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Mammory Tumors PDF Print E-mail


  •  Most common tumor in the female dog.
  • 50-52% of all tumors in intact bitch.
  • Median age between 10 -11 years old.
  • Rare in any dog less than 5 years of age.
  • Breeds with increased risk include: toy poodle, mini breeds, English spaniel, Brittany spaniel, English setter, pointer, fox terrier, Boston terrier, cocker spaniel, and labs.
  • Breeds with a DECREASED risk include: boxers, greyhounds, beagles, and Chihuahuas.


  •  If dog spayed prior to first heat, 0.05% chance of developing mammary tumor.
  • After the first heat, 8% chance of developing mammary tumor.
  • After the second heat, 26% chance.


  •  40-60% of all malignant MGT's have estrogen receptors (ER), and 70% of the benign tumors have ER's.
  • Well-differentiated tumors tend to have more ER's than poorly differentiated ones (more malignant).
  • Prolonged administration of estrogen has not been shown to increase the incidence of MGT in dogs.
  • Administration of progesterone may increase the formation of benign MGTs.
  • Another study found that among spayed dogs, the incidence of MGT was lower in dogs that were thin between the ages of 9 -12 months.


  •  50% of MGT in bitch are malignant, 50% benign.
  • Suspicious signs: firmly adhered, weight loss, cough, rapidly growing mass, irregular heat cycles.
  • May have multiple benign ones.
  • Need biopsy because needle aspirate and cytology is not diagnostic - only tells tissue type.
  • Carcinomas (95%) are much more common than sarcomas (5%).


  •  75% occur as single masses. 25% occur as multiple nodules.
  • 65-70% of canine tumors occur in glands 4 and 5 (dogs have 5 pairs of glands). The cranial 2 drain to the axillary lymph node. The caudal 2 drain to the inguinal lymph node. Number 3 drains to either the axillary or inguinal lymph node.

Diagnostic techniques and work up:

  •  Physical exam.
  • CBC/chemistry profile/urinalysis.
  • Coagulation profile if suspect inflammatory carcinoma due to DIC potential.
  • Thoracic radiographs, abdominal ultrasound or radiographs to evaluate the sublumbar lymph nodes if caudal glands involved.
  • Do rectal to check internal iliac nodes.
  • Excisional biopsy best way to diagnose MGT. Fine needle aspirate is very insensitive.
  • Lymph node aspirate.


  •  Size
  • Clinical evidence of invasiveness.
  • Evidence of inflammatory carcinoma.
  • Ulceration.
  • Recent rapid growth.
  • Most common metastatic sites: lung, sublumbar lymph nodes, liver, rarely bone.
  • 50% of dogs with malignant MGT will metastasize.


  •  Surgical excision remains the treatment of choice.
  • Spay first.
  • Radical mastectomy verses lumpectomy: no studies prove that one is more beneficial than the other except for sarcomas.
  • Lumpectomy or nodulectomy - good for small (<0.5 cm) masses.
  • Single mastectomy - good for >1 cm masses.
  • Regional mastectomy - remove glands 1,2, and 3 en bloc with draining lymph nodes (axillary and cranial sternal lymph nodes); glands 4 and 5 should be removed together and they drain to superficial inguinal lymph nodes which drain to the medial iliac lymph nodes.
  • Unilateral or bilateral radical mastectomy only done for convenience if many MGT's exist in a row.
  • Axillary nodes should only be remove if presence of metastatic disease is confirmed; they are rarely involved with cancer.
  • Inguinal node should always be removed with gland 5.


  •  Use based on tumor histology and size.
  • No single chemotherapy protocol has been reported to be effective in dogs .
  • Adriamycin/cytoxan or cisplatin has shown some anti-tumor activity against canine mammary tumor cell lines.


  • Tumor size is the most important prognostic indicator for mammary adenocarcinomas. Good = <3 cm, well-circumscribed, lymph node negative.
  • Dogs with sarcomas will be dead of their disease within 1 - 2 years. 

Feline Mammary tumors:

  •  80 - 95% MGT in cats are malignant.
  • More common in females.
  • Age 10 - 12 years.
  • Increase risk in Siamese cats.
  • Survival10 - 12 months.