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Feline mammary carcinomas (FMCs) are heterogeneous neoplasms often comprising multiple histological patterns within a single case. The current practice of focusing on a prominent subtype hinders prognostic accuracy and potentially produces suboptimal therapeutic decisions. This retrospective study proposed criteria for grouping these mixed patterns and evaluated their prognostic significance in FMCs. The criteria were based on the tubular carcinoma composition and adopted from those of human breast cancer. Eighty-seven cats with FMCs were divided into groups according to tubular carcinoma composition: pure tubular carcinoma (PTC, n = 34), tubular carcinoma mixed with other subtypes (TMC, n = 16), and nontubular carcinoma (NTC, n = 37). Prognostic factors and overall survival (OS) were compared among the groups. The PTC and TMC groups were significantly associated with early-stage FMCs (P = .03) and low argyrophilic nucleolar organizer region values (P = .02). Conversely, NTCs had the poorest prognosis (P = .03) and were associated with incomplete surgical margins (P = .004), advanced clinical stage (P < .001), and high histological grade (P < .001) on multivariable analysis. Median OS for the PTC, TMC, and NTC groups was 221, 242, and 147 days, respectively. Considering individual histological subtypes, cats with adenosquamous carcinoma had a shorter OS (60 days) than those with comedocarcinoma (147 days) and tubular carcinoma (240 days, P < .001). These criteria were associated with FMC outcome in this cohort, providing a potentially useful framework for prognosis. However, further validation through prospective studies is needed to fully establish their clinical utility.