Rat Mammary Tumors

Yarmouth Veterinary Center

75 Willow Street
Yarmouth , ME 04096





Most masses on rats are benign mammary tumors called fibroadenomas. 

Rat mammary glands extend along the underline from the neck to the pelvis, so mammary tumors can occur anywhere along the length of the body. They are common in male and female rats and can appear as early as a few months of age. 

Less than 10% of rat mammary tumors are malignancies, the most common of which is adenocarcinoma. The only way to distinguish fibroadenomas from adenocarcinomas is with a biopsy.


Rat mammary tumors are typically soft, freely moving growths on the underside of the patient. They may grow rapidly, sometimes an inch or more every few days. They are usually not painful when touched and do not bother the rat until they become so large that they make walking difficult. Occasionally they will obstruct adjacent body parts or ulcerate through the skin, creating a more complicated problem.


The diagnosis is based simply on finding the tumor. While it is very likely that a growth on a rat is a fibroadenoma, the only way to confirm this for any particular patient is with a biopsy. 

Rat mammary tumors are almost always located outside of the abdomen or chest, but may become so large that they interfere with internal organs. In some cases, particularly when surgical removal is planned, imaging with x-rays and ultrasound is done to determine the extent of the tumor. 


The treatment of choice is surgical removal of the tumor. This is not a minor surgery; even small tumors require a relatively extensive surgical procedure. Rats generally tolerate surgery very well, but our experience has been that the rate of complications, including death, is higher than the complication rate for similar surgeries in dogs and cats.

We recommend but do not require that the removed tumor(s) be sent to the pathology lab as a biopsy.


After the appearance of one tumor, It is very, very common for rats to develop additional mammary tumors, at the same or different locations. We have treated some rats in this situation with multiple surgeries. 

There is some evidence that spaying or neutering reduces the incidence of mammary tumors in rats when done as a preventive measure early in life, or after one or more tumors have been removed. It is this author's feeling that spaying or neutering is minimally helpful in preventing these tumors. We are always glad to discuss this option and provide the service if the rat's owner is interested. (Spaying or neutering can provide other benefits unrelated to the tumor)