We read with interest the article by Stocchettiet al on the accuracy and feasibility of the ellipsoid and the Cavalieri method in assessment of the volume of intracranial mass lesions in patients with severe head injury.1 We agree with the authors that the volume of intracranial lesions, and its change over time, is important in the diagnosis and management of patients with head injury and in the evaluation of clinical trials.2

However, the methodology used in the study raised our concern. We have several comments on their statements, because they are potentially misleading.

(1) The statement that computer based reading of mass lesions is the choice when accurate volume estimation is necessary, is insufficiently founded. Tracing CT lesions on a digitised screen automatically calculating area and hence volume, is a hazardous task: delineating hyperdense and hypodense lesions from normal surrounding intracranial structures cannot always be performed reliably, due to isodensity of normal brain tissue at some edges and due to partial volume effects. Moreover, lesion tracing is the same as area estimation using a simple device such as a point counting grid with sufficient grid points, and is in fact not superior at all.3

(2) Volume estimations based on Cavalieri's principle have to fullfill one absolute requirement: randomness.4 5 The volume of any object may be estimated from randomised and parallel sections separated by a known distance by summing up the areas of all cross sections of the object and multiplying this sum by the known intersection distance. The total area of all cross sections may be estimated by a stereological point counting method.3 A systematic array of grid intersection points is superimposed on …

Dr N Stocchetti