Sick Baby Girl

My cat is 11 years old and very muched love. Recently, over the last month she has 
lost a lot of weight and spends much of her tim sleeping. A trip to or Vet and an 
X-ray proved that she had a large growth that could not be operated on.
We don't think she is in pain, but are not sure. 
We know the time will come when it will be nessiary to put her down, but we are not 
sure if she will be able to tell us or, if we will know when that time has come. Can 
you help?