My Megan, now 13 , developed a fungating mammary tumour about 3 years ago. It grew so quickly , almost within a few days ,and had she not rolled over to show me it, I may not have noticed it for ages. It had started oozing nasty stinky stuff but did not seem to cause her any distress. We were in France at the time but due to return to the UK within a couple of days so we decided to have the surgery done in the UK. Our vet in France seemed pretty pessimistic and gave her a farewell cuddle. Anyway, she had the tumour removed and there has been no spread or recurrence. She has other age related health issues but none regarding tumour. Oh yes, she was spayed about 6 months after the tumour -she developed pyometra, just before we were planning to have her spayed - full recovery from that too.
Hope this is reassuring.