This is how she looks nowadays:

She looks healthy, her fur is shiny and soft, but she's slowly fading away. When she came eight years ago she weighed six kilos, nowadays she's barely four and even though she eats well (canned food 2-3 times a day and some dry food when she feels like it) she's slowly getting thinner.

I am quite certain she has cancer, but why I don't take her to see a vet? Because she's almost 15 years old the chances are thin that she would get operated or if she did her chances to survive through the operation would be as thin.
She's not in pain, she's happy, she eats well and takes care of herself, so I have decided that even she probably has cancerous tumor somewhere I am not even trying to cure her: she is an old cat and very old Siamese and I believe that she has the right to live rest of her life with dignity, without operations which could kill her and without me knowing for sure that she is terminally ill.
If/ when she becomes too weak to take care of herself or has apparent pains I will take her to the vet and do the right thing.

I just believe that she has a right to enjoy the rest of her days without any excess stress.