Proliferative activity of canine mast cell tumours evaluated by bromodeoxyuridine incorporation and Ki-67 expression

The proliferative activity of 91 canine mast cell tumours was assessed on the basis of the Ki-67 positive index (Ki-67 PI) and mitotic index (MI) and, in 15 cases, also by the labelling index of bromodeoxyuridine (BrdU; an analogue of tritiated thymidine) incorporated in vivo into S-phase cells. BrdU and Ki-67 were detected immunohistochemically. The tumours were graded histologically (I, II or III). The BrdU labelling index (BrdU LI) tended to increase as the grade became higher. In terms of the mean values of Ki-67 PI, significant differences were found between histological tumour grades I and II (P < 0.01) and between grades II and III (P < 0.01). In terms of mean MI, grades I and II were found to differ significantly (P < 0.05). With Spearman rank correlation coefficient and linear regression analysis, the BrdU LI and Ki-67 PI showed a highly significant correlation. This strong correlation indicated that Ki-67 was, like BrdU, a useful marker for proliferative potential in canine mast cell tumours; moreover, its use did not require the prior administration of any reagent to the live animal.