Transrectal ultrasound versus magnetic resonance imaging in the estimation of prostatic volume.

OBJECTIVE

To establish which method of determining prostatic volume (transrectal ultrasound [TRUS] or magnetic resonance imaging [MRI]) and which calculation formula give the most exact and least variable results; to determine the size and the source of the variability: and to establish which method is the more sensitive to drug-induced changes in prostate volume.

PATIENTS AND METHODS

Prostatic size was estimated by TRUS and MRI in 21 patients treated medically (either active treatment or placebo) for benign prostatic hyperplasia. Each patient was examined at baseline, and after 3 months and 6 months of treatment. Prostatic volume was calculated at every visit using different formulae proposed in the literature.

RESULTS

With some of these formulae, including the classical ellipsoid formula, there was a strong correlation (r > 0.8) between TRUS and MRI volume estimates. For others the correlation was much weaker, suggesting unreliability. MRI gave a significantly larger volume than TRUS because of larger values for the cephalocaudal and anteroposterior diameters. For patients on placebo the visit-to-visit variability of the prostate volume was 10-12% of the mean volume, whether calculated by TRUS or MRI. Part of this variability was apparently due to natural variation of prostate size.

CONCLUSION

The classical ellipsoid formula is adequate for determining prostate volume. MRI and TRUS give different volumes. Visit-to-visit variability is similar for both methods and is partly due to real, natural variation. MRI is better able than TRUS to detect drug-induced changes in prostate volume.