A review of 17 cases of horses with primary liver tumors revealed that equine and human tumors are similar and that the classification scheme currently employed in human medicine could be applied to equine tumors.

The primary malignant liver tumors hepatoblastoma (HB) and hepatocellular carcinoma (HCC) are rare. According to Dorothee Bienzle, DVM, PhD, Dipl. ACVP and colleagues from the Ontario Veterinary College's Department of Pathobiology, only 17 Canadian case reports have been described and only nine of these were ultimately diagnosed as HB. The remaining primary liver tumors diagnosed in fetuses and juvenile horses were mixed hamartoma, mesenchymal hamartoma, and HCC.

The cause of HB remains largely unknown. In humans, tumors can be classified according to the scheme described by the International Childhood Liver Tumours Strategy Group. This scheme involves microscopic analysis of the tumor cells to assess the normal and cancerous hepatocytes (liver cells).

Bienzle et al. found that primary hepatic tumors in the horse are "diverse in morphology and include subtypes compatible with classification criteria applied to human tumors