Correlation between histologic diagnosis mean nucleolar organizer region count and prognosis in canine mammary tumors

In this study, surgically excised mammary tumors from 98 bitches were graded histologically, and the grade was compared with the mean nucleolar organizer region (NOR) count in silver-stained paraffin-embedded sections. Histologically benign tumors, papillary adenocarcinomas, and intraductal carcinomas showed relatively little variation; the mean count for each category was between three and four NOR per nucleus. There was, however, a significant increase in the NOR counts in tubular and solid carcinomas. This increase was most pronounced for tumors that showed evidence of infiltration into the surrounding connective tissues. The mean NOR count for noninfiltrative carcinomas was 5.1, and that for invasive carcinomas was 7.3 (P less than 0.03). The mean NOR count for individual carcinomas ranged from 2.0 to 12.3, and a significant correlation was found between an increased NOR count and tumor-related death during the first post-surgical year. The 39 bitches in which the tumor had an NOR count less than 8.0 had a generally favorable prognosis; only six (15%) died as a result of the original neoplasm. In contrast, 18/21 dogs (85%) with a carcinoma having an NOR count greater than 8.0 died from the tumor during the first post-surgical year. A similar, although less pronounced result was obtained specifically for invasive carcinomas, in which 3/12 (25%) tumors with an NOR count less than 6.0 resulted in the death of the host, compared with 17/20 (85%) that had an NOR count greater than 6. By using this technique, it is possible to identify a subgroup of bitches with invasive mammary carcinomas that have a very poor prognosis following apparently adequate surgical ablation of the primary tumor.