Accuracy of prostatic volume calculation in transrectal ultrasonography.

Abstract

Estimation of the prostatic volume by transrectal ultrasonography is now widely used to evaluate the effect of conservative treatment and to improve the specificity of prostate specific antigen. Accurate measurement of prostatic volume is necessary for the calculation of prostate specific antigen density (PSA/prostatic volume ratio). Three dimensional models of the prostatic contour were constructed from transrectal ultrasonotomograms taken at 5 mm intervals in 20 cases. Using these models, the accuracy of ellipsoid volume calculation and prolate ellipse volume calculation were examined. For prolate ellipse volume calculation, prostatic length was measured in two ways; length (Lv) vertical to the transverse section, and length (Lp) parallel to the prostatic urethra. Ellipsoid volume calculation showed errors of about 20%. Moreover, ellipsoid volume calculation fluctuated markedly (up to 50%) in accordance with the change of the scanning angle of the transverse section. The error of prolate ellipse volume calculation using Lp for length increased by an average of 25% as the angle moved away from the perpendicular. The prolate ellipse volume calculation using Lv for length was the most reliable method. The degree of error was least dependent on the angle between the plane and the axis. Although prolate ellipse volume calculation was more accurate than ellipsoid volume calculation, it is essential to have the three axes cross at right angles to obtain an accurate value by the prolate ellipse volume calculation.