Mammary tumours are common in female dogs, with 35-60% being malignant. They occur in male dogs but are rare and likely to be malignant.
Mammary neoplasia is uncommon in cats, but when it occurs >80% are malignant.
Common sites of metastasis are lymph nodes and the lung. Other sites include the liver, kidney, bone and heart.
Aetiology of the tumour is unknown but a hormonal influence is likely as 70% of benign tumours and 50% of malignant canine tumours contain oestrogen and progesterone receptors. Metastatic tumours tend to lose both oestrogen and progesterone receptors.
Feline tumours sometimes contain progesterone receptors but rarely contain oestrogen receptors.
Risk factors include: old age, obesity early in life, progestagen treatment, intact status or bitches spayed after 2.5 years of age.
Possible mammary tumours include:
- Benign: adenoma/fibroadenoma, benign mesenchymal tumour, benign mixed tumour
- Malignant: carcinoma (solid, tubular, papillary, inflammatory), sarcoma, carcinosarcoma, adenocarcinoma
The animal will present with a history of the owner noticing a lump, or it may be discovered during routing physical examination. It is rare for the animal to present with signs referable to metastases such as coughing or lameness.
On physical examination, masses may be very variable in size and multiple different masses may be found in one or both mammary chains. It is essential to palpate all glands.
Masses may be moveable or fixed, and possibly ulcerated.
Axillary or inguinal lymph nodes may be enlarged. Rectal exam may reveal enlarge sublumbar lymph nodes.
Canine mammary tumours tend to occur most commonly in the caudal glands.
Inflammatory carcinomas present as a diffuse swelling, often ulcerated, with a systemically ill animal.
Feline tumours are often not a discrete mass, and they occur most commonly in the cranial glands.
Mammary neoplasms cannot be diagnosed visually. The aim is to obtain a diagnosis and to stage the disease using the TNM system:
- Tumour size: T1 <3cm, T2 3-5cm, T3 >5cm
- Regional lymph node: N0 no metastasis, N1 metastasis
- Distant metastasis: M0 not present, M1 present
Haematology and biochemistry should be performed to make sure geriatric patients are safe to anaesthetise.
Radiography of the thorax and abdomen to check for distant metastases is recommended.
Abdominal ultrasound to check the sublumbar and inguinal lymph nodes is also useful.
Fine needle aspiration and cytological analysis of the mass is an essential tool: it may reveal cells with criteria for malignancy, with distinct nucleoli, high nuclear-cytoplasmic ratio and moderate anisocytosis and anisokaryosis.
It may be difficult to differentiate benign from malignant tumours on cytology alone, as the features of malignancy may be subtle or inflammation may result in cells which may mimic malignancy.
Surgical biopsy and histopathology will provide the definitive diagnosis. It is usually excisional and performed at the same time as treatment.
Immunohistochemistry can be performed to determine the presence of hormone receptors in the tumour, but this is an expensive test which does not really alter the treatment plan.
Other diseases which may mimic mammary masses and which should be ruled out include:
Mastitis, mammary hyperplasia, skin/subcutaneous tumours, foreign body reactions, granulomas
Medical treatment is rarely used, and surgical removal is the treatment of choice for all mammary tumours except inflammatory carcinomas.
Surgery may be curative or palliative depending on the diagnosis.
Principles include: excising the tumour with at least 2cm margins, excising the underlying fascia if the tumour has invaded the subcutaneous tissue, excising a portion of abdominal muscle if the tumour has invaded the abdominal wall, placing a drain in the wound if dead space has been created.
In cats: the minimum recommendation is a unilateral mastectomy as tumours are usually aggressive.
In dogs: the tumour should be removed entirely, in different ways depending on tumour size, location and number. This may involve a lumpectomy, a simple mastectomy, a regional mastectomy, a unilateral mastectomy or a bilateral mastectomy.
The inguinal lymph nodes are often excised together with the caudal glands if they are removed.
Post-operative care involves analgesia and supportive care.
Complications unclude: seroma, wound infection and dehiscence, hindlimb oedema, recurrence of the disease and metastatic spread.
In dogs, prognostic factors include:
- Tumour size: <3cm 30% recurrence, >3cm 85% recurrence
- Histologic type: sarcomas are worse than carcinomas or mixed types
- Grade/differentiation: 90% mortality at 2 years if the cells are poorly differentiated, 24% mortality at 2 years if the cells are well differentiated.
- Lymph node involvement: if present, there is 80% recurrence.
- Distant metastases have a poorer prognosis
- Presence of progesterone/oestrogen receptors holds a better prognosis.
There is no evidence that site or number of tumours is a prognostic indicator.
Inflammatory carcinomas have a grave prognosis with <4 weeks survival time. Malignant tumours incompletely excised hold a 75% mortality within the year. Dogs with malignant tumours completely excised and with no evidence of metastases are expected to live 1-2 years. Complete excision of benign tumours is curative.
In cats, prognostic factors include:
- Tumour size: >3cm medial survival time <6 months, <2cm median survival time 3 years.
- Extent of surgery: there is 66% recurrence if local surgery rather than radical mastectomy is performed
- Histologic grading: well differentiated tumours with no evidence of lymphatic invasion have a better prognosis.
The overall median survival time is <1 year and the disease hold a poor prognosis overall in cats.
Mammary neoplasia is almost preventable by early spaying (before 2.5 years in dogs)
In dogs, the relative risk if spayed before their first season is 0.05%. After the first season it is 8%, after the second it is 26% and after the third season there is no protective effect.
In cats the relative risk if spayed before 1 year is 0.6% compared to intact cats.
Also see Mammary Neoplasia - Rat
Lipscomb, V. (2009) Mammary gland neoplasia RVC student notes
Merck and co (2008) Merck veterinary manual Merial
Withrow, S. (2001) Small animal clinical oncology Saunders
Fossum, T. (2007) Small animal surgery Mosby
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