Classification and behavior of canine mammary epithelial neoplasms based on life-span observations in beagles

As part of a study of the effects of low-level radiation, 1,343 Beagles, in cluding 671 males and 672 females, were evaluated over their full lifetime for the occurrence of mammary neoplasia; there were 139 control males and 1 38 control females and 532 irradiated males and 534 irradiated females. All nodules found in surgical specimens or at necropsy were evaluated histolog ically. The overall incidence, metastasis and recurrence rates, and contrib ution to mortality of mammary neoplasms were determined. Based on this uniq ue opportunity to correlate morphologic characteristics with ultimate biolo gical behavior of all mammary tumors in a defined canine population, we pro pose a histogenetically based reclassification of epithelial mammary tumors . Of the 672 female dogs, 70.8% (476) had at least one mammary neoplasm; 60 .7% (408) had more than one. Two male dogs had mammary neoplasms. Of 1,639 mammary carcinomas in the 672 females, 18.7% (307) were classified as ductu lar carcinomas (arising from the small interlobular or intralobular ductule s), whereas 80.7% (1,322) were classified as adenocarcinomas of other histo genetic origin. Of 73 fatal carcinomas, ductular carcinomas accounted for 4 8 fatalities (65.8%), whereas other adenocarcinomas accounted for only 20 f atalities (27.4%). Radiation had no effect on this ratio. Ductular carcinom as also had a higher rate of metastasis than did adenocarcinomas. Existing classifications of mammary carcinomas do not recognize the characteristic m orphologic features, the degree of malignancy, and the prognostic importanc e of these ductular carcinomas. Metastasis rates did not differ between sim ple and complex carcinomas or between those lesions and adenocarcinomas in mixed tumors. True carcinosarcomas metastasized more frequently (100%, or 5 /5) than did adenocarcinomas in mixed tumors (34.4%, or 22/64), emphasizing the importance of not lumping these tumors under the classification of mal ignant mixed tumors.