First pathological study of canine primary breast lymphoma and the description of its clinicopathological characteristics as an animal model for human primary breast lymphoma.

Canine breast cancer (BC) and human BC are the most prevalent tumors in female dogs and humans, respectively. Several studies have indicated that canine BC is a good model for human BC. Unlike breast carcinomas, human primary breast lymphoma (PBL) is a rare tumor, but no case of canine PBL has been reported thus far. The current study presents a case of canine MC of the primary non-Hodgkin lymphoma (NHL) type for the first time and subsequently questions the theory of considering it as a model for human PBL. A 2-cm tumor was surgically removed from the left caudal abdominal mammary gland of a 6-year-old female dog of the terrier breed. Microscopic examination did not show any sign for the epithelial origin of the tumor. By contrast, histomorphological view and molecular pathological evaluation by immunohistochemistry showed that the tumor was of the diffuse large B-cell lymphoma (DLBCL) type [cluster of differentiation 19+ (CD19+), CD20+, CD10+, B-cell lymphoma 6+, CD3-, CD15-]. According to the World Health Organization classification, DLBCL is considered to be an NHL. Canine NHL is common in dogs and certain investigators believe that the biological behavior and clinical course is extremely similar to human NHL, and therefore, consider it as a model of human NHL. To the best of our knowledge, the current study is the first report of canine PBL. As the most significantly reported human PBL histotype is the DLBCL type, the histomorphological and immunophenotyping characteristics of canine PBL in the study considerably match with human PBL and raise the hypothesis that it can be a model for human PBL.