Canine mammary tumours: protective effect of late ovariectomy and stimulating effect of progestins.

Ovariectomy, even when performed at an advanced age, was found to be to some extent protective against mammary tumour development in dogs. Bitches treated with progestins had a slightly higher risk for mammary tumours (all types, benign and malignant) than controls. Progestin treatment did not increase the risk of mammary cancer. Benign tumours in (treated and untreated) dogs appeared earlier than malignant ones. Progestin treatment resulted in earlier appearance of both benign and malignant tumours than in controls. The ratio solitary/multiple mammary tumours was not significantly different between treated and untreated dogs.