Ovariectomy reduces the risk of tumour development and influences the histologic continuum in canine mammary tumours

Abstract

Influence of neutering on canine mammary tumorigenesis has been a source of vivid discussion over the last decades. The purpose of this retrospective study was to describe the association between neuter status, tumour size and degree of malignancy in a large population of 625 female dogs with altogether 1459 removed mammary tumours (MTs). MT‐bearing dogs were predominantly intact (80.3%) and intact dogs were overrepresented in the tumour population compared to the control group of >19 000 females (p < .0001). Multiple MT occurred in 340 patients (54.4%) and were significantly more common in intact dogs (57.8% vs. 40.7% spayed). Neutered dogs were not only significantly more likely to have a malignant MT (p < .0001) but were significantly more often affected by more aggressive tumour subtypes (p < .0001). Positive correlation between increasing tumour size and increasingly malignant phenotype was slightly stronger in spayed (rs = .217; p = .021) compared to intact (rs = .179; p = .0003) patients. After ovariectomy, progression from benign to malignant occurs in smaller size tumours, as MT ≥2 cm in diameter were malignant in 86.9% of the spayed patients, compared to 62.0% in intact patients (p = .0002). Intact bitches have a higher risk for MTs and tumour multiplicity. MTs in neutered females are more often malignant and belong to more aggressive subtypes compared to MTs in intact dogs. In neutered bitches, histologic progression from benign to malignant and further along the cancer progression continuum occurs at smaller tumour sizes.

Risk factor analysis and clinicopathological characteristics of female dogs with mammary tumours from a single-center retrospective study in Poland

“…Routine OH is often performed because of its protective value against reproductive tract disorders and CMTs. Depending on the age of the dog at the time of OH, potentially fatal CMTs may be preventable 14 , 17 , 18 . However, some studies did not confirm such a beneficial effect 15 , 66 , 67 .…”

“…), also considering the adverse effects of early OH 15 – 17 . Moreover, a recent study suggests that OH performed in adulthood (≥ 4 years of age) may still decrease the risk of CMT development; however, a reduction in the prevalence of benign tumours has been observed in neutered dogs 18 . Even late-spayed dogs (after 2 years of age) had a fourfold lower risk of CMT-related death compared with intact dogs 17 .…”

“…These differences in the percentages can be due to different spaying rates in each country. If there is a high spaying rate, there will be a decreased total CMT rate but an increase in malignant tumours among the CMT 18 .…”

Canine Mammary Tumors: Classification, Biomarkers, Traditional and Personalized Therapies

“…In these regions, preventive neutering is not a common procedure in dogs [ 6 ]. Currently, it is known that ovariohysterectomy significantly reduces the risk of cancer development [ 8 ].…”

Epidemiology of mammary tumours in bitches under veterinary care in the UK in 2016

“…Other risk factors, such as neuter status, time of neutering and breed, were also identified, 6 with entire bitches, bitches of an advanced age (i.e., greater than 12 years) at neutering and breeds such as Boston Terrier, Cocker Spaniel, English Springer Spaniel, German Shorthaired Pointer, Labrador Retriever, Dachshund and Poodle showing increased risk of mammary tumours 7–11 . More recently, it has been reported that early ovariohysterectomy had a protective effect against mammary tumours 12 . However, there remains limited current evidence directly relevant to UK bitches.…”