- Email: [email protected]
Determination of Prostatic Volume with Transrectal Ultrasound: A Study of Intra-Observer and Interobserver Variation
0022-5347/96/1554-1299$03.00/0
THE J W R N A OF ~ . UROLOGY Copyright 0 1996 by AMERICAN URoLoCiru ASSOCIATION,INC.
Vol. 155, 1299-1300, April 1996 Printed in U.S.A.
DETERMINATION OF PROSTATIC VOLUME WITH TRANSRECTAL ULTRASOUND: A STUDY OF INTRA-OBSERVER AND INTEROBSERVER VARIATION T. S. BATES, J. M. REYNARD, T. J. PETERS AND J. C. GINGELL From the Bristol Urological Institute, Southmead Hospital and Department of Social Medicine, Health Care Evaluation Unit, University of Bristol, Bristot, United Kingdom
ABSTRACT
Purpose: We assessed the intra-observer and interobserver variation in measurements of prostatic volume using transrectal ultrasound. Materials a n d Methods: Two volume estimations were performed by 1 observer in 40 patients and by 2 observers i n 75. Results: Mean prostatic volumes measured ranged from 18.9 to 87.0 ml. (median 39.8) for 1 observer a n d from 15.5 t o 95.4 ml. (median 42.4) for 2 observers. The mean difference plus or minus standard deviation and limits of agreement between the paired volume estimations were 0.1 ? 3.3 ml. and -6.5 to 6.7 ml., respectively, for 1observer, and -0.3 ? 5.3 ml. and -10.9 to 10.3ml., respectively, for 2 observers. The mean difference was 9.8%for 2 observers compared to 4.6% for 1. Conclusions: The results show that there is considerable variation in repeated measurements of prostatic volume a n d the variation is greatest for 2 observers compared t o 1 observer. KEYWORDS:prostate, ultrasonography, observer variation the mean and standard deviation of these differences, and determining the limits of agreement between the observations as described by Bland and Altman.7 The limits of agreement are defined as the mean difference 22 standard deviations of the differences and are interpreted as the range over which 95% of paired observations would be expected to differ. In addition, for each series we calculated the mean and range of the percentage volume difference, defined as the difference between observations divided by the mean of the 2 volume estimations. The analyses were performed using computer software.
Prostatic volume estimation using transrectal ultrasound is a common clinical procedure. Uses include preoperative assessment of prostate size, interpretation of elevated prostate specific antigen levels (prostate specific antigen density) and, in the field of research, measurement of the effects of prostate shrinking drugs. Several methods of volume determination have been developed and the accuracy of each has been assessed.*-6 Currently, the prolate ellipse formula (height x width x length x d6)is the most commonly used in clinical practice since it is rapid, more easily performed than planimetry and has been shown to have high correlation coefficients with actual prostate volume.2 To our knowledge no study has assessed the variation between observers and between repeated measurements by 1 observer with this technique.
RESULTS
MATERIALS AND METHODS
We studied 115 patients 42 to 81 years old (mean age 65) undergoing transrectal ultrasound for routine urological assessment. All examinations were performed using an ultrasound scanner with a 7.5 MHz. probe. The prostate was scanned in the transverse and sagittal planes, and the height, width and length were measured. Intra-observer variation was assessed in 40 patients. Two measurements were performed by 1 observer (T.S. B.) at the start and end of scanning but before any biopsies were obtained. Each individual measurement was immediately stored in the scanner memory. Volumetric calculations were performed only after all measurements had been stored and the patient had left the department. For interobserver variation 75 patients Were assessed by 2 observers (T.S.B. and J. M. R.). Measurements were taken consecutively with onIy 1 observer in the room a t any time and both observers were blinded to the results of each other. Both observers were urological trainees with considerable experience in transrectal ultrasound. Volumes were estimated using the prolate ellipse formula. The data were analyzed by examining paired differences, and Accepted for publication October 20, 1995.
Transrectal ultrasound was well tolerated by all patients and prostatic volumes determined during both parts of the study were comparable. Using the mean across the 2 observers, prostatic volumes in the interobserver variabilities group ranged from 15.5 to 95.4 ml. (median 42.4,75 patients). In the 1 observer group, using the mean of the 2 measurements, the volumes ranged from 18.9 to 87.0 ml. (median 39.8,40 patients). Interobserver variation. Mean differences between observers in the measurements of height, width and length were 0.0 5 2.7 mm., -0.1 2 2.6 mm. and 0.1 2 3.9 mm., respectively, demonstrating that most interobserver variation occurred in the measurement of prostatic length. The overall mean prostatic volumes were similar between the 2 observers (mean 44.2 ? 18.2ml. for observer 1 and 44.5 -C 19.7ml. for observer 2).The paired differences between the volume measurements derived from the 2 observers ranged from - 16.1 to 12.7 ml. Mean difference in volume measurement between the 2 observers was only -0.3 ml., demonstrating that there was no consistent bias. The standard deviation of these differences was 5.3 ml., and the limits of agreement (mean 2 2 standard deviations) for the differences in prostatic volume were - 10.9 to 10.3 ml. for the entire range (fig. 1). For mean prostatic volumes less than 50 ml. the limits of agreement were -8.1 to 7.2ml. Mean difference in volume estimation in this series was 9.8% (range 0 to 28.9).
1299
1300
DE'EXMINATION OF PROSTATIC VOLUME WITH TRANSRECTAL ULTRASOUND
* ..
,I
~... ..........................................
0
........... . : :- .
*
-'O
-16
m .
..?..
0......* .... ......................................... * *.. * * +
I
................................
o
to
m
a
ua*2sD I
*:
**'
w-nM)
.........................
. . . a**. --... . . 4l ..:.2: . . ..................................... o
o
w
10
m
m
I_
Yn-m
l
tm
mId) I
the variation is less marked when measurements are performed by 1observer (limits of agreement -6.5 and 6.7 ml. compared to - 10.9 and 10.3 ml. for 2 observers). As expected, the variations in measurement in both groups increased in proportion with gland size. Differences between 2 observers were most marked in the measurement of prostatic length (standard deviation 3.9 compared to 2.7 and 2.6 for height and width, respectively). No such difference was observed in measurements by 1observer, which presumably reflects differences in interpretation of the exact location of the bladder neck and prostatic apex. With 2 observers the mean difference in volume estimation was approximately 10%(range 0 to 28.9), which was more than twice that for 1 observer. CONCLUSIONS
FIG. 1. Difference in prostatic volumes (VI measured by 2 observers against mean prostatic volume. SD,standard deviation.
These results demonstrate that there is considerable variation in repeated prostatic volume measurements, as determined by the prolate ellipse formula, using transrectal ultrasound. It is interesting to note that the differences in Intra-observer variation. Mean differences among height, prostatic volume measurement obtained by 2 observers are width and length measurements were 0.4 2 2.1 mm., 0.5 ? similar in magnitude to those reported to occur with drug 1.6 mm. and -0.7 ? 2.1 mm.,respectively. The paired dif- treatments that shrink the prostate.8 We conclude that for ferences between the 2 volume measurements made by 1 longitudinal studies of prostatic volume 1observer should be observer ranged from -6.6 to 10.6 ml. Mean difference in used when possible. The results of any such study should volume measurement between the 2 observations was 0.1 2 be interpreted in light of these variations. Finally, we recom3.3 ml. Therefore, the limits of agreement (mean 2 2 stan- mend more research into the causes of variation so that dard deviations) for the entire range of prostatic volumes procedures to reduce them can be developed and evaluated. were -6.5 to 6.7 ml. (fig. 2). For mean prostatic volumes less REFERENCES than 50 ml. the limits of agreement were -4.3 to 4.5 ml. Mean difference in volume estimation in this series was 4-6% 1. Hastak, S. M.,Gammelgaard, J. and Holm, H. H.: Transrectal ultrasonic volume determination of the prostate-a preopera(range 0 to 15.7). DISCUSSION
Measurement of prostatic volume using transrectal ultrasound is easy to perform and well tolerated by patients. The accuracy of the procedure in relation to actual measured prostatic volume has been addressed previously.2-6 We have shown that there is variation in measurements performed serially by 1observer and by 2 different observers. However,
m 16
I
-11
1
-I
-'a
1I
FIG. 2. Differencein prostatic volumes (v) measured by 1 observer against mean prostatic volume. SD,standard deviation.
tive and postoperative study. J. Urol., 127: 1115,1982. 2. Tems, M. K. and Stamey. T. A.: Determination of prostate volume by transrectal ultrasound. J. Urol., 145: 984,1991. 3. Jones, D.R., Roberta,E. E., Griffiths, G. J., Parkinson, M. C., Evans, K. T. and Peeling, W. B.: Assessment of volume measurement of the prostate using per-rectal ultrasonography. Brit. J. Urol., &4: 493,1989. 4. Myschetzky, P.S., Suburu, R. E., Kelly, B. S., Jr., Wilson, M. L., Chen, S. C. and Lee, F.: Determination of prostate gland volume by transrectal ultrasound: correlation with radical prostateetamy specimens. Scand. J. Urol. Nephrol., suppl., 137: 107, 1991. 5. Yip, Y. L., Chan, C. W., Li, C.K., Chu, V. and Lau, M. L.: Quantitative analysis of the accuracy of linear array transrectal ultrasound in measurement of the prostate. Brit. J. Urol., 67: 79,1991. 6. Littrup, P. J., Williams, C. R., Egglin, T. K. and Kane, R. A,: Determination of prostatic volume with transrectal US for cancer screening. Part 11. Accuracy of in vitro and in vivo techniques. Radiology, 179 49, 1991. 7. Bland, J. M. and Altman, D. G.: Statistical methods for the assessing agreement between two methods of clinical measurement. Lancet, 1: 307, 1986. 8. Gormely, G. J., Stoner, E., Bruskewitz, R. C., Imperato-McGinley, J., Walsh, P. C., McConnell, J. D., Andriole, G. L., Geller, J., Bracken, B. R., Tenover, J. S., Vaughan, E. D., Pappas, F., Taylor, A,, Binkowitz, B. and Ng, J.: The effect of Finasteride in men with benien Drostatic hyperplasia. The Finasteride Study Group. New Eigl: J. Med., 327: 1185,1992.
THE J W R N A OF ~ . UROLOGY Copyright 0 1996 by AMERICAN URoLoCiru ASSOCIATION,INC.
Vol. 155, 1299-1300, April 1996 Printed in U.S.A.
DETERMINATION OF PROSTATIC VOLUME WITH TRANSRECTAL ULTRASOUND: A STUDY OF INTRA-OBSERVER AND INTEROBSERVER VARIATION T. S. BATES, J. M. REYNARD, T. J. PETERS AND J. C. GINGELL From the Bristol Urological Institute, Southmead Hospital and Department of Social Medicine, Health Care Evaluation Unit, University of Bristol, Bristot, United Kingdom
ABSTRACT
Purpose: We assessed the intra-observer and interobserver variation in measurements of prostatic volume using transrectal ultrasound. Materials a n d Methods: Two volume estimations were performed by 1 observer in 40 patients and by 2 observers i n 75. Results: Mean prostatic volumes measured ranged from 18.9 to 87.0 ml. (median 39.8) for 1 observer a n d from 15.5 t o 95.4 ml. (median 42.4) for 2 observers. The mean difference plus or minus standard deviation and limits of agreement between the paired volume estimations were 0.1 ? 3.3 ml. and -6.5 to 6.7 ml., respectively, for 1observer, and -0.3 ? 5.3 ml. and -10.9 to 10.3ml., respectively, for 2 observers. The mean difference was 9.8%for 2 observers compared to 4.6% for 1. Conclusions: The results show that there is considerable variation in repeated measurements of prostatic volume a n d the variation is greatest for 2 observers compared t o 1 observer. KEYWORDS:prostate, ultrasonography, observer variation the mean and standard deviation of these differences, and determining the limits of agreement between the observations as described by Bland and Altman.7 The limits of agreement are defined as the mean difference 22 standard deviations of the differences and are interpreted as the range over which 95% of paired observations would be expected to differ. In addition, for each series we calculated the mean and range of the percentage volume difference, defined as the difference between observations divided by the mean of the 2 volume estimations. The analyses were performed using computer software.
Prostatic volume estimation using transrectal ultrasound is a common clinical procedure. Uses include preoperative assessment of prostate size, interpretation of elevated prostate specific antigen levels (prostate specific antigen density) and, in the field of research, measurement of the effects of prostate shrinking drugs. Several methods of volume determination have been developed and the accuracy of each has been assessed.*-6 Currently, the prolate ellipse formula (height x width x length x d6)is the most commonly used in clinical practice since it is rapid, more easily performed than planimetry and has been shown to have high correlation coefficients with actual prostate volume.2 To our knowledge no study has assessed the variation between observers and between repeated measurements by 1 observer with this technique.
RESULTS
MATERIALS AND METHODS
We studied 115 patients 42 to 81 years old (mean age 65) undergoing transrectal ultrasound for routine urological assessment. All examinations were performed using an ultrasound scanner with a 7.5 MHz. probe. The prostate was scanned in the transverse and sagittal planes, and the height, width and length were measured. Intra-observer variation was assessed in 40 patients. Two measurements were performed by 1 observer (T.S. B.) at the start and end of scanning but before any biopsies were obtained. Each individual measurement was immediately stored in the scanner memory. Volumetric calculations were performed only after all measurements had been stored and the patient had left the department. For interobserver variation 75 patients Were assessed by 2 observers (T.S.B. and J. M. R.). Measurements were taken consecutively with onIy 1 observer in the room a t any time and both observers were blinded to the results of each other. Both observers were urological trainees with considerable experience in transrectal ultrasound. Volumes were estimated using the prolate ellipse formula. The data were analyzed by examining paired differences, and Accepted for publication October 20, 1995.
Transrectal ultrasound was well tolerated by all patients and prostatic volumes determined during both parts of the study were comparable. Using the mean across the 2 observers, prostatic volumes in the interobserver variabilities group ranged from 15.5 to 95.4 ml. (median 42.4,75 patients). In the 1 observer group, using the mean of the 2 measurements, the volumes ranged from 18.9 to 87.0 ml. (median 39.8,40 patients). Interobserver variation. Mean differences between observers in the measurements of height, width and length were 0.0 5 2.7 mm., -0.1 2 2.6 mm. and 0.1 2 3.9 mm., respectively, demonstrating that most interobserver variation occurred in the measurement of prostatic length. The overall mean prostatic volumes were similar between the 2 observers (mean 44.2 ? 18.2ml. for observer 1 and 44.5 -C 19.7ml. for observer 2).The paired differences between the volume measurements derived from the 2 observers ranged from - 16.1 to 12.7 ml. Mean difference in volume measurement between the 2 observers was only -0.3 ml., demonstrating that there was no consistent bias. The standard deviation of these differences was 5.3 ml., and the limits of agreement (mean 2 2 standard deviations) for the differences in prostatic volume were - 10.9 to 10.3 ml. for the entire range (fig. 1). For mean prostatic volumes less than 50 ml. the limits of agreement were -8.1 to 7.2ml. Mean difference in volume estimation in this series was 9.8% (range 0 to 28.9).
1299
1300
DE'EXMINATION OF PROSTATIC VOLUME WITH TRANSRECTAL ULTRASOUND
* ..
,I
~... ..........................................
0
........... . : :- .
*
-'O
-16
m .
..?..
0......* .... ......................................... * *.. * * +
I
................................
o
to
m
a
ua*2sD I
*:
**'
w-nM)
.........................
. . . a**. --... . . 4l ..:.2: . . ..................................... o
o
w
10
m
m
I_
Yn-m
l
tm
mId) I
the variation is less marked when measurements are performed by 1observer (limits of agreement -6.5 and 6.7 ml. compared to - 10.9 and 10.3 ml. for 2 observers). As expected, the variations in measurement in both groups increased in proportion with gland size. Differences between 2 observers were most marked in the measurement of prostatic length (standard deviation 3.9 compared to 2.7 and 2.6 for height and width, respectively). No such difference was observed in measurements by 1observer, which presumably reflects differences in interpretation of the exact location of the bladder neck and prostatic apex. With 2 observers the mean difference in volume estimation was approximately 10%(range 0 to 28.9), which was more than twice that for 1 observer. CONCLUSIONS
FIG. 1. Difference in prostatic volumes (VI measured by 2 observers against mean prostatic volume. SD,standard deviation.
These results demonstrate that there is considerable variation in repeated prostatic volume measurements, as determined by the prolate ellipse formula, using transrectal ultrasound. It is interesting to note that the differences in Intra-observer variation. Mean differences among height, prostatic volume measurement obtained by 2 observers are width and length measurements were 0.4 2 2.1 mm., 0.5 ? similar in magnitude to those reported to occur with drug 1.6 mm. and -0.7 ? 2.1 mm.,respectively. The paired dif- treatments that shrink the prostate.8 We conclude that for ferences between the 2 volume measurements made by 1 longitudinal studies of prostatic volume 1observer should be observer ranged from -6.6 to 10.6 ml. Mean difference in used when possible. The results of any such study should volume measurement between the 2 observations was 0.1 2 be interpreted in light of these variations. Finally, we recom3.3 ml. Therefore, the limits of agreement (mean 2 2 stan- mend more research into the causes of variation so that dard deviations) for the entire range of prostatic volumes procedures to reduce them can be developed and evaluated. were -6.5 to 6.7 ml. (fig. 2). For mean prostatic volumes less REFERENCES than 50 ml. the limits of agreement were -4.3 to 4.5 ml. Mean difference in volume estimation in this series was 4-6% 1. Hastak, S. M.,Gammelgaard, J. and Holm, H. H.: Transrectal ultrasonic volume determination of the prostate-a preopera(range 0 to 15.7). DISCUSSION
Measurement of prostatic volume using transrectal ultrasound is easy to perform and well tolerated by patients. The accuracy of the procedure in relation to actual measured prostatic volume has been addressed previously.2-6 We have shown that there is variation in measurements performed serially by 1observer and by 2 different observers. However,
m 16
I
-11
1
-I
-'a
1I
FIG. 2. Differencein prostatic volumes (v) measured by 1 observer against mean prostatic volume. SD,standard deviation.
tive and postoperative study. J. Urol., 127: 1115,1982. 2. Tems, M. K. and Stamey. T. A.: Determination of prostate volume by transrectal ultrasound. J. Urol., 145: 984,1991. 3. Jones, D.R., Roberta,E. E., Griffiths, G. J., Parkinson, M. C., Evans, K. T. and Peeling, W. B.: Assessment of volume measurement of the prostate using per-rectal ultrasonography. Brit. J. Urol., &4: 493,1989. 4. Myschetzky, P.S., Suburu, R. E., Kelly, B. S., Jr., Wilson, M. L., Chen, S. C. and Lee, F.: Determination of prostate gland volume by transrectal ultrasound: correlation with radical prostateetamy specimens. Scand. J. Urol. Nephrol., suppl., 137: 107, 1991. 5. Yip, Y. L., Chan, C. W., Li, C.K., Chu, V. and Lau, M. L.: Quantitative analysis of the accuracy of linear array transrectal ultrasound in measurement of the prostate. Brit. J. Urol., 67: 79,1991. 6. Littrup, P. J., Williams, C. R., Egglin, T. K. and Kane, R. A,: Determination of prostatic volume with transrectal US for cancer screening. Part 11. Accuracy of in vitro and in vivo techniques. Radiology, 179 49, 1991. 7. Bland, J. M. and Altman, D. G.: Statistical methods for the assessing agreement between two methods of clinical measurement. Lancet, 1: 307, 1986. 8. Gormely, G. J., Stoner, E., Bruskewitz, R. C., Imperato-McGinley, J., Walsh, P. C., McConnell, J. D., Andriole, G. L., Geller, J., Bracken, B. R., Tenover, J. S., Vaughan, E. D., Pappas, F., Taylor, A,, Binkowitz, B. and Ng, J.: The effect of Finasteride in men with benien Drostatic hyperplasia. The Finasteride Study Group. New Eigl: J. Med., 327: 1185,1992.