
There was a study carried out a few years ago and I ahve personal expereince to confirm this, that showed that spaying after a lump had already appeared had no protective effect against developing any more.
My oldest girls now 13 years old had a small mammary tumour just before I had planned to have another litter from her (she had had two). She was 5 1/2 at the time.
I had it removed and tested and it was a well defined carcinoma about the size of a baked bean. I thought having her spayed would be helpful, so did. It was a couple years after this that a Veterinary article appeared in teh canine press showing that it wasn't helpful.
At about 9 years of age I noticed a small hard lump in the same place, and this time we didn't bother testing it but had her rear two golands removed on that side as they were attached to the same lymph system.
A year later I found a soft lump on her chest and in panic asked for it to be removed and testred, and it was just a fatty tissue lump that older dogs often get and the name of which I have forgotten. She has had one or two of these apear under her skin but have left them alone now as know they are benign and you wouldn't know they were there unless you carefuly masaged her trunk.
She is now getting a bit gaunt in the quarters and thin and greyer in the face, but still at 13 1/2 years old does 2 to 4 miles walking and tootling about off lead.