Annie I did some reading when you first posted about Lottie and if the cancer hasn't spread, the prognosis is usually excellent with most dogs living a normal life span. Not all mammary cancers spread and surgery can be the only treatment needed. The way one vet put it was mammary cancer in dogs is a 50/50/50 cancer. This means 50% of tumours will be benign, 50% will be malignant and 50% of malignant cancers will have spread. So in theory because you know Lottie's tumour is malignant, there's a 50/50 chance of it having spread.
On the Blue Cross website it says this about breast tumours -
Dogs have five breasts on each side of the tummy, visible as two rows of nipples, and tumours may occur in one or more. About half of these tumours are benign, while the rest are malignant cancers. The choices for surgery are removal of the lump alone, or removal of some or all of the rest of the breast tissue. Removing more tissue does not appear to prevent internal spread of cancers. These often spread to the lungs, so chest x-rays are advisable prior to surgery, although early spread may not be visible. Spaying a bitch at or after breast surgery could reduce the chance of recurrence.
It also says -
Ultrasound or MRI scanning may be suggested. These may help in discovering whether the cancer has spread to other areas of the body – a process that vets refer to as “staging”. They can also give an indication of your pet’s general health, which affects the ability to withstand treatment.
Full Blue Cross information
Another option I thought you could ask the vet about is the possibility starting the surgery, but not proceeding further if the cancer has spread. She'd still have to go through the anesthetic and have stitches, but she should recover from these quite quickly. Also, your vet can refer you to an Oncologist vet in Wales, so that might be worth thinking about.
I don't mean to pry, but the treatment is likely to be expensive, so do you have pet insurance? Sending you gentle {{{HUGS}}}
