Ovarian Tumors

By Malcolm Weir, DVM, MSc, MPH; Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM

What is an ovarian tumor?

An ovarian tumor is a type of tumor that develops from the uncontrolled disordered growth of cells found within the ovary. The cells of the ovary contain germ (primordial) cells and epithelial (skin-like) cells amongst many others, and tumors may develop from any one of these cell types. Many ovarian tumors are malignant (cancerous). Some of the more common malignant ovarian tumors include teratomas (or teratocarcinomas), carcinomas, and adenocarcinomas.

Because many dogs and cats in North America undergo ovariohysterectomy (spay surgery), the incidence of this type of cancer is quite low. In pets that are intact (not spayed), the occurrence of malignant ovarian tumors is 6.25% in dogs and even less (0.7-3.6%) in cats.

What causes ovarian tumors?


The reasons why a particular pet may develop these or any other kinds of tumors is not straightforward. Very few cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. In the case of ovarian cancer, there is no known cause. With the exception of teratomas, most cases of ovarian cancer develop in older, and of course, intact dogs. Some breeds may be predisposed to the development of these tumors, including German Shepherds, Boxer Dogs, Yorkshire Terriers, Poodles, Pointers, Boston Terriers, and possibly Shetland sheepdogs, Australian cattle dogs, and the spitz. There is no reported breed predilection in cats.

What are the signs that my pet has ovarian cancer?

The signs of ovarian cancer vary depending on the type of tumor and what areas of the body it may affect. Ovarian tumors may metastasize (spread elsewhere in the body), and some are capable of producing hormones.

Many ovarian tumors are asymptomatic, meaning there are no clinical signs whatsoever. Signs may only become evident when they grow very large in size. In such cases the clinical signs can include fluid build-up in the abdomen, causing a rounded belly. If the tumor spreads to the lungs, fluid can build-up in the chest and cause difficulty with breathing. As some ovarian tumors produce estrogen and progesterone, there may be signs related to the excess of these hormones. Such signs include vulvar enlargement, vaginal discharge, persistent estrus (menstruation and heat), pyometra (pus in the uterus), masculinization, lethargy, lack of appetite, weight loss, and thinning of the haircoat.

How is this cancer diagnosed?

These tumors are generally difficult to diagnose initially and may be found with a physical exam or routine bloodwork. On physical examination, your veterinarian may palpate (feel) a noticeable mass in the abdomen, notice some outward signs, such as a swollen vulva or vaginal discharge, and may note abnormalities in bloodwork. If your pet has a pyometra (infection in the uterus) or other diseases that can develop as a result of higher and abnormal hormone production, your veterinarian may consider the possibility of an ovarian tumor.

In some cases, elevated calcium may be observed on bloodwork which may indicate further diagnostics tests (i.e., X-rays or abdominal ultrasound), at which point the tumor may be found.

"By far, ovarian cancer is most commonly diagnosed by abdominal ultrasound or during a spay procedure."

Some tumors will produce hormones that not only cause your pet’s vulva to look larger than normal but will also be apparent on bloodwork. For example, anti-Mullerian hormone (AMH) will be elevated in dogs and cats with a certain form of ovarian cancer called a granulosa cell tumor (GCT).

By far, ovarian cancer is most commonly diagnosed by abdominal ultrasound or during a spay procedure (ovariohysterectomy). If a spay procedure is performed, any abnormal tissues will be submitted to a pathologist for review, called histopathology. This allows the pathologist to look at the tissues under a microscope and determine the type of tumor and if it is cancerous or not.

How does this cancer typically progress?

In the majority of cases, ovarian tumors act more locally aggressive (meaning they penetrate local tissues) with a low rate of metastasis (spread to other parts of the body). Only in about 20-30% of cases does metastasis occur. Your veterinarian may recommend full staging (searching for potential spread to other locations in the body) prior to surgery. This may include bloodwork, urinalysis, X-rays of the lungs, and possibly an abdominal ultrasound. If any abnormalities are discovered, these areas may be sampled prior to, or at the time of, surgery to determine if the cancer has spread elsewhere.

What are the treatments for this type of tumor?

Ovariohysterectomy (spay) is by far the most common and pursued treatment option. In the case of benign or locally growing tumors, it is the treatment of choice as the tumors are completely removed.

If staging and other diagnostic tests have indicated metastasis (spread to other areas in the body), then a spay procedure may still be performed with removal of other affected tissues or lymph nodes at the same time. Follow-up with chemotherapy may be pursued, though there is limited knowledge regarding its success.

Is there anything else I should know?

These types of tumors are quite rare given that the majority of pet owners in North America spay their pets. Breeding animals, given repeated estrus cycles and pregnancies, with exposure to fluctuating sex hormones over an extended period of time, may be more prone to developing these tumors. Many of these cancers are treatable but spaying typically must be pursued.

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