They can occur in the rats’ groin or armpit, along her side or on her back; rats have mammary tissue in unexpected places. They do not usually cause any distress until they either seriously impede the rat’s movement or start to ulcerate and become sore, or outgrow their blood supply and become gangrenous.
If your rat develops a tumor then you can either have it surgically removed, or to have her put to sleep when she becomes unhappy. You do not need to put her to sleep as soon as a tumor appears – she may have many months of happy life ahead of her before it starts to hurt, and as the rat’s owner you will be the best person to decide when she is no longer enjoying life. If you decide to have the tumor removed and it is benign, the operation is relatively simple and need not be stressful for the rat if she is otherwise healthy. It is helpful to find a vet with experience in this area.
However, bear in mind that a rat who is prone to tumors may well develop others after a first tumor is removed. This does not mean that it is not worth having the operation done – the rat could well gain at least an extra 3 or 4 months of life, which is comparable to 6-8 years for a human – but you need to take into account her overall health and your vet’s opinion as to whether the tumor can be operated on. It is easier to remove tumors while they are still small.