Age incidence and site distribution of mammary dysplasias in young beagle bitches

The age incidence and site distribution of 2,142 mammary dysplasias were documented for 39 beagle bitches 6 months to 4 years of age. Lesion onset was at 2-3 years of age, at which time more than 50% of the females had dysplasias. Dysplasias appeared before palpable tumors. Posterior mammae developed more lesions than did anterior mammae. Thus the gradient for early onset of lesions coincided with the gradient for tumor frequency reported previously; a preneoplastic potential is suggested for (some) dysplasias. Problems of defining normal tissue are discussed.