Mammary tumours in dogs

Overview

Benign and malignant tumours

These tumours originate in mammary gland tissue. They are among the common tumours of female dogs. Around 50% are malignant. Benign tumours can grow slowly locally. Malignant tumours can invade surrounding tissue.

One dog can have benign and malignant lumps simultaneously. A lump can affect one gland or several sites along the chains. Tumours occur more often in the rear glands.

Among insured Swedish bitches, 13% had a diagnosed mammary tumour by age 10. Swiss cancer records found 14.50% of pathology laboratory-diagnosed tumours originated in mammary glands.

A study from northern Europe examined 71 affected dogs: 28 had only benign mammary tumours. Twenty-seven had only malignant tumours. Sixteen had both. Twenty-eight had more than one mammary tumour.

Causes

Hormonal effects on mammary tissue

Genetic and hormonal factors contribute to development. Ovarian hormones normally direct mammary growth. After oestrus, glands are exposed to high progesterone. Oestrogen can promote cell division. Tumours may have receptors through which sex hormones affect their cells.

One tumour can contain several cell types. Carcinoma is a malignant tumour originating from mammary epithelium, the gland's lining tissue. A malignant component can develop within a benign tumour over time.

Continued cell division creates more opportunities for genetic alterations. Hormonal exposure repeats across oestrous cycles. Highly altered tumour cells can have few or no hormone receptors.

Risk factors

Tumours particularly affect unspayed bitches. They are rare in males. Most affected females are over eight. Higher frequencies have been reported in Cocker and Springer Spaniels and Boxers, for example. Repeated phantom pregnancies have been linked to increased risk. Excess weight at one year has also been linked to increased risk.

Schnauzer reports particularly describe older pepper-and-salt females. A Swedish Schnauzer health survey reported tumours in 15% of pepper-and-salt dogs. The proportion in black dogs was 3%.

Mean diagnosis age in insured Swedish bitches was 8.0 years. Females accounted for 97.74% of recorded Swiss mammary tumours.

Typical symptoms

A tumour may feel like a well-defined subcutaneous nodule. It may be painless.

• A firm lump beside a nipple.
• Several separate nodules along the mammary chain.
• Ulceration of skin over a lump.
• Nipple discharge.
• Mammary swelling and tenderness.
• Weight loss with advancing disease.

Ulcerated tumours can hurt. Painful ulceration can reduce appetite. Advanced disease can impair general condition.

Some lumps adhere to surrounding tissue. Inflammatory carcinoma can cause redness extending around the tumour. This cancer's signs can worsen rapidly. A broader swollen area may then be palpable beneath skin.

Complications and consequences

Metastases and recurrence

Malignant tumours can spread to nearby lymph nodes. Beyond regional nodes, metastases most often occur in lungs. Liver and bone can also be affected. Cells remaining after surgery can cause local recurrence. Separate new tumours can develop in remaining mammary tissue.

Tumours over 3 cm are associated with poorer prognosis. High histological grade is also associated with poorer prognosis. Surgery alone can suffice for small, localised tumours. A postoperative cohort with invasive mammary carcinoma had median survival of 11.4 months. In this group, 41.5% died of mammary carcinoma within a year of surgery.

Tumour cells in lymphatic or blood vessels increase metastatic risk. Follow-up after carcinoma surgery found progression in 27% within the first year. At two years, this was 36%. Progression included recurrence at or near the surgical site and distant metastases.

What survival figures describe

A histology-based study followed 433 bitches after carcinoma removal. With cells in vessels or nodes, prognosis also depended on histological tumour size. Median time to mammary carcinoma-related death was 348 days for tumours no larger than 20 mm with spread. For tumours over 20 mm with cells in vessels or nodes, it was 163 days.

For local invasive carcinomas no larger than 20 mm without demonstrated vascular or nodal spread, median time to carcinoma-related death was four years and 8.5 months. The same classification's local tumours over 20 mm had a median of three years and 2.8 months.

Diagnosis at the vet

Palpation, needle sampling and biopsy

The vet feels the full length of both mammary chains. Every lump's size and location are recorded. Attachment to deeper tissue affects surgical planning. Palpation alone cannot identify tumour nature. Fine-needle sampling can help distinguish mammary from other subcutaneous tumours. Microscopic tissue examination, or histology, determines malignancy. Separate removed lumps are examined individually because types can differ.

The pathologist assesses differentiation: how closely cells resemble normal mammary cells. Division frequency influences grade. Cells at a specimen's edge show the tumour reached the surgical margin.

Assessing spread

TNM classification describes extent. T depends on diameter. Tumours under 3 cm are T1. Those 3–5 cm are T2. Those over 5 cm are T3. Stage IV indicates regional lymph node spread without distant metastases. Distant metastases indicate stage V.

Axillary or groin nodes can contain tumour cells despite normal size. Enlargement can also reflect inflammation. Needle sampling can seek tumour cells. Three-view chest X-rays often assess lung metastases. Abdominal ultrasound can investigate spread when needed.

Blood tests assess general health before surgery. CT can provide more detailed staging information.

Clinical stages I–III describe different-sized tumours without identified nodal or distant spread. Examination of surrounding tissue invasion helps characterise malignant growth.

Treatment

Surgical removal

Surgery is first-line treatment for most canine mammary tumours. Extent depends partly on lump numbers. A single tumour can be removed locally with sufficient healthy tissue. Location may warrant removing the whole affected gland. Several tumours along one side may warrant removing that entire chain.

Margin width for malignancy depends on type and location. Depending on size and growth pattern, 1–3 cm of healthy surrounding tissue may be removed. Deep invasion may require removal of attached abdominal fascia. Cells at margins prompt assessment of further surgery.

Pain after extensive chain removal can need hospital monitoring.

Additional treatment and concurrent spaying

Nodal spread may prompt chemotherapy alongside surgery. Survival benefit is not established for every mammary cancer form. Surgery generally does not prolong survival with inflammatory carcinoma. Pain-relieving treatment may then be prescribed.

Ovary removal can be considered during carcinoma surgery in an unspayed bitch. A randomised 60-dog study found no statistically significant overall survival extension from concurrent spaying. It also found no significant difference in time to a new tumour or progression.

Removing ovaries and uterus also prevents pyometra.

Prevention

Early spaying may reduce mammary tumour risk. A 1969 estimate placed carcinoma risk after spaying before first oestrus at 0.5% of the risk in entire bitches. Spaying between first and second oestrus gave an estimate of 8% of that risk. Between second and third, it was 26%.

A 2025 review found six of 13 studies did not demonstrate reduced risk with early spaying. Timing decisions also consider breed susceptibility to other diseases becoming more common after surgery. Certain hormonal oestrus suppressants can increase benign tumour risk.

Important for dog owners

New lumps and recovery

Feel the mammary chains regularly for new nodules. Record location and size so growth rate can be assessed. A collar usually prevents wound licking for 10–14 days after surgery. Activity is usually restricted for around two weeks while the wound heals. Watch for redness and swelling. Report discharge immediately.

Prescribed pain medication helps recovery. After malignant tumour removal, the site and nearby nodes may be checked every three months during the first year. Later checks may be six-monthly.

Wounds near a hind leg may be more prone to opening.

Frequently asked questions

Is a small mammary lump always benign?

Even small lumps need veterinary investigation. Palpation alone cannot reliably establish malignancy. Small tumours can be easier to treat without metastases. Microscopic examination of removed tissue gives the most reliable identification.

Can a spayed dog develop mammary tumours?

Yes. Early spaying may reduce risk. New nodules need assessment even if small and painless in a spayed dog. They may lie beneath skin immediately beside a nipple.

Must the whole chain be removed?

A single lump or gland may suffice for an isolated tumour. Several same-side tumours can warrant the whole chain. The pathologist checks margins for cells afterwards. This helps assess whether surgery was sufficient.

Can tumours return after surgery?

Local recurrence is possible. Cells left at margins increase this risk. A separate new lump can also develop in another gland. Regularly feel remaining chains for new lumps.

How is spread identified?

The vet examines regional nodes because tumours can spread there. Sampling can seek metastases despite normal node size. Chest X-rays investigate visible lung spread. Abdominal organs can also be examined by ultrasound when needed.