Incidentally diagnosed mammary gland tumors are less likely to be malignant than nonincidental mammary gland tumors

OBJECTIVE To compare malignancy rates of canine mammary gland tumors diagnosed incidentally and nonincidentally. ANIMALS 96 female dogs from which mammary gland tumors were removed. METHODS Medical records of all female dogs from which mammary gland tumors were removed at a privately owned referral institution between 2018 and 2021 were reviewed. Data were obtained on signalment for each dog, histopathologic results for each tumor, and the primary reason for each dog's presentation to the hospital. The proportion of malignant tumors was compared between dogs that were presented with nonincidental MGTs and dogs that were presented for a different primary condition and had incidental MGTs found on examination. RESULTS A total of 195 tumors were removed from the 96 dogs in this study. In dogs with incidental MGTs, 82 of 88 (93%) tumors were benign and 6 of 88 (7%) were malignant. In dogs with nonincidental MGTs, 75 of 107 (70%) tumors were benign and 32 of 107 (30%) were malignant. Nonincidental MGTs were significantly (OR, 5.83; 95% CI, 2.31 to 14.73; P =.001) more likely to be malignant compared with incidental MGTs. Dogs with nonincidental MGTs were 6.84 times as likely to have a malignant MGT removed compared with dogs with incidental MGTs (OR, 6.84; 95% CI, 2.47 to 18.94; P <.001). The likelihood of malignancy increased by 5% for each 1-kg increase in body weight (OR, 1.05; 95% CI, 1.01 to 1.09; P =.013). Larger tumors were more likely to be malignant than smaller tumors (P =.001). CLINICAL RELEVANCE Most incidentally diagnosed MGTs are benign and allow for a good prognosis after excision. Small dogs and dogs with MGTs < 3 cm in diameter are the least likely to have a malignancy.