A large number of studies have investigated feline mammary tumors in an attempt to identify prognostic markers and generate comparative analyses with human breast cancer. Nevertheless, a retrospective base of assessments and the lack of standardization in methodology and study design have caused weakness in study results, making comparison difficult. We examined feline mammary tumor publications and evaluated postulated prognostic parameters according to the recently published ''Recommended Guidelines for the Conduct and Evaluation of Prognostic Studies in Veterinary Oncology.'' Using these criteria, we determined with statistically significant reliability that prognostic parameters for feline mammary tumors are tumor grading and lymph node/lymphovascular invasion. Furthermore, tumor subtype, size, and staging are worthy of further standardized investigation. We present statistical significance for each studied parameter as well as its relevance to disease progression and survival. Our evaluation suggests that marker expression (ie, Ki67, HER2, ER) may provide relevant information applicable for therapeutic predictions; however, consensus efforts and protocol standardization are needed. We identify and discuss major points of concern-such as sample preservation and selection, standardization of immunohistochemical protocols, and evaluation of results-to provide support for subsequent reliable analyses.

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BackgroundHuman breast cancer is a heterogeneous disease classified by molecular subtyping into luminal A, luminal B, HER2-overexpressing, basal-like, claudin-low and normal-breast like. The routinely applied and standardized immunohistochemical-based surrogates of this classification group together the last three entities as triple-negative breast cancer (TNBCs) that show the most diverse and complex heterogeneity and represent a therapeutic challenge.In the present work 156 feline mammary lesions consisting of feline mammary carcinomas (FMCs), benign neoplasms, and hyperplastic/dysplastic tissues were evaluated histologically and by immunohistochemistry for expression of basal and luminal cytokeratins (CK), vimentin, alpha-smooth muscle actin, calponin, estrogen receptor (ER) alpha (a), and progesterone receptor (PR). Thirty-seven FMCs with 27 matched non-neoplastic controls were also investigated for gene expression of ERa, ER beta, PR, and HER2.ResultsA large group of hormone receptors (HRs)-negative aggressive carcinomas - that did not overexpress HER2 - could be distinguished from the less aggressive (10.8%) and benign (8%) HRs + tumors, that showed bilineage (luminal and myoepithelial) differentiation. Immunohistochemical evaluations of cytoplasmic filaments indicated that HRs- FMCs are vimentin+, CK14+, and CK5_6+ carcinomas that may resemble the TNBCs (basal like/claudin low) described in women. The identification of luminal and myoepithelial progenitors within the mammary ductal system suggested potential cells/sites of origin of these tumors. A diffuse and never previously described CKs/vimentin luminal cell co-expression was detected in the non-neoplastic ducts, indicating a potential bilineage progenitor.ConclusionsThese results indicate and potentially explain the high incidence of triple-negative, vimentin + aggressive tumors in cats that may used to elucidate some of the challenging features of TNBCs in women.

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When compared with the canine species, feline mammary tumors (FMTs) are much less heterogeneous, with a predominance of simple malignant neoplasm. Benign FMTs are rare, and it is unclear if complex and mixed tumors exist in the feline. In this study, we selected for immunohistochemical analyses 12 FMTs that had unusual histologic features. A group of 8 (2 benign and 6 malignant) FMTs showed a biphasic epithelial/myoepithelial population and a very regular cord-like distribution in a "Chinese lettering" pattern, within ectatic ducts. A second group (2 benign and 2 malignant) had an intraductal epithelial papillary growth pattern with a basally located monolayer of myoepithelial cells and a supporting fibrovascular stroma. The myoepithelial component always produced a standard immunohistochemical signature. All malignancies were grade I, and the subjects were all alive at 1 year postdiagnosis. On the basis of their morphology, we propose that they be classified as feline ductal adenoma/carcinoma and feline intraductal papillary adenoma/carcinoma, respectively. They overlap with their canine counterparts and lack the typical myoepithelial differentiation patterns seen in canine complex neoplasms, and therefore, the term complex should be avoided in felines. This study will add new information on FMT classification and be useful for prognostic studies.