Nucleolar organiser regions in iris melanocytic tumours: an accurate predictor?

A silver staining technique which demonstrates the nucleolar organiser region (NOR) was used in paraffin sections of iris naevi and melanomas. The technique shows argyrophilic NOR associated proteins (AgNORs) which are seen in nuclei as black dots. In nine iris naevi the AgNOR count ranged from 1.54 to 3.82 (mean 2.73), in 21 melanomas from 1.89 to 8.31 (mean 4.67). Mean AgNOR counts greater than four dots per nucleus were only seen in malignant lesions, thereby differentiating between benign and malignant tumours, whenever high AgNOR counts were found. We subsequently examined three tumours in the intermediate group of aggressive naevi, Jakobiec group 6: there were counts averaging 4.08 but with a wide standard deviation of counts indicating that the behaviour of this group of aggressive tumours is likely to depend on the percentage of cells bearing the higher numbers of NORs, which may represent mitotic potential or increased metabolic rate.