Mammary tumours are probably the most common tumours that bitches (female dogs) get. They feel like hard lumps and can occur anywhere in the mammary glands. They are more common than breast cancer in humans but that is largely because dogs have ten mammary glands. Luckily about two thirds of tumours are benign, at least at the early stages although some may become malignant later.
The most effective way of treating mammary tumours is to remove them and this may need quite extensive surgery. Sometimes lymph nodes have also to be removed as well. There are generally no long term consequences of surgery. Because of the increasing malignancy with size it is important to have mammary tumours removed as soon as they are detected.
Very occasionally mammary tumours can be highly malignant and spread not only locally but to other organs as well. The spread is most often to the lungs and there are usually thousands of new tumours. This is one instance where the tumour itself is usually very painful and so sadly euthanasia is the only sensible course of action.
The good news is that it is very easy to prevent mammary tumours. Because there is a strong hormone influence on them, spaying before a first season is virtually guaranteed to prevent any tumours occurring.