could do to help her. The vet gave me the option to do the biospy or just let her live the rest of her life out. So as long as she still has quality of life I will keep doing what I am doing. I just want to make sure that there is nothing else I can do. Thanks