Mammary Tumours in Dogs and Cats: The Lump Under the Skin, Explained

A small, firm nodule under the skin of your dog's or cat's belly is one of the most common reasons pet owners bring an older, unspayed female in for examination — and mammary tumours are, in fact, among the most frequently diagnosed neoplasms (cancers/tumours) in companion animals. This article explains what is happening at the cellular level, who is most at risk, how we tell a harmless lump from a dangerous one, and what the road to recovery looks like.

The mammary gland is, by design, a hormonally responsive tissue. With every oestrous cycle, the surges of oestrogen and progesterone that prepare the body for a possible pregnancy also stimulate cycles of proliferation in the mammary ductal and glandular epithelium. In an intact female dog or cat that is never bred, this pattern repeats itself unopposed, cycle after cycle, year after year — and repeated proliferation is, fundamentally, repeated opportunity for something to go wrong at the DNA level.

Figure 2. The same hormonal drivers described below also produce entirely normal, physiological change: this pregnant bitch shows a visibly enlarged, pigmented mammary chain and prominent teats — the expected result of oestrogen and progesterone preparing the gland for lactation. It is this same proliferative machinery that, over repeated cycles and years, creates the opportunity for the malignant changes described in this article.

At the centre of this process sits the oestrogen receptor alpha (ERα), a protein inside mammary epithelial cells that oestradiol binds to. Once activated, ERα moves into the cell's nucleus and switches on genes that drive cell division, cell survival, and blood vessel growth toward the tissue (Torres et al., 2021). In a normal cycle, this is a tightly regulated, temporary event. In canine mammary carcinogenesis, prolonged or dysregulated exposure to oestradiol—both from the ovaries and, importantly, from oestrogen synthesised locally within the mammary tissue itself—is believed to drive genetic damage and clonal expansion of cells carrying oncogenic mutations over time (Torres et al., 2021). Obesity compounds this risk: fat tissue expresses the enzyme aromatase, which converts androgens into oestrogen, so overweight dogs are exposed to higher local oestrogen concentrations within the mammary gland itself (Torres et al., 2021).

Genetics also plays a role. Mutations in the tumour-suppressor genes BRCA1 and BRCA2 — the same genes implicated in hereditary human breast cancer — have been identified in dogs with mammary tumours, and this shared molecular biology is part of why canine mammary cancer is increasingly used as a natural model for studying human breast cancer (Klopfleisch et al., 2011). As tumours progress, they tend to lose their dependence on, and expression of, ERα — benign and low-grade malignant tumours are typically ER-positive, while high-grade, aggressive carcinomas frequently become ER-negative, a shift that mirrors the pattern seen in more aggressive human breast cancers (Torres et al., 2021).

Beyond hormone signalling, malignant mammary cells also retool their basic energy metabolism. Rather than relying primarily on the efficient, oxygen-dependent process normal cells use, many tumour cells shift toward drawing energy from glucose fermentation even when oxygen is plentiful — a pattern first described in human tumours nearly a century ago and now also documented in canine mammary tumours, alongside altered fat and amino acid handling that helps fuel rapid cell division (Tamarindo et al., 2023). This metabolic shift is thought to be a genuine vulnerability rather than just a side effect, and it is an active area of research into new veterinary cancer treatments that could starve tumour cells of the specific fuel sources they have come to depend on.

The practical, clinical consequence of this biology is well established: in a landmark retrospective study, bitches spayed before their first oestrus carried only around 0.5% of the mammary cancer risk of an intact dog; that figure rose to roughly 8% in dogs spayed after one heat cycle and 26% after two or more, with little further benefit once a dog was spayed past about two and a half years of age (Schneider et al., 1969). These figures have been repeated in veterinary teaching for over fifty years and remain a useful illustration of the hormonal principle at work — though it is worth noting, in the interest of giving a balanced picture, that a later systematic review found much of the underlying evidence base for this precise dose-response relationship to be of variable quality and cautioned against treating the exact percentages as firm guarantees for an individual dog (Beauvais et al., 2012). The broader conclusion that early spaying is protective, and that protection declines the longer a dog remains intact, is nonetheless well supported (Sorenmo et al., 2000; Nosalova et al., 2024).

Mammary tumours can occur in any intact female dog, but certain breeds are consistently over-represented in clinical and epidemiological studies, pointing to an underlying genetic susceptibility even though a single causative mutation has not been identified (Nosalova et al., 2024). Reported predisposed breeds vary somewhat by country and study population, but several names recur across the literature:

Age itself is one of the strongest and most consistent predictors: risk rises steadily from around seven years of age and continues to climb through the early-to-mid teens, and older dogs are more likely to present with larger, higher-grade, malignant tumours than younger dogs (Burrai et al., 2020).

Figure 4. Multiple discrete mammary nodules of varying size in a cat — a reminder that, as in dogs, more than one mass is often present, and each should be assessed on its own merits rather than assuming they share a single diagnosis.

Figure 5. A more advanced feline mammary mass, with ulceration and haemorrhage — the kind of aggressive, locally invasive presentation that is disproportionately common in cats compared with dogs, and one of the reasons any feline mammary lump deserves prompt attention.

Mammary tumours usually present as one or more firm, discrete nodules that an owner or veterinarian feels while palpating the gland chain. In dogs, the caudal-most pair of glands (glands 4 and 5, nearest the groin) is involved in roughly 65–70% of cases, and more than half of affected dogs have more than one tumour at diagnosis — so we palpate and record findings for every gland, not just the obvious lump (Sorenmo et al., 2011; Rodríguez-Bejarano et al., 2026). Cats most often present with a mass in the caudal glands as well and may show ulceration, discharge, or a rapidly enlarging, inflamed mass in more advanced disease.

Figure 7. Two firm, well-circumscribed mammary nodules on a dog, skin-prepped ahead of biopsy or surgery. A discrete, slow-growing mass like this is more typical of a benign or low-grade tumour — but appearance alone cannot confirm that, which is exactly why histopathology remains essential (see below).

Most dogs with mammary tumours are otherwise entirely well at the time of diagnosis. Signs such as weight loss, lethargy, coughing, or lameness are uncommon and, when present, usually reflect more advanced, metastatic disease rather than the primary mammary mass itself. Around half of malignant mammary tumours will have already spread to the regional lymph nodes by the time they are diagnosed, which is why we palpate those nodes carefully at every examination — and, notably, the pattern of metastatic spread in dogs closely mirrors that of human breast cancer, reinforcing why this disease is so widely studied as a natural comparison for the human condition (Rodríguez-Bejarano et al., 2026).

Figure 8. A mammary mass from a patient examined at The Andys Veterinary Clinic. The mass shows central ulceration, a raw and haemorrhagic surface, and surrounding tissue swelling — the kind of ill-defined, inflamed presentation described above that warrants a prompt veterinary evaluation approach.

Fine-needle aspiration (FNA) and cytology can help rule other conditions in or out and can sometimes suggest malignancy, but it cannot reliably distinguish benign from malignant mammary tumours or determine histological grade on its own. Histopathological examination of the surgically removed mass remains the gold-standard diagnostic and prognostic tool, since it is the only method that establishes both the tumour type and its histological grade — the single most reliable indicator of biological behaviour (Goldschmidt et al., 2011).

"Mammary tumour" is a broad umbrella term rather than a single diagnosis. Histopathology sorts each mass into one of many recognised subtypes, and the subtype has a real bearing on how the tumour is expected to behave and respond to treatment (Goldschmidt et al., 2011; Withrow & Vail, 2007):

Because a single dog can carry a mix of benign and malignant masses at the same time — and because a benign-looking mass is not guaranteed to stay that way — every removed mass is submitted for histopathology, not just the one that looks most suspicious clinically (Goldschmidt et al., 2011).

For carcinomas specifically, the pathologist also assigns a histological grade, using a three-parameter scoring system adapted from the Elston–Ellis (Nottingham) method originally developed for human breast cancer and validated for prognostic use in dogs (Peña et al., 2013). Each of three features is scored from 1 to 3, and the three scores are added together:

The combined score (3–9 points) sets the grade:

Grade I (3–5 points, well differentiated) carries the best prognosis;

Grade II (6–7 points, moderately differentiated) is intermediate; and

Grade III (8–9 points, poorly differentiated) is associated with the shortest survival times and highest metastatic risk (Peña et al., 2013).

This grade, reported alongside the subtype, is one of the figures owners most often ask us to explain after surgery — it is, in effect, the pathologist's best estimate of how aggressively that specific tumour is likely to behave.

Because malignant mammary tumours can spread to regional lymph nodes and to the lungs, we recommend clinical staging before major surgery in any patient with a mass suspicious for malignancy. A typical work-up includes:

These findings are combined into a clinical stage using a TNM (tumour–node–metastasis) system, which, alongside the eventual histopathology report, guides decisions about the extent of surgery and whether additional (adjuvant) treatment is warranted (Cassali et al., 2020).

In dogs, a large randomised trial found no survival benefit to radical mastectomy (removing the entire gland chain) over a more conservative approach removing only the affected gland(s) with adequate margins, provided the mass is completely excised (Sorenmo et al., 2011). This allows us, in many canine cases, to tailor surgery to the individual tumour rather than defaulting to the most extensive operation.

Cats are managed more aggressively because of their higher malignancy rate: a unilateral or bilateral radical mastectomy (removing the entire affected chain, sometimes staged as two separate surgeries for bilateral disease) is generally recommended, since more conservative excisions in cats are associated with a substantially shorter disease-free interval than radical surgery — roughly 300–325 days versus 575–1,300 days in one comparison (Torres et al., 2021).

The equivalent debate exists in dogs, though the evidence is less clear-cut than in cats. One study found that after a more limited (regional) mastectomy for a single tumour, a new mass developed in the same mammary chain in 58% of dogs, and most of those needed a second surgery (Rodríguez-Bejarano et al., 2026). This is part of why some surgeons favour a more extensive first operation in dogs with multiple risk factors for new tumours, even though, as noted above, a wider first surgery has not been shown to extend overall survival once a tumour is completely removed.

Figure 10. A mammary mass draped and secured in sterile towels, ready for excision. Isolating the mass this way protects the surgical field and keeps the margins clean — the practical, unglamorous groundwork behind the surgical options described above.

If your pet is still intact, we will usually recommend ovariohysterectomy (spaying) at the time of, or shortly after, mass removal. This does not reduce the risk from the tumour already present, but it removes the ongoing hormonal stimulus to the remaining mammary tissue and has been shown to improve survival time in dogs with oestrogen-receptor-positive mammary carcinoma (Torres et al., 2021; Kristiansen et al., 2016).

Chemotherapy is not routinely required for benign tumours or low-grade, node-negative carcinomas removed with clean margins, but it is considered for high-grade tumours, tumours with lymphovascular invasion, or cases with lymph node involvement at diagnosis, based on the histopathology report (Cassali et al., 2020). In cats, no chemotherapy protocol has yet been shown to reliably extend survival on its own, which is part of why early, complete surgical excision is emphasised so strongly in this species (Torres et al., 2021).

For tumours that cannot be fully removed — inflammatory mammary carcinoma, recurrent disease, or macroscopic metastatic spread — small-molecule tyrosine kinase inhibitors are an emerging option. Toceranib phosphate, a drug licensed for canine mast cell tumours, blocks growth-factor receptors (including VEGFR and PDGFR) involved in tumour blood-vessel formation and proliferation. In dogs with inflammatory mammary carcinoma, a 2024 study combining toceranib with the anti-inflammatory carprofen achieved disease stabilisation in 60% of cases and meaningfully improved comfort, appetite, and mobility, even though it did not shrink the tumours outright (Garcia-de la Virgen et al., 2024). A separate 2024 study in cats with inoperable or recurrent mammary adenocarcinoma found toceranib produced a measurable response in over a third of cats and clinical benefit in nearly two-thirds, with a manageable side-effect profile (Del Portillo Miguel et al., 2024). These are not cures, and larger trials are still needed, but they offer a genuine quality-of-life option for patients who have run out of surgical choices.

Separately, many malignant mammary tumours in both dogs and cats overexpress an inflammatory enzyme called cyclooxygenase-2 (COX-2)—one study found it in essentially all of the invasive canine carcinomas it examined—which is part of the rationale for including a non-steroidal anti-inflammatory drug (NSAID) in the supportive treatment plan for suitable patients, both for pain control and for its mild anti-angiogenic effect on the tumour itself (Millanta et al., 2006). As with any long-term NSAID use, this requires screening for suitability and monitoring, and we tailor the choice and dose to each patient.

Prognosis for mammary tumours is highly variable and depends far more on the individual tumour's characteristics than on the species alone. The most consistently reported prognostic factors are:

For dogs with malignant tumours managed with appropriate surgery, many will do well long-term, particularly when the tumour is small, low-grade, and node-negative. The clinical stage at diagnosis (see Staging, above) is itself strongly predictive of outcome in dogs, as shown in the table below:

These ranges are compiled from published surgical case series in dogs with malignant mammary tumours (Chang et al., 2005; Liptak, 2017); the Stage V figures are based on smaller numbers of cases than the earlier stages and should be read with that caveat in mind. Cats with completely excised, small, low-grade tumours can also have a good outcome, but the overall one- and two-year survival figures reported for feline mammary carcinoma—roughly 47–50% and 32%, respectively, when surgery is used alone—underscore just how much earlier detection changes the picture in this species (Torres et al., 2021).

Figure 12. A mammary mass left unaddressed for an extended period, now large and pendulous. Tumour size at diagnosis is one of the strongest predictors of outcome in both dogs and cats — a mass this size carries a far more guarded prognosis than the same tumour would have at a fraction of the size, which is the central argument for early evaluation.

If you have felt a new lump along your dog's or cat's abdomen, or your pet is due for a wellness examination, please contact us at The Andys Veterinary Clinic in Loresho, Nairobi, or in Nanyuki. Early evaluation is the single most useful step you can take.