Hormonal background of canine and feline mammary tumours.

Ovarian steroid hormones and their synthetic derivatives may enhance mammary tumorigenesis in dogs and cats. In toxicity studies of synthetic progestagens a dose-related effect has been observed in the dog, with low-dose exposure sometimes being protective against mammary tumour development. There is some evidence that steroid dependence, as reflected by the presence of steroid receptors (that are nearly always present in normal mammary tissue and benign mammary tumours), is decreased in advanced stages of malignant disease, both in the dog and cat. However, this difference in steroid receptor expression between benign and malignant conditions is not related to any significant alterations in the concentration of receptors for epidermal growth factor. Progestagens have been suggested to promote mammary tumorigenesis in the dog by their induction of growth hormone overproduction; however, there is no conclusive evidence that this effect is necessary for mammary tumour induction. Basal levels of growth hormone and of prolactin were found to be similar in tumour-bearing dogs and age-matched controls.