Errors in transrectal ultrasonic planimetry of the prostate: computer simulation of volumetric errors applied to a screening population.

Three systemic errors in routine ultrasonic planimetric volume measurements of the prostate were assessed. A computer model using ellipsoids was used to simulate the step section technique and different forms of rotational movements of the prostate during planimetry. The planimetric volume was up to 12% smaller than the exact volume, depending on the degree of rotational movement, the shape, and the length of the ellipsoid. In vivo study of a screening population showed that it is worthwhile to compare caliper length with the number of planimetric steps, as the difference might be an indication of the difference between planimetric and caliper measured volume. In shorter prostates the planimetric volume was smaller than the prolate spheroid volume when compared to longer prostates, as was seen in the computer simulation.