Small canine mammary tumours have different prognostic factors: A study of 3,470 tumours

Mammary gland neoplasms are the most frequent tumour in female dogs and most are malignant. Pet longevity and improvement in healthcare with early diagnosis increases the prevalence of cancer in animals. Tumour size is used as a prognostic factor to determine TNM (Tumour, Node, Metastasis) staging and considers tumours <3 cm, 3-5 cm and >5 cm. Due to the high rate of malignancy of mammary gland neoplasms, this study aimed to evaluate canine mammary tumours <3 cm and correlate the findings with clinicopathological parameters. A total of 3,470 mammary tumours were included and were classified as group 1 (G1, <1.5 cm) or group 2 (G2, 1.5-3 cm) as well as by prognosis - good, intermediate, poor or indeterminate. Clinicopathological parameters such as age, histological type, grade and lymph node status were analysed. The results revealed a significantly higher association of G1 tumours with non-neoplastic or benign diagnoses (P <0.001), a higher prevalence of histological grade I (P <0.001) and more frequent involvement of M2, M3 and M4 mammary glands (P <0.001). G2 tumours were significantly associated with a malignant diagnosis (P <0.001), higher prevalence of grades II and III (P <0.001), presence of lymph node metastasis (P <0.001) and involvement of the M5 mammary gland (P <0.001). Tumours with a good prognosis were most prevalent in dogs under 6 years old (mean 10 ± 2.8; P <0.0001) while a poor prognosis was significantly higher in dogs over 14 years old (mean 11.5 ± 2.8; P <0.0001). Tumours with an average size of 1.1 ± 0.7 cm had a significantly better prognosis (P <0.0001) and those with an average size of 1.6 ± 0.8 cm had a worse prognosis (P <0.0001). Histological type had an important role even in small tumours. Canine mammary tumours <1.5 cm and >1.5 cm had important differences in prognosis and highlight the need for a review of the minimal tumour size established in TNM staging for canine mammary gland tumours.