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Classification of tubulo‐papillary renal cortical tumours using estimates of nuclear volume

The classification of renal cortical tumours is problematic, with no clear division of benign from malignant tumours. Unbiased stereological estimates of volume‐weighted nuclear volume (nuclearv̄v) were obtained by point sampling of nuclear intercepts in a retrospective study of 36 variably sized tubulo‐papillary basophilic cell renal cortical tumours. There was no clear pattern of evolution of nuclearv̄v with increasing macroscopic tumour diameter. Estimates of nuclearv̄v could not distinguish between 21 tumours classified as renal adenomas with macroscopic diameters < 3 cm (average nuclearv̄v= 241 μm3) and 15 tumours classified as renal cell carcinomas with diameters > 3 cm, or aggressive histological pattern (average nuclearv̄v= 229 μm3) (2p= 0.68). In this subtype of renal cortical tumours, estimates of nuclearv̄v do not support the historical convention of using a 3 cm tumour diameter as the dividing line between adenomas and carcinomas, but support the theory of a single group of tumours. As most of the truly incidental renal cortical tumours are less than 1 cm in diameter, this limit could be considered. Such small benign cortical nodules have never been reported to metastasize, and would thus be excluded from being diagnosed and registered as malignant. Although this dividing line is again arbitrary, and cannot be justified by the stereological measurements, it is a practical solution to a clinical problem. There were too few examples of disease progression to assess the prognostic significance of nuclearv̄v in these tumours.