Hepatocellular carcinoma is the most common primary liver cancer in dogs, arising from hepatocytes — the main functional cells of the liver. It occurs in three distinct forms: massive (a single large tumour in one liver lobe, about 55% of cases), nodular (multiple tumour nodules across the liver, about 30%), and diffuse (cancer spread throughout the liver, about 15%). This distinction is critically important because the massive form can often be cured by surgically removing the affected liver lobe, whereas the nodular and diffuse forms are generally not amenable to complete surgical removal. Many hepatocellular carcinomas are discovered incidentally during imaging for other reasons, as they can grow to a substantial size before causing clinical signs. When symptoms do develop, they may include lethargy, reduced appetite, weight loss, or abdominal distension. The liver has remarkable regenerative capacity, and dogs typically recover well from surgical removal of even a large portion.