Canine Cancer: Mammary Tumors– What Pet Parents Need to Know About Them

What is a Mammary Tumor in Dogs?

A mammary tumor is when an abnormal replication of cells happens in the breast tissue. There are different kinds of mammary tumors, but they are commonly associated with breast cancer in female dogs.1 In fact, mammary tumors are the most commonly diagnosed of all tumors found in female dogs. The highest incidence of mammary tumors occurs in dogs within the age group of 8 to 11 years.6,7

There are three common types of mammary tumors: adenomas, adenocarcinomas, and carcinomas. Adenomas are less severe because they are non-cancerous, then there are also adenocarcinomas which are malignant, and the most commonly seen form is carcinomas, which are also malignant.2 

About 50% of all mammary tumors found will be cancerous, and of those 50% will have already spread to other parts of the body. The least common kind of malignant tumor found is an inflammatory mammary carcinoma, which is highly malignant and often the most painful of the carcinomas.4 Inflammatory carcinomas make up less than 5% of all mammary tumors.

Mammary tumors are often found by the owner by chance, as many of them do not cause major symptoms and are located through regular petting or grooming, or sometimes by a vet at a regular check up. If you have a female dog who is either intact—having never been fixed—or was spayed after their first heat cycle, it never hurts to regularly check their tummies for lumps and bumps. Like any cancer, finding, diagnosing, and treating early is always going to lead to the best outcomes. 

What Causes Mammary Tumors?

Mammary tumors develop due to hormones in the body and the types of hormones can determine whether the tumor that forms will be benign or malignant. Male dogs are almost never affected by mammary tumors. 

The risk factor for many female dogs is based on when they were spayed within their lifespan. For dogs that were not spayed at all, or spayed after age 2, they are significantly more likely to develop a mammary tumor. 

The risk for a dog spayed before their first heat, around 6 months of age, is very low, typically around 0.5%. Dogs spayed at this young age haven’t reached a level of sexual maturity that allows their body to start producing the necessary hormones for the development of a mammary tumor. The risk continues to climb for dogs who are spayed after that age, with dogs spayed after their first heat being around an 8% chance, after their second heat being 26%.3 

There are also some breeds that have been noted to be predisposed to developing mammary tumors, which include Mini Poodles, Spaniels, and German Shepherds.3

Signs and Symptoms of Mammary Tumors in Dogs

Mammary tumors don’t often have symptoms aside from the tumor itself, and the best time to find a mammary tumor is when there are no other symptoms, because that is often a sign that the tumor is either benign, or has not yet spread the cancer to other parts of the body.

These tumors can appear in many different ways, they can vary in size, shape, feel, color, and can be either movable or attached to the breast tissue. The location of these tumors is also not  always consistent. They can develop in any of the glands, as well as in the ductal or alveolar areas, though the most common are the fourth and fifth glands which are closest to the groin.1,6 

Some dogs may develop just one tumor, or they may develop several in different glands or even in the same one.2 A dog developing multiple tumors, or even one that has a history of mammary tumors, doesn’t mean that all will be the same type. There can be benign and malignant tumors appearing on the body at the same time, and if a dog has a history of one that doesn’t mean that they will always develop that same type. 

More advanced tumors may cause the skin to open and bleed, and some tumors may develop discharge. Both of those changes can cause the dog to lick the affected glands. The gland may become warm to the touch and it may start to cause the dog pain. 

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If the tumor has spread to other parts of the body, other signs may appear like general unwellness, loss of appetite, lack of energy, and weight loss. If the lungs have been affected by the spread, there may be signs of trouble breathing or a cough.1 

Inflammatory mammary carcinoma is a specific type of mammary tumor that can cause severe swelling, redness, and pain in the affected gland that can extend all the way through the entire chain of glands on the affected side or affect both sides. 

How are Mammary Tumors Diagnosed 

From the beginning stages of a mammary tumor diagnosis it is important to keep in mind that benign and malignant tumors of this kind are practically indistinguishable before the right tests are done. Though thankfully, benign tumors typically won’t progress into malignant tumors.6 Because of their similarities, your vet will treat every tumor as if it is malignant to make sure all bases are covered. This doesn’t always mean that you need to worry, especially if your dog has shown no signs of spread; it is a precaution. 

The most commonly used initial test for diagnosing a mammary tumor is a Fine Needle Aspirate (FNA), wherein your vet will take a small, standard needle and insert it into the affected area to pull a sample of the affected cells out. The cells are then examined under a microscope to identify if the tumor is benign or malignant. In cases where the FNA is not entirely clear, a standard tissue biopsy should be performed on the removed tumor.2 

For a dog that has been diagnosed with a malignant tumor, your vet may also recommend additional tests to stage the tumor and check for potential spread of the cancer. These tests may include bloodwork, urinalysis, x-rays of the lungs, abdominal ultrasounds, and an FNA of the local lymph nodes—even if they appear normal.3,5 

These tests allow the vet to know how to safely proceed in treatment for your pet, and know what kind of follow up treatment they will need after the tumor is directly addressed. 

How are Mammary Tumors Treated?

Surgery is the best treatment for a majority of mammary tumors. If there is only one small tumor, under 3cm, and no evidence of spread, surgery can be all that is needed to treat it. Dogs who have multiple tumors may need to have all of the associated glands removed, or in some cases all glands removed.  

Some vets may recommend that when getting surgery on the tumors, an unspayed dog should also have the spaying procedure. Because being spayed can have a great impact on the likelihood of a tumor returning in the same area, it can be seen as beneficial to add the procedure.1 However, at this time there is no conclusive evidence that the procedure helps or harms when offered at the same time as a gland removal surgery.

Some dogs who have already shown signs of spreading or have a high risk for spread may have chemotherapy recommended after surgery to maintain their health and defend any development of cancer in other parts of the body. 

Radiation therapy is not a commonly used treatment, though it can be helpful as a preoperative irradiation on large tumors or a postoperative irradiation of completely removed tumors. In rare cases, radiation therapy can also be used as palliative care for inoperable tumors.2

For dogs with inflammatory mammary carcinomas, surgery is not recommended because it does not improve survival rate. There is, unfortunately, no effective treatment for inflammatory carcinomas yet. Radiation therapy in combination with a non-steroidal anti-inflammatory drug (NSAIDs) has been shown to offer the most effective pain relief, but unfortunately prognosis for this type of cancer is still not good.3 

What to Expect After Treatment

Dogs who developed benign tumors have an excellent prognosis after surgery. Because the tumor was not cancerous, there is no threat of spread to the rest of the body. Once that pooch has rested and healed from the surgery, they should be good to go on happily with the rest of their lives. 

For malignant tumors, the prognosis comes down to whether that tumor has spread or not. In about 50% of cases where a malignant tumor has been diagnosed, it has not yet spread to other parts of the body, and treatment can lead to great prognosis after surgery. Unfortunately, the other 50% of cases are diagnosed after the cancer has accelerated, where there will be a high risk of spread by the time of treatment. In these instances, your pet will need supplemental treatment after surgery to determine their prognosis.2

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Tumor size, shape, type, grade, and pathological changes will directly impact the dog’s prognosis. If the tumor is smaller in size at the time of surgery, it is less likely that it will come back or spread.5 Dogs who have multiple tumors that have well differentiated carcinomas are often more likely to be benign, whereas dogs with less differentiated carcinomas are often malignant with shorter survival times.3 Female dogs in the initial clinical stage have a longer overall survival time. However, the overall survival is lower (median 504 days) for female dogs with neoplasms with histological grade III.  The female dogs with regional lymph node and distant metastases have median survival times of 331 days, and 236 days, respectively.7

Many dogs will live several years after complete removal of certain malignant tumors. 

Unfortunately, inflammatory carcinomas are highly aggressive, and with no currently effective treatment available, the prognosis is very poor, ranging anywhere from weeks to months.4 Early detection is the only protection for patients with this kind of mammary tumor. 

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— Update: 05-01-2023 — found an additional article Clinical staging in bitches with mammary tumors: Influence of type and histological grade from the website for the keyword end stages of breast cancer in dogs.


Previous studies have reported tumor metastases in less than 50% of dogs, which corresponds with the findings of this study. A retrospective study of 99 dogs in the United States reported 25% of the animals at stage I, 12% at stage II, and 15% at stage III of the disease (13). However, a study in Brazil that examined 36 dogs described 22.2% of the animals as stage I and 75% as stage II of the disease; none of the animals were classified as stage III (14).

Given that the differentiation between stages I, II, and III is based only on the tumor size, another study conducted in Brazil reported that 35% of malignant tumors were smaller than 5.0 cm (11), while other studies have reported that neoplasms measured less than 3.0 cm [48.81% (15) and 44.7% (16), respectively].

A study involving human female subjects reported that 55.3% of patients presented tumors smaller than 3 cm and that the tumor diameter negatively affected the chance of survival despite resection (7). The smaller tumor size in these female subjects might have been due to early diagnosis. A similar correlation was observed herein which might be due, in part, to the owners seeking medical attention for swelling in the breasts sooner than usual.

With regard to the bitches diagnosed with the lymph node invasion in the present study (29.7%), previous studies conducted in Portugal and Colombia showed similar frequencies of occurrence [298% (16) and 30% (17), respectively]. Another study, however, had classified only 15.46% of the animals as stage IV (13).

The frequency of distant metastases in the present study was 7.9%. Previous studies have also reported similarly low frequencies of bitches at this stage of the disease; Ribas et al (18) and Gomes et al (17) reported that 8.3% and 16.7% of dogs were diagnosed with pulmonary metastasis, respectively. However, a negative radiological finding does not prove the absence of metastases, since the detection of tumors is challenging owing to their smaller sizes (19). Moreover, highly accurate diagnostic methods such as magnetic resonance imaging (MRI) and positron emission tomography (PET) scans are rarely practiced in veterinary medicine. Another reason for the fewer number of dogs at stage V of the disease is the fact that these animals are often already at advanced stages of the disease without having received any surgical treatment.

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With regard to the frequency of tumors according to histological type, previous studies have reported that tubular carcinoma is the most common (20,21), which corresponds to the findings in this study. However, the incidences of other types of tumors reported by different studies vary. The different histological classification systems used for the canine mammary tumors makes it difficult to compare the results of the various studies. The solid carcinoma has been reported as the second most frequent type of carcinoma in a previous study (20); however, another study reported the complex carcinoma as being the second most frequent (21). Carcinoma in mixed tumors also appears as one of the more frequent types of carcinomas (22), and in the present study, it was found to be the third most frequent.

The simple carcinomas were found to have the highest frequency of occurrence in this study when the tumors were grouped on the basis of their histological origin. The simple carcinomas have also been reported as the most frequent type of tumors in studies conducted in Japan (23), Brazil (24), and Iran (21).

Several authors have reported that simple carcinomas are more aggressive and have a worse prognosis than complex carcinomas (25,26). Therefore, the clinical staging of simple carcinomas in the present study was unexpected and can be attributed to the fewer number of the animals at the stages II, III, and V.

However, the staging of the carcinomas in mixed tumors in this study revealed the lower aggressiveness of these tumors, as has previously been described by Cassali et al (26), who also reported that these tumors have a better prognosis compared to other carcinomas. However, in the present study, the statistical corroboration of this fact was hindered because none of the animals with carcinomas in mixed tumors were at stage V.

With regard to the complex carcinomas, we did not expect a large number of the animals to present lymph node invasions (stage IV) since Rasotto et al (27) had previously shown that 66.6% of the simple carcinomas in their study had metastasized to the lymph nodes and in 81.01% of the cases, invasion of the lymphatic vessels was observed; whereas, among the complex carcinomas, only 11.11% had metastasized to the lymph nodes and 20% showed lymphatic invasion.

The time-evolution of the tumor might affect the occurrence of the metastases. The larger tumors usually have a longer development duration, which increases the likelihood of metastasis (28). Unfortunately, the determination of the duration of tumor progression depends on the owner’s observation of the condition and is therefore often unclear.

In the histological grading findings, complex carcinomas showed less differentiation compared to simple carcinomas, as has been reported previously (27,10). The degree of differentiation of simple carcinomas is a matter of debate; it has been reported that this histological type of carcinoma has no association with the degree of differentiation (27) and that their classification as grade III (10) is frequent. The discrepancies between the results of the various studies might be because of the subjectivity of the gradation method, which can vary according to the reviewer. Another hypothesis is that a greater number of tubular tumors were classified as carcinomas in this study since tubule formation is one of the parameters considered for gradation, and tubular carcinomas tend to attain lower scores in the evaluation.

Histological grade is a factor that can influence the clinical staging of a tumor. A study in Slovenia reported that 64.71% of animals had stage I and grade I tumors, a result that is comparable to what we presented in this study (29). Similarly, a study conducted in Madrid described a significant association between the histological grade and the clinical staging of tumors (30). Other previous studies have also correlated the type and histological grade of the tumors with the overall survival rate (16).

A limitation of the present study was the fewer number of animals at stage V of the disease. Although mixed tumor carcinomas typically show less aggression, fewer metastases, and lower cell differentiation, whereas simple carcinomas are more aggressive, this association could not be proven statistically because of the fewer number of animals at stage V. It can, however, be concluded that the histological grading proved to be the best parameter for the prognostic evaluation of breast carcinomas.


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About the Author: Tung Chi