Histopathological findings and proliferative activity of canine sebaceous gland tumours with a predominant reserve cell population

Sebaceous gland tumours represent the third most common skin tumours in dogs, but diagnostic criteria for tumours with basal differentiation (i.e. sebaceous epithelioma) are poorly defined and there is lack of correlation with biological behaviour. The aim of this study was to identify the main histological criteria associated with malignancy in 30 canine sebaceous gland tumours with a predominant reserve cell population. For each case, tumour proliferative activity was assessed by determining mitotic index and the Ki67/MIB-1 index. Additional histological features included endophytic or exophytic growth, proportion of reserve/intermediate/mature cells, connection to the epidermis, nuclear characteristics, peripheral invasion, neoplastic emboli and necrosis. Mitotic and Ki67 indexes were variable, but correlated (R = 0.66; P <0.001), and both were significantly higher in infiltrative tumours (P = 0.018 and P <0.001, respectively). No significant difference in histological features was observed between tumours comprised of more or less than 90% reserve cells, nor among tumours showing proliferative activity in sebocytes. This study suggests that high proliferative activity and peripheral invasion should be considered the most significant parameters for the differentiation between benign and malignant sebaceous gland tumours. Furthermore, the incidence of circumanal gland and testicular tumours in these dogs was significantly higher compared with an age-matched control population, suggesting a potential androgen-related pathway for the tumourigenesis of canine sebaceous gland neoplasms.