Not a cavalier, but my Benji, a small collie cross had a cancer on his forehead.
At first we thought the slight lump, which suddenly appeared between his eyes, was a bump, caused by catching his face on the corner of the shed, during a mad gallop. However, just by looking, I think the vets knew what it was and a biopsy confirmed. Because of it's location and the fact that the growth extended into his nasal cavity, there was nothing to do but try to slow things down.
He was given a high dose of steroids, which did slow things down (The lump had grown noticably in the short time we waited for test results) but made him put on a massive amount of wait, taking him from a very dainty collie to a dog with the bulk of a sheep.
Despite this, he enjoyed life, though he only had eight months in total. The vet assured me that Benji had no pain and that he would tell me when it was time. We chose to let him go before any pain started and the time was when the size of growth pulled on his eyelids, threatening to make completely closing difficult.
He only had eight months, but they were happy ones. His last morning was spent eating his favourite food, romping in the woods and being allowed to chew upa small branch. It was very sad to let him go, but then I felt a surge of joy that he'd gone, unknowing and before pain.
Barbara, Monty, Joly and Teddy.