This icon indicates that an image is available. Click to open. In the bitch, mammary tumours may be benign or malignant, the incidence of each being roughly similar. This is a feature of so-called complex or mixed mammary tumours. Histologically, ma rgins of the tumour are irregular, with local invasion and spread via lymphatics.
Further spread is often to the lungs and sometimes bone.
Even though tumours do not develop until later, most commonly after six years of age, some hormonal change occurs early in life which affects subsequent tumour development. Bitches spayed before about 2 years old have a greatly reduced risk of subsequently developing mammary tumours. In women, some breast tumours are hormone dependent and respond to antihormone treatment. In some laboratory animals, oestrogens are important in the development and maintenance of mammary carcinomas but there is no evidence that this is important in dogs or cats. Various hormone receptors have been demonstrated in canine mammary tumour cells.