Classification of primary hepatic tumours in the cat

•Histological and immunohistochemical examination of primary hepatic neoplasms in the cat can be used to differentiate hepatocellular adenomas and carcinomas, cholangiocellular carcinomas as well as small cell carcinomas with HPC characteristics and neuroendocrine carcinomas.•In contrast to the dog, feline hepatocellular carcinomas showed no HPC characteristics but maintained their hepatocellular characteristics.•Instead, within the small cell carcinomas or carcinoids a K19 negative group of true neuroendocrine carcinomas and a K19 positive group with HPC characteristics were recognized.•These differences between canine and feline primary hepatic neoplasms justify a separate, new classification scheme for the cat.•This new classification could be used as a basis for assessment of future therapeutic interventions.

Hepatic tumours in dogs have recently been re-classified to follow a revised human classification system that takes account of identified hepatic progenitor cells. This study investigated the presence and relative frequency of morphological types of feline primary hepatic neoplasms and aimed to determine whether a similar new classification scheme could be applied in cats. Feline primary liver tumours (n = 61) were examined histologically and with a series of immunohistochemical markers.Six cases of nodular hyperplasia and 21 tumours of hepatocellular origin were diagnosed. The latter were subdivided into hepatocellular tumours that were well differentiated and had no evidence of metastases (n = 18) and tumours that showed poorly differentiated areas with marked cellular and nuclear pleomorphism and had intrahepatic and, or, distant metastases (n = 3). These malignant feline hepatocellular tumours maintained their hepatocellular characteristics (HepPar-1, MRP2, pCEA positive) and were negative, or only <5% positive, for K19. Twenty-five cholangiocellular tumours were diagnosed and all had intrahepatic and, or, distant metastases. Eight NSE positive small cell carcinomas (carcinoids) were diagnosed and subdivided into small cell carcinomas with HPC characteristics (K19 positive) and neuroendocrine carcinomas (K19 negative). In addition, one squamous cell carcinoma originating from the distal part of the choledochal duct was recognised.Feline primary hepatic neoplasms can be sub-divided into benign and malignant hepatocellular tumours, cholangiocellular carcinomas, small cell carcinomas with HPC characteristics, neuroendocrine carcinomas and squamous cell carcinomas. The marked species difference justifies a specific classification for feline primary hepatic neoplasms.