Mammary tumours are common in entire female cats, with malignancy rates up to 90%. Neutering dramatically reduces risk if performed before two years of age.
Feline mammary tumours cause significant welfare harm from tumour growth, ulceration, metastasis, and treatment. The extremely high malignancy rate means prognosis is often poor even with surgery. Prevention through early neutering is overwhelmingly preferable to treating established tumours. Neutering at the correct age is one of the most impactful welfare decisions for female cats.