OK, second part to a previous question?

It is well doented that mammary cancer can be prevented more than 98% of the time by spaying before the first heat-- this is happening because your dog was middle aged when she was spayed. I don't know of any scientific proof that it is because of the dog food you fed her. Not all mammary tumors are malignant, but most are. If you decide to do surgery, be sure you get chest rads done first, because if the tumor has spread to her lungs, there is no point in putting her through surgery. Also, check the other mammary glands- rub them between your fingers and feel for small hard lumps. In the early stages, they feel like bb's, then marbles, and these tumors often spread to the nearby teats. The one you describe has probably been there a long time, and I would be surprised if it is the only one she has. If it is, it probably isn't malignant. If she has more than one, she may need to have the whole mammary chain taken out, which is a very big surgery. Even if it hasn't spread, the surgery will be traumatic at her age, and she probably won't live much longer with the tumor removed than she will if you leave it alone. As long as she is eating, sleeping, pooping, etc like she normally does and seems to enjoy her life, leave her alone. Nothing you can do can make her live forever, and it might not be the fairest thing to put her through a painful and difficult surgery at her age. It's always hard to lose an old friend, but we can let them have dignity at the end of their lives. Spoil her and enjoy the time you have left-- don't make her spend it in pain.