Myeloid metaplasia in canine mixed mammary tumors: occurrence and characterization

Abstract

Background: Mixed tumors are among the most frequent mammary neoplasms in female dogs. Some of these tumors present bone marrow associated with the newly formed osseous tissue, characteristic of myeloid metaplasia. Objective: To evaluate the occurrence of these lesions in a series of mixed tumors, and determine its histomorphological characteristics. Animals and methods: In total, 384 canine mammary mixed tumors from 289 animals have been reviewed. The lesions were classified according to the presence of osseous metaplasia associated with myeloid metaplasia or extramedullary hematopoiesis. Myeloid metaplasia characterization was determined from the morphological characteristics and organization of the cells and adjacent tissues. Cytoplasmic staining for CD31 and Factor VIII were used as a criterion to confirm the presence of blood vessels and megakaryocytes, respectively. Results: The 384 cases included 206 benign and 178 carcinomas in mixed tumors. Osseous metaplasia was present in 16.1% and calcified areas exclusively in 3.1% lesions. Among all osseous metaplasia, 33.9% presented some type of extramedullary hematopoiesis, of which 71.4% were classified as myeloid metaplasia and 28.6% as extramedullary hematopoiesis. Myeloid metaplasia cases consisted of 67% benign mixed tumors and 33% carcinomas in mixed tumors. CD31 and Factor VIII expression occurred in all myeloid metaplasia, confirming the presence of blood capillaries and megakaryocytes. Myeloid metaplasia was observed in 24% of mixed tumors containing osseous metaplasia and in 4% of all mixed tumors analyzed. Conclusion: Despite the low frequency of this lesion, additional studies are needed to understand the implications of myeloid metaplasia in canine mammary mixed tumors.