The first sign of the tumor is a small hard lump in the mammary tissue. These can often go unnoticed until they reach a significant size. Multiple tumors within the effected or neighboring gland is common. Faster growing tumors have a greater tendency to be malignant and are more likely to have already spread than slow growing tumors.

Because it is not possible to determine the nature of individual mammary tumors by their appearance and feel all tumors should be considered potentially cancerous and removed as soon as they are discovered. There are three surgical methods of dealing with a mass in the mammary tissue. 1- Removal of the tumor from the mammary tissue. 2- Removal of the gland containing the tumor (mastectomy). 3 Removal of all the mammary glands on the side of the body effected (radical mastectomy). The logic behind a radical mastectomy is to remove all of the lymph glands which drain the site of the tumor and all areas of potential local spread. Where a simple mastectomy or lump removal is performed it is recommended that the tumor be analyzed. Should the results identify the tumor as malignant a radical mastectomy can then be performed.

The long term outlook for dogs with mammary tumors varies greatly depending on the type of tumor and the stage at which it is treated. Around half the dogs diagnosed with malignant tumors are cured by surgery showing no evidence of spread or recurrence. The carcinoma group of tumors which accounts for 85% of mammary cancers are more likely to spread to other parts of the body than sarcomas. Sarcomas however invade locally and have a very high recurrence rate following surgery.

Chemotherapy has been advocated as an additional treatment for dogs with malignant mammary tumors however the results of trials with these drugs have been variable.

Desexing of females before their first heat period reduces the risk of mammary tumors to almost zero. If desexing is delayed until after the first heat the benefit gradually reduces and by two and one half years there is no significant reduction of risk. Desexing at the time of tumor removal has not been shown to be of any benefit in reducing the rate of recurrence or death associated with cancerous tumors.