I'm so sorry you are going through this. If this were my cat, I would still continue the force feeding...as the cat will get weaker and more dehydrated and will get worse otherwise. To me the cat still has quality of life.
Here is a general article on cancer in cats and dogs with an additional link at the end. http://www.pets.ca/dogs/tips/cancer-...s-pet-tip-230/
Here is an older thread on cats, cancer and nutrition just in case it's cancer
Hope this might help