12. Aus, G., Bergdahl, S., Hugosson, J. et al: Volume determinationsof the whole prostate and of adenomas by transrectal ultra-sound in patients with clinically benign prostatic hyperplasia:correlation of resected weight, blood loss and duration of op-eration. Br J Urol, 73: 659, 1994

13. Hendrikx, A. J., van Helvoort-van Dommelen, C. A., van Dijk,M. A. et al: Ultrasonic determination of prostatic volume: acadaver study. Urology, 34: 123, 1989

14. Watanabe, H., Igari, D., Tanahashi, Y. et al: Measurements ofsize and weight of prostate by means of transrectal ultrasono-tomography. Tohoku J Exp Med, 114: 277, 1974

15. Terris, M. K. and Stamey, T. A.: Determination of prostatevolume by transrectal ultrasound. J Urol, 145: 984, 1991

EDITORIAL COMMENTS

The importance of the transition zone of the prostate is increas-ingly understood by urologists. In BPH transition zone volume maybe used to choose medical or surgical treatment. Although mosturologists continue to measure the total volume of the prostate, BPHin fact represents the hypertrophied transition zone of the prostate.Transition zone volume is also important for calculating PSA densityof the transition zone, which was demonstrated to be useful fordiscriminating benign from malignant prostatic disease in patientswith an intermediate PSA level.

By measuring transition zone volume with transrectal ultrasoundbefore prostatectomy these authors show that there was an excellentcorrelation (0.95) of prostate adenoma weight with estimated transitionzone volume. They state that because there was a statistically signifi-cant difference between mean adenoma weight and transition zonevolume estimated by ultrasound, further agreement should be obtainedbefore the clinical use of transition zone measurements. They evenrecommend not using transition zone volume measurement in clinicalpractice. This suggestion is clearly arguable and hardly acceptablewhen analyzing this study in detail. The data presented may be inter-preted in exactly the opposite way. All urologists continue to use rou-tinely total prostate volume but numerous studies have shown thatthere is wide interobserver and intra-observer variability of this meas-urement. An underestimation or overestimation by transrectal ultra-sound that is sometimes close to 30% has been reported.1 Althoughtransition zone volume is certainly subject to observer variability, it ismore reliable than total volume estimation by transrectal ultrasound(reference 10 in article).

Figure 3 in the article shows a remarkable correlation of adenomaweight with transition zone volume. If one removes from this figureprostate adenomas larger than 100 gm. (implying that the wholeprostate was even larger), which are now uncommon in most westerncountries, the correlation is even better. Eyeballing this correlationindicates that it may well have been that for prostates smaller than100 gm. no statistically significant difference would have been ob-served between the estimated and measured transition zones. There-fore the conclusion that transition zone volume measurementsshould not be used is hardly supported by the data presented exceptin cases of an extremely large prostate. In these cases there is adifference but the clinical relevance is probably much less important.

As recommended by the authors, international agreements shouldcertainly be obtained to improve the reliability of transition zone meas-urements. However, in contrast to what they claim, this study supportsthe use of transition zone volume measurement in clinical practice.

Alexandre ZlottaService d’UrologieHopital ErasmeBrussels, Belgium

REFERENCE

1. Matthews, G. J., Motta, J. and Fracehia, J. A.: The accuracy oftransrectal ultrasound prostate volume estimation; clinicalcorrelations. J Clin Ultrasound, 24: 501, 1996

These authors make a strong argument for clear agreements onhow best to estimate transition zone volume by transrectal ultra-sound. They found high correlations (r2 5 91%) between the weightof 50 prostatic adenomas (transvesical enucleation) and calculatedtransition zone volume using the prolate ellipsoid method, width 3height 3 cephalocaudal length 3 0.52. However, the intercept infigure 3 shows that the transrectal ultrasound prolate ellipsoid

method substantially overestimates the enucleated weight of theadenoma. The authors are careful to note several potential causes ofthis discrepancy, including incomplete transvesical enucleation ofthe adenoma as well as our finding years ago that it is difficult toestimate the cephalocaudal length of the prostate as an ellipsoidshape because of inaccuracies imposed by the vesical neck and distalapex of the prostate (reference 15 in article). For these reasons wehave always preferred the simpler prolate spheroid formula, p/6 3(transverse dimension)2 3 anteroposterior dimension, whether weare estimating total prostate or transition zone volume. Using theseestimates we have recently shown that calculated transition zonevolume (or total prostate volume) is equivalent to free-to-total orcomplex-to-total serum PSA ratios in receiver operating character-istics curves in terms of separating true biopsy positive fractions(sensitivity) from true biopsy negative fractions (specificity).1

The authors are correct that the issue of how best to estimate accu-rately transition zone volume by transrectal ultrasound is importantand should be resolved with a definitive study. After all, the mostcommon cause of elevated serum PSA greater than 4 ng./ml. is notprostate cancer but BPH. Roehrborn et al estimated that about a thirdof the men with BPH have a serum PSA of greater than 4 ng./ml. andsubstantial numbers with BPH clearly have serum PSA in the 2 to 4ng./ml. range.2 The diagnostic improvement in sorting out men withprostate cancer from men with BPH by factoring serum PSA by trans-ition zone volume (or total prostate volume) comes not from someunknown property of the cancer, but from the changes in serum PSAdirectly related to the size of the BPH. The same thing is true forfree-to-total serum PSA ratios. Men with less than 10% free-to-totalPSA have such minimal amounts of BPH that little free PSA is liber-ated into the serum. Thus, knowing transition zone volume (or totalprostate volume) is just as useful as knowing the free-to-total serumPSA ratios, as we have previously shown.1, 3 Since transition zone sizereflects the amount of BPH and largely determines free-to-total PSAratios, we agree with the authors that anything we do to improve theaccuracy of transrectal ultrasound estimates of transition zone volumeat the time of prostate biopsy would be helpful.

Thomas A. StameyDepartment of UrologyStanford University Medical CenterStanford, California

REFERENCES

1. Stamey, T. A. and Yemoto, C. E.: Examination of the 3 molecularforms of serum prostate specific antigen for distinguishingnegative from positive biopsy: relationship to transition zonevolume. J Urol, 163: 119, 2000

2. Roehrborn, C. G., Boyle, P., Gould, A. L. et al: Serum prostate-specific antigen as a predictor of prostate volume in men withbenign prostatic hyperplasia. Urology, 53: 581, 1999

3. Prestigiacomo, A. F. and Stamey, T. A.: Can free and totalprostate specific antigen and prostatic volume distinguish be-tween men with negative and positive systematic ultrasoundguided prostate biopsies? J Urol, 157: 189, 1997

REPLY BY AUTHORS

The correlation coefficient measures the degree of association be-tween the continuous variables but it does not measure how closelythey agree. A high correlation coefficient cannot be interpreted as anindication of good agreement. There are several reasons why corre-lation is an inappropriate analysis. The correlation coefficient is ameasure of the strength of the linear association between 2 vari-ables, which is not the same as a measure of agreement. We mayhave a high degree of correlation when the agreement is clinicallypoor. Although we found a significant and clear correlation betweenthe adenoma weight and estimated transrectal ultrasound volumefor transition zone in our study, this does not mean that in anindividual case the estimated transrectal ultrasound volume will beexactly the same as the adenoma weight. According to our studythere is a significant difference between these 2 variables (p ,0.001),which is why we calculated the regression equation and concludedthat international agreements should be obtained to determine thetransition zone volume more precisely. On the other hand, we knowthat as with all ultrasonographic measurements, there will alwaysbe discrepancies, and interobserver and intra-observer variation inthe measurement of transition zone volume. With this in mind, onecan of course use transition zone volume measurement in clinicalpractice.

COMPARISON OF TRANSITION ZONE VOLUME WITH ENUCLEATED PROSTATE ADENOMA WEIGHT 75