Mammary cancer in cats?

After finding an approx. one inch lump 3 weeks ago on my 8 years old cat's belly I contacted my vet. She said my cat needs immediate surgery and that the lump is probably a mammary tumor. I've been crying all day yesterday and searched the internet all night long for answers. My cat has been spayed when she was about 7 months old and I took good care of her. I just don't know why, and I'm very sad about the outcome. In the articles on the internet I found that most vets do x-rays to determine if the cancer has spread, but my vet did not do any x-rays nor bloodwork, she said we need to do surgery and the lump needs to come off. My question..has anyone else experienced the same I'm going through? Please I need some advise and help. Her surgery is scheduled for this coming Wednesday.