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Determination of Prostate Volume by Transrectal Ultrasound
0022-534 7/91/1455-0984$03.00/0 THE JOURNAL OF UROLOGY Copyright© 1991 by AMERICAN UROLOGICAL ASSOCIATION, INC.
Vol. 145, 984-987, May 1991 Printed in U.S.A.
DETERMINATION OF PROSTATE VOLUME BY TRANSRECTAL ULTRASOUND MARTHA K. TERRIS*
AND
THOMAS A. STAMEY
From the Department of Urology, Stanford University, Stanford, California
ABSTRACT
Estimation of prostate gland volume with transrectal ultrasound may provide important information in the evaluation of benign and malignant prostatic diseases. To determine the most accurate means of volume estimation 150 patients underwent transrectal ultrasound with 15 separate methods of volume estimation. All patients underwent subsequent radical prostatectomy or cystoprostatectomy. Prostate specimen weights were compared with the results of each volume estimation method. Step-section planimetry, previously assumed to be the most accurate means of volume measurement, exhibited a Pearson correlation coefficient of 0.93. The elliptical volume, widely used as an alternative to planimetry, demonstrated a correlation coefficient of 0.90. The most accurate method to estimate prostate weight (r = 0.94) was a variation of the prolate spheroid formula, expressed as 1r/6 (transverse dimension) 2 (anteroposterior dimension). When different volume ranges were considered, this prolate spheroid formula provided the closest estimate of weight in glands of less than 40 gm. and those in the 40 to 80 gm. range. The most accurate method to estimate prostates weighing greater than 80 gm. was the formula 1r/6 (transverse dimension)3. KEY WORDS: ultrasonography, prostate
Estimation of prostate volume may be useful in a variety of clinical settings. For example, a precise estimate of the amount of benign prostatic hyperplasia would help to determine the appropriate therapy as well as assist in the interpretation of serum prostate specific antigen levels for the presence of cancer.1·2 Also, the decrease in prostate mass after hormonal manipulation or-radiation therapy can be used as an indication of therapeutic efficacy. 3- 9 An accurate estimation of prostate volume by transrectal ultrasound has been the goal of many researchers. Several studies have been performed but each uses only 1 or 2 methods to estimate volume. 1-20 The majority either have no controls, or are based upon prior ultrasound examinations or transurethral resection weights and not entire prostates. 1 - 19 Many evaluate only nonhuman subjects. 18• 19 Most of the studies reported to date have estimated prostate volume by using low frequency transabdominal probes rather than contemporary high frequency transrectal ultrasound. 2 •10 • 11· 20 In this study we evaluate all of the currently used methods of transrectal sonographic volume determination as well as several derivations from those methods. In each of 150 patients 15 ultrasound volume estimates are compared to the actual prostate weights obtained from radical prostatectomy or radical cystoprostatectomy specimens.
At the area of greatest transverse diameter in the axial plane the anteroposterior (fig. 1) and transverse (fig. 2) dimensions of the prostate were measured and recorded. The axial transducer was then mounted onto a ratcheted stepping device (Brue! & Kjaer model UA 0651) allowing measurement of the cephalocaudal dimension of the prostate by recording axial images from the base to the apex of the prostate at 2 mm. intervals (fig. 3). The distance traversed could be read from the scale on the stepping device. Surface area measurements were generated by the ultrasound unit computer from tracings of the prostate image circumference at each of these 2 mm. stepped intervals (fig. 3). Care was taken not to include the seminal vesicles in these measurements. Sagittal scanning was then performed using the 7.0 MHz. sector scanner (Bruel & Kjaer model 8537). Cephalocaudal measurement was repeated by measuring the distance from the base to the apex in the midline sagittal view (fig. 4). All measurements were performed by a single sonographer before any prostate biopsies or other manipulations that might introduce artifacts. A constant volume of 30 cc was used in the water path balloon to avoid variations in measurement by varying degrees of prostatic compression by the balloon. An
METHODS AND MATERIALS
In 150 men 31 to 79 years old (average age 65.2 years) transrectal ultrasound of the prostate was performed followed by radical prostatectomy for adenocarcinoma of the prostate (123) or radical cystoprostatectomy for transitional cell carcinoma of the bladder (27). To avoid selection bias all patients who presented to our urology clinic for transrectal ultrasound evaluation underwent complete volumetric analysis. The first 150 patients to undergo removal of the prostate were entered into the study. The Bruel & Kjaer 1846 console was used for all ultrasound examinations. Each patient was initially scanned transversely with the 7.0 MHz. axial transducer (Bruel & Kjaer model 1850). Accepted for publication October 12, 1990. Supported in part by the Richard M. Lucas Cancer Foundation. *Requests for reprints: Department of Urology (8287), Stanford University Medical Center, Stanford, California 94305-5118. 984
I
I
I/
FIG. 1. On axial images section of approximately largest transverse diameter is identified. In this view anteroposterior dimension is measured in midline from rectal surface of gland to most anterior aspect.
UL'TRASOUl'lD DETERM!f~ATION- OF PROSTATE VVEIGHT
985
\
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FIG. 2. Transverse diameter also is measured in axial plane on same image from which anteroposterior dimension was obtained. Distance from most lateral aspect of gland, right to left, is noted.
0.2 cm \
\ \ \ \
FIG. 3. Cephalocaudal dimension in axial plane is measured by mounting ultrasound probe in ratcheted stepping device, which allows images to be taken in 2 mm. increments. Total distance traversed by stepping device when imaging from base to apex then is measured providing cephalocaudal dimension. To obtain volume by step-section planimetry, this process is repeated. At each 2 mm. stepped image area of prostate is calculated by tracing its circumference. Sum of these areas is multiplied by 2 mm. stepping interval to calculate volume.
optional approach to this problem would be the use of a biplane probe that maintains constant fluid volume content during transition from transverse to sagittal views. From the aforementioned measurements, a variety of methods were applied to calculate the prostatic volume. The cephalocaudal dimension as measured in the axial plane was used in these formulas. Step-section planimetry. In this method volume is calculated by taking the sum of the prostate image surface areas measured at each step and multiplying by the stepping interval (2 mm.). Elliptical volume. The formula n/6 (transverse diameter X anteroposterior diameter x cephalocaudal diameter) was applied to the dimensions measured during the ultrasound examination based on the assumption that the prostate gland has an elliptical shape. Spherical volume. This formula, 1r/6 (diameter) 8 , assumes that the prostate is a sphere. Each of the sonographically measured dimensions, transverse, anteroposterior and cephalocaudal, was separately used in this formula as the spherical diameter. This formula also was applied to a mean diameter computed for all possible combinations of 2 of these 3 dimen sions (transverse and anteroposterior, anteroposterior and ce-
FIG. 4. As alternative to stepping device, cephalocaudal measurements can be obtained in sagittal plane. Distance from base at junction with seminal vesicles to apex at junction to distal urethra is measured in midline view.
phalocaudal, and cephalocaudal and transverse) and the mean diameter of all 3 dimensions. Prolate spheroid volume. Based on the assumption that the prostate is a prolate spheroid (oblong), the formula 1r/6 (major axis) 2 (minor axis) was applied to all possible combinations of the sonographically determined dimensions. Thus, 15 different sonographic volume estimates were prospectively calculated for each of the 150 patients. Each calculation resulted in a volume expressed in cubic centimeters. After radical prostatectomy the seminal vesicles were removed at the prostatic base and the prostate was weighed on a calibrated digital scale (Mettler PE 3600). After radical cystoprostatectomy the bladder and seminal vesicles were removed, and the prostate was likewise weighed. Since the specific gravity of prostatic tissue is 1.050, 14 the prostate weight in grams was directly compared to each of the volume estimates in cubic centimeters calculated from the ultrasound images. The dimensions of the prostate specimens were also measured. The greatest transverse and anteroposterior dimensions, and the midline cephalocaudal dimension were recorded. Pearson correlation coefficients, average error and standard deviation for each volume estimate were computed. Pearson correlation coefficients between the sonographically determined dimensions and the actual specimen dimensions were also calculated. Since the prostate gland appears to acquire different configurations with different volume ranges, the cases were subdivided into those less than 40, 40 to 80 and greater than 80 gm. Pearson correlation coefficients are subject to artifactual depression when subpopulations are observed separately from the entire population. Therefore, only the mean and standard deviation of the absolute error were used to compare these volume groups. RESULTS
The transrectal ultrasound measurement of the transverse diameter averaged 4. 7 cm. (range 1.8 to 7.2 cm.), the average anteroposterior diameter was 3.0 cm. (range 1.3 to 6.6 cm.), the average axial cephalocaudal diameter was 4.0 cm. (range 3.0 to 7.2 cm.) and the average· sagittal cephalocaudal diameter was 3.4 cm. (range 2.0 to 5.6 cm.). When correlated with the
986
TERRIS AND STAMEY
dimensions of the actual prostate specimens the transverse and anteroposterior dimensions correlated favorably with coefficients of 0.78 and 0.79, respectively. The cephalocaudal dimension, by either method of sonographic measure, correlated poorly with the cephalocaudal dimension of the specimen. The cephalocaudal dimension by axial determination exhibited a correlation coefficient of 0.4 7, whereas that obtained in the sagittal plane correlated with a coefficient of 0.37. The volume computation methods were applied to these dimensions for each of the 150 patients. The prostatectomy specimens ranged from 14 to 149.3 gm., with an average of 44.9 gm. The correlation of each of the 15 methods of volume determination with the prostate weight is shown in the table. The step-section planimetry method correlated with the prostate volume, with a correlation coefficient of 0.93. The prolate spheroid volume using the transverse diameter as the major axis and the anteroposterior diameter as the minor axis demonstrated the best over-all correlation with the prostate weight, with a correlation coefficient of 0.94. The average error of stepsection planimetry was greater than that of the prolate spheroid volume (9.4 and 8.8 gm., respectively). The step-section planimetry volume underestimated the weight of the prostate in 129 of 150 cases (86%), whereas 111 (74%) were underestimated by the prolate spheroid volume. The elliptical volume formula, the calculation most widely used to estimate prostate volume, exhibited a correlation coefficient of only 0.90 when compared to the prostate gland weight. In contrast to the more accurate methods, this formula tended to overestimate the volume in 135 of 150 cases (90%), with an average error of 12.1 gm. Some of the error encountered may be a result of a decrease in prostatic blood volume after ligation of the blood supply to the gland during surgery. As seen with the over-all correlations, prostates weighing less than 40 and those 40 to 80 gm. were best estimated by the prolate spheroid formula: 1r/6 (transverse diameter) 2 (anteroposterior dimension). The spherical formula 1r/6 (transverse
dimension) 3 was most accurate for glands weighing more than 80 gm. DISCUSSION
The step-section planimetry method of volume determination is accurate (correlation coefficient 0.93) but it is extremely time-consuming, is tedius for the sonographer and prolongs the discomfort of the examination for the patient. In addition, this method requires sophisticated, expensive software and cumbersome hardware to execute. This study reveals that various formula-derived volumes can be used to calculate volume. The ellipsoid volume calculation, 1r/6 (transverse diameter X anteroposterior diameter X cephalocaudal diameter), has been used extensively as an easy alternative to step-section planimetry. The fact that it is slightly less accurate has been assumed but not documented. A large portion of the error in the application of this ellipsoid formula is the inclusion of the cephalocaudal dimension. This measurement is technically difficult, since the point of juncture between the prostatic apex and distal urethra frequently is poorly visualized. Likewise, the definition between the base of the prostate, and the seminal vesicles and bladder neck is not often well seen. These problems are compounded by the fact that many sonographers obtain this measurement during sagittal scanning with a sector transducer, which introduces inaccuracy due to the inherently poor lateral resolution and distortion from refraction. We attempted to avoid these problems by measuring the cephalocaudal dimension with axial stepped images instead of a sector scanner. Regardless of this adjustment, the cephalocaudal dimension still exhibited an extremely poor correlation with that of the specimen. It is not surprising, therefore, that the best methods to calculate prostate weight did not include the cephalocaudal dimension at all. The best formula to estimate prostate volume is a variation of the prolate spheroid formula using the transverse diameter as the major axis and the anteroposterior diameter as the minor axis. The formula is expressed as 1r/6 (transverse diameter) 2
Correlation of transrectal ultrasound volume determinations with prostate specimen weights Vol. Category Total Correlation Coefficient
Av. Error ± Standard Deviation (gm.)
<40 cc Av. Error ± Standard Deviation (gm.)
40-80 cc Av. Error ± Standard Deviation (gm.)
>80 cc Av. Error ± Standard Deviation (gm.)
0.93 0.90
9.4 ± 7.5 12.1 ± 8.8
8.6 ± 7.0 8.4 ± 5.0
10.9 ± 8.1 14.5 ± 7.2
17.6 ± 9.8 26.4 ± 13.8
0.88 0.75 0.61 0.90
17.6 26.6 17.1 11.7
0.85
18.1 ± 12.0
12.3 ± 6.5
22.2 ± 8.6
39.1 ± 20.2
0.84
11.0 ± 9.2
11.0 ± 9.1
10.1 ± 8.9
17.0 ± 9.3
0.91
10.8 ± 8.2
7.4 ± 4.8
13.0 ± 7.0
24.4 ± 13.6
0.94
8.8 ± 7.1
6.6 ± 4.8
9.8 ± 6.4
16.2 ± 9.2
0.88
18.9 ± 10.3
14.6 ± 5.5
21.8 ± 8.6
34.7 ± 19.1
0.87
10.8 ± 8.9
11.5 ± 9.3
10.1 ± 8.3
23.2 ± 13.8
0.86
22.3 ± 11.6
16.4 ± 5.9
26.3 ± 8.0
44.1 ± 19.3
0.76
12.7 ± 10.5
11.1 ± 8.8
11.7 ± 10.2
27.7 ± 11.0
0.80
16.6 ± 12.0
10.9 ± 6.4
20.2 ± 9.1
39.1 ± 19.0
Method of Measure
Step section ,r/6(transverse diameter x anteroposterior diameter X cephalocaudal diameter) ,r/6(transverse diameter)' ,r /6(anteroposterior diameter)' ,r/6(cephalocaudal diameter)' ,r/6[(transverse diameter+ anteroposterior diameter)/2] 3 ,r/6[(anteroposterior diameter+ cephalocaudal diameter)/2] 3 ,r/6[(transverse diameter+ cephalocaudal diameter)/2] 3 ,r/6[(transverse diameter+ anteroposterior diameter + cephalocaudal diameter)/3] 3 ,r/6(transverse diameter )2 (anteroposterior diameter) ,r /6(anteroposterior diameter )2( transverse diameter) ,r/6(transverse diameter )2 ( cephalocaudal diameter) ,r /6(anteroposterior diameter )2 ( cephalocaudal diameter) ,r/6(cephalocaudal diameter )2 ( transverse diameter) ,r /6(cephalocaudal diameter )2 ( anteroposterior diameter)
± ± ± ±
12.6 12.6 14.0 8.1
16.4 20.7 13.1 8.7
± ± ± ±
11.9 5.8 10.5 4.7
18.2 30.8 17.7 13.5
± ± ± ±
13.0 9.1 12.7 7.4
15.9 47.7 42.1 23.9
± 9.1 ± 25.0 ± 14.7
± 14.1
Average weight (range) was 44.9 gm. (14.0 to 149.3 gm.) for 150 total specimens, 30.8 gm. (14.0 to 39.5 gm.) for 83 specimens less than 40 cc, 51.5 gm. (40.7 to 76.6 gm.) for 56 specimens 40 to 80 cc and 110.2 gm. (87.0 to 149.3 gm.) for 11 specimens greater than 80 cc.
ULTRASOUND DETERMINATION OF PROSTATE WEIGHT
diameter). This formula also was optimal for prostates weighing less than 80 gm. In large glands, more than 80 gm., the spherical volume formula using the transverse diameter, 7r/6 (transverse diameter)'\ provided the best estimation of prostate weight. The anteroposterior and transverse dimensions are easily and rapidly obtained by ultrasound equipment with even the simplest of software. With the calculations presented, prostate volume can be quickly estimated. The importance of the transverse dimension in these calculations is interesting, since this is the dimension roughly measured by digital rectal examinations. Therefore, the fact that many experienced urologists can estimate volume well by digital examination is not surprising, especially in glands of more than 80 gm. Prostate gland volume estimations have proved to be useful in the clinical evaluation of benign hyperplasia, and response to hormonal and radiation therapy. 1-9 Other potential uses include the evaluation of patients with elevated prostate specific antigen levels, which may be explanable by the prostate size alone, and the pre-treatment determination of the volume of cancer within the prostate gland, which can provide important prognostic information and influence treatment decisions.21 Drs. Christos E. Constantinou, Iain Johnstone and Mark Mitchell assisted in the computations performed in this study. REFERENCES 1. Jakobsen, H., Torp-Pederson, S. and Juul, N.: Ultrasonic evaluation of age-related human prostatic growth and development of benign prostatic hyperplasia. Scand. J. UroL NephroL, suppl., 107: 26, 1988. 2. Roehrborn, C. G., Chinn, H. K. W., Fulgham, P. F., Simpkins, K. L. and Peters, P. C.: The role of transabdominal ultrasound in the preoperative evaluation of patients with benign prostatic hypertrophy. J. UroL, 135: 1190, 1986. 3. Clements, R., Griffiths, G. J., Peeling, W. B. and Edwards, A. M.: Transrectal ultrasound in monitoring response to treatment of prostate disease. Urol. Clin. N. Amer., 16: 735, 1989. 4. Stone, N. N.: Flutamide in treatment of benign prostatic hypertrophy. Urology, 34: 64, 1989. 5. Brandt, B., Menu, G., el Khansa, A. and Lardennois, B.: Monitoring of prostate volume by ultrasound in hormonally treated prostate cancer. Prog. Clin. Biol. Res., 243B: 13, 1987. 6. Peters, C. A. and Walsh, P. C.: The effect of nafarelin acetate, a luteinizing-hormone-releasing hormone agonist, on benign prostatic hyperplasia. New Engl. J. Med., 317: 599, 1987.
987
7. Fujino, A. and Scardino, P. T.: Trans,ectal ultrasonography for prostatic cancer: its value in staging and monitoring the response to radiotherapy and chemotherapy. J. Urol., 133: 806, 1985. 8. Watanabe, H., Ohe, H., Ando, K., Sawamura, Y., Niijima, T., Nakamura, S., Orikasa, S., Tanahashi, Y., Imamura, K. and Yoshida, H.: The effect of estramustine phosphate on prostatic cancer estimated by transrectal ultrasonotomography. Prostate, 2: 155, 1981. 9. Carpentier, P. J., Schroeder, F. H. and Schmitz, P. I. M.: Transrectal ultrasonometry of the prostate~the prognostic relevance of volume changes under endocrine management. World J. U rol., 4: 159, 1986. 10. Henneberry, M., Carter, M. F. and Neiman, H. L.: Estimation of prostatic size by suprapubic ultrasonography. J. Urol., 121: 615, 1979. 11. Walz, P.H., Wenderoth, U. and Jacobi, G. H.: Suprapubic transvesical sonography of the prostate: determination of prostate size. Eur. Urol., 9: 148, 1983. 12. Hastak, S. M., Gammelgaard, J. and Holm, H. H.: Transrectal ultrasonic volume determination of the prostate~a preoperative and postoperative study. J. Urol., 127: 1115, 1982. 13. Miyazaki, Y., Yamaguchi, A. and Hara, S.: The value of transrectal ultrasonography in preoperative assessment for transurethral prostatectomy. J. Urol., 129: 48, 1983. 14. Watanabe, H., Igari, D., Tanahashi, Y., Harada, K. and Saitoh, M.: Measurements of size and weight of prostate by means of transrectal ultrasonotomography. Tohoku J. Exp. Med., 114: 277, 1974. 15. Styles, R. A., Neal, D. E. and Powell, P. H.: Reproducibility of measurement of prostatic volume by ultrasound. Comparison of transrectal and transabdominal methods. Eur. Urol., 14: 266, 1988. 16. Okafor, P. I. S., Wild, S. R., Beynon, L. L. and Chisholm, G. D.: Progress in transrectal ultrasonography for prostatic disease. Brit. J. UroL, 55: 721, 1983. 17. Bartsch, G., Egender, G., Hiibscher, H. and Rohr, H.: Sonometrics of the prostate. J. Urol., 127: 1119, 1982. 18. Miyashita, H., Watanabe, H., Ohe, H., Saitoh, M., Oogama, Y. and Iijima, S.: Transrectal ultrasonotomography of the canine prostate. Prostate, 5: 453, 1984. 19. Berry, S. J., Sterner, R., Coffey, D. S. and Ewing, L. L.: Methods for monitoring canine prostate size: internal and external caliper measurements. Prostate, 6: 303, 1985. 20. Vilmann, P., Haneke, S., Strange-Vognsen, H. H., Nielsen, K. and Sesrensen, S. M.: The reliability of transabdominal ultrasound scanning in the determination of prostatic volume. An autopsy study. Scand. J. Urol. Nephrol., 21: 5, 1987. 21. Stamey, T. A. and Kabalin, J. N.: Prostate specific antigen in the diagnosis and treatment of adenocarcinoma of the prostate. I. Untreated patients. J. Urol., 141: 1070, 1989.
Vol. 145, 984-987, May 1991 Printed in U.S.A.
DETERMINATION OF PROSTATE VOLUME BY TRANSRECTAL ULTRASOUND MARTHA K. TERRIS*
AND
THOMAS A. STAMEY
From the Department of Urology, Stanford University, Stanford, California
ABSTRACT
Estimation of prostate gland volume with transrectal ultrasound may provide important information in the evaluation of benign and malignant prostatic diseases. To determine the most accurate means of volume estimation 150 patients underwent transrectal ultrasound with 15 separate methods of volume estimation. All patients underwent subsequent radical prostatectomy or cystoprostatectomy. Prostate specimen weights were compared with the results of each volume estimation method. Step-section planimetry, previously assumed to be the most accurate means of volume measurement, exhibited a Pearson correlation coefficient of 0.93. The elliptical volume, widely used as an alternative to planimetry, demonstrated a correlation coefficient of 0.90. The most accurate method to estimate prostate weight (r = 0.94) was a variation of the prolate spheroid formula, expressed as 1r/6 (transverse dimension) 2 (anteroposterior dimension). When different volume ranges were considered, this prolate spheroid formula provided the closest estimate of weight in glands of less than 40 gm. and those in the 40 to 80 gm. range. The most accurate method to estimate prostates weighing greater than 80 gm. was the formula 1r/6 (transverse dimension)3. KEY WORDS: ultrasonography, prostate
Estimation of prostate volume may be useful in a variety of clinical settings. For example, a precise estimate of the amount of benign prostatic hyperplasia would help to determine the appropriate therapy as well as assist in the interpretation of serum prostate specific antigen levels for the presence of cancer.1·2 Also, the decrease in prostate mass after hormonal manipulation or-radiation therapy can be used as an indication of therapeutic efficacy. 3- 9 An accurate estimation of prostate volume by transrectal ultrasound has been the goal of many researchers. Several studies have been performed but each uses only 1 or 2 methods to estimate volume. 1-20 The majority either have no controls, or are based upon prior ultrasound examinations or transurethral resection weights and not entire prostates. 1 - 19 Many evaluate only nonhuman subjects. 18• 19 Most of the studies reported to date have estimated prostate volume by using low frequency transabdominal probes rather than contemporary high frequency transrectal ultrasound. 2 •10 • 11· 20 In this study we evaluate all of the currently used methods of transrectal sonographic volume determination as well as several derivations from those methods. In each of 150 patients 15 ultrasound volume estimates are compared to the actual prostate weights obtained from radical prostatectomy or radical cystoprostatectomy specimens.
At the area of greatest transverse diameter in the axial plane the anteroposterior (fig. 1) and transverse (fig. 2) dimensions of the prostate were measured and recorded. The axial transducer was then mounted onto a ratcheted stepping device (Brue! & Kjaer model UA 0651) allowing measurement of the cephalocaudal dimension of the prostate by recording axial images from the base to the apex of the prostate at 2 mm. intervals (fig. 3). The distance traversed could be read from the scale on the stepping device. Surface area measurements were generated by the ultrasound unit computer from tracings of the prostate image circumference at each of these 2 mm. stepped intervals (fig. 3). Care was taken not to include the seminal vesicles in these measurements. Sagittal scanning was then performed using the 7.0 MHz. sector scanner (Bruel & Kjaer model 8537). Cephalocaudal measurement was repeated by measuring the distance from the base to the apex in the midline sagittal view (fig. 4). All measurements were performed by a single sonographer before any prostate biopsies or other manipulations that might introduce artifacts. A constant volume of 30 cc was used in the water path balloon to avoid variations in measurement by varying degrees of prostatic compression by the balloon. An
METHODS AND MATERIALS
In 150 men 31 to 79 years old (average age 65.2 years) transrectal ultrasound of the prostate was performed followed by radical prostatectomy for adenocarcinoma of the prostate (123) or radical cystoprostatectomy for transitional cell carcinoma of the bladder (27). To avoid selection bias all patients who presented to our urology clinic for transrectal ultrasound evaluation underwent complete volumetric analysis. The first 150 patients to undergo removal of the prostate were entered into the study. The Bruel & Kjaer 1846 console was used for all ultrasound examinations. Each patient was initially scanned transversely with the 7.0 MHz. axial transducer (Bruel & Kjaer model 1850). Accepted for publication October 12, 1990. Supported in part by the Richard M. Lucas Cancer Foundation. *Requests for reprints: Department of Urology (8287), Stanford University Medical Center, Stanford, California 94305-5118. 984
I
I
I/
FIG. 1. On axial images section of approximately largest transverse diameter is identified. In this view anteroposterior dimension is measured in midline from rectal surface of gland to most anterior aspect.
UL'TRASOUl'lD DETERM!f~ATION- OF PROSTATE VVEIGHT
985
\
\
\
\
\
\
"------~~\
\
\
\
\
\ \ \
\
FIG. 2. Transverse diameter also is measured in axial plane on same image from which anteroposterior dimension was obtained. Distance from most lateral aspect of gland, right to left, is noted.
0.2 cm \
\ \ \ \
FIG. 3. Cephalocaudal dimension in axial plane is measured by mounting ultrasound probe in ratcheted stepping device, which allows images to be taken in 2 mm. increments. Total distance traversed by stepping device when imaging from base to apex then is measured providing cephalocaudal dimension. To obtain volume by step-section planimetry, this process is repeated. At each 2 mm. stepped image area of prostate is calculated by tracing its circumference. Sum of these areas is multiplied by 2 mm. stepping interval to calculate volume.
optional approach to this problem would be the use of a biplane probe that maintains constant fluid volume content during transition from transverse to sagittal views. From the aforementioned measurements, a variety of methods were applied to calculate the prostatic volume. The cephalocaudal dimension as measured in the axial plane was used in these formulas. Step-section planimetry. In this method volume is calculated by taking the sum of the prostate image surface areas measured at each step and multiplying by the stepping interval (2 mm.). Elliptical volume. The formula n/6 (transverse diameter X anteroposterior diameter x cephalocaudal diameter) was applied to the dimensions measured during the ultrasound examination based on the assumption that the prostate gland has an elliptical shape. Spherical volume. This formula, 1r/6 (diameter) 8 , assumes that the prostate is a sphere. Each of the sonographically measured dimensions, transverse, anteroposterior and cephalocaudal, was separately used in this formula as the spherical diameter. This formula also was applied to a mean diameter computed for all possible combinations of 2 of these 3 dimen sions (transverse and anteroposterior, anteroposterior and ce-
FIG. 4. As alternative to stepping device, cephalocaudal measurements can be obtained in sagittal plane. Distance from base at junction with seminal vesicles to apex at junction to distal urethra is measured in midline view.
phalocaudal, and cephalocaudal and transverse) and the mean diameter of all 3 dimensions. Prolate spheroid volume. Based on the assumption that the prostate is a prolate spheroid (oblong), the formula 1r/6 (major axis) 2 (minor axis) was applied to all possible combinations of the sonographically determined dimensions. Thus, 15 different sonographic volume estimates were prospectively calculated for each of the 150 patients. Each calculation resulted in a volume expressed in cubic centimeters. After radical prostatectomy the seminal vesicles were removed at the prostatic base and the prostate was weighed on a calibrated digital scale (Mettler PE 3600). After radical cystoprostatectomy the bladder and seminal vesicles were removed, and the prostate was likewise weighed. Since the specific gravity of prostatic tissue is 1.050, 14 the prostate weight in grams was directly compared to each of the volume estimates in cubic centimeters calculated from the ultrasound images. The dimensions of the prostate specimens were also measured. The greatest transverse and anteroposterior dimensions, and the midline cephalocaudal dimension were recorded. Pearson correlation coefficients, average error and standard deviation for each volume estimate were computed. Pearson correlation coefficients between the sonographically determined dimensions and the actual specimen dimensions were also calculated. Since the prostate gland appears to acquire different configurations with different volume ranges, the cases were subdivided into those less than 40, 40 to 80 and greater than 80 gm. Pearson correlation coefficients are subject to artifactual depression when subpopulations are observed separately from the entire population. Therefore, only the mean and standard deviation of the absolute error were used to compare these volume groups. RESULTS
The transrectal ultrasound measurement of the transverse diameter averaged 4. 7 cm. (range 1.8 to 7.2 cm.), the average anteroposterior diameter was 3.0 cm. (range 1.3 to 6.6 cm.), the average axial cephalocaudal diameter was 4.0 cm. (range 3.0 to 7.2 cm.) and the average· sagittal cephalocaudal diameter was 3.4 cm. (range 2.0 to 5.6 cm.). When correlated with the
986
TERRIS AND STAMEY
dimensions of the actual prostate specimens the transverse and anteroposterior dimensions correlated favorably with coefficients of 0.78 and 0.79, respectively. The cephalocaudal dimension, by either method of sonographic measure, correlated poorly with the cephalocaudal dimension of the specimen. The cephalocaudal dimension by axial determination exhibited a correlation coefficient of 0.4 7, whereas that obtained in the sagittal plane correlated with a coefficient of 0.37. The volume computation methods were applied to these dimensions for each of the 150 patients. The prostatectomy specimens ranged from 14 to 149.3 gm., with an average of 44.9 gm. The correlation of each of the 15 methods of volume determination with the prostate weight is shown in the table. The step-section planimetry method correlated with the prostate volume, with a correlation coefficient of 0.93. The prolate spheroid volume using the transverse diameter as the major axis and the anteroposterior diameter as the minor axis demonstrated the best over-all correlation with the prostate weight, with a correlation coefficient of 0.94. The average error of stepsection planimetry was greater than that of the prolate spheroid volume (9.4 and 8.8 gm., respectively). The step-section planimetry volume underestimated the weight of the prostate in 129 of 150 cases (86%), whereas 111 (74%) were underestimated by the prolate spheroid volume. The elliptical volume formula, the calculation most widely used to estimate prostate volume, exhibited a correlation coefficient of only 0.90 when compared to the prostate gland weight. In contrast to the more accurate methods, this formula tended to overestimate the volume in 135 of 150 cases (90%), with an average error of 12.1 gm. Some of the error encountered may be a result of a decrease in prostatic blood volume after ligation of the blood supply to the gland during surgery. As seen with the over-all correlations, prostates weighing less than 40 and those 40 to 80 gm. were best estimated by the prolate spheroid formula: 1r/6 (transverse diameter) 2 (anteroposterior dimension). The spherical formula 1r/6 (transverse
dimension) 3 was most accurate for glands weighing more than 80 gm. DISCUSSION
The step-section planimetry method of volume determination is accurate (correlation coefficient 0.93) but it is extremely time-consuming, is tedius for the sonographer and prolongs the discomfort of the examination for the patient. In addition, this method requires sophisticated, expensive software and cumbersome hardware to execute. This study reveals that various formula-derived volumes can be used to calculate volume. The ellipsoid volume calculation, 1r/6 (transverse diameter X anteroposterior diameter X cephalocaudal diameter), has been used extensively as an easy alternative to step-section planimetry. The fact that it is slightly less accurate has been assumed but not documented. A large portion of the error in the application of this ellipsoid formula is the inclusion of the cephalocaudal dimension. This measurement is technically difficult, since the point of juncture between the prostatic apex and distal urethra frequently is poorly visualized. Likewise, the definition between the base of the prostate, and the seminal vesicles and bladder neck is not often well seen. These problems are compounded by the fact that many sonographers obtain this measurement during sagittal scanning with a sector transducer, which introduces inaccuracy due to the inherently poor lateral resolution and distortion from refraction. We attempted to avoid these problems by measuring the cephalocaudal dimension with axial stepped images instead of a sector scanner. Regardless of this adjustment, the cephalocaudal dimension still exhibited an extremely poor correlation with that of the specimen. It is not surprising, therefore, that the best methods to calculate prostate weight did not include the cephalocaudal dimension at all. The best formula to estimate prostate volume is a variation of the prolate spheroid formula using the transverse diameter as the major axis and the anteroposterior diameter as the minor axis. The formula is expressed as 1r/6 (transverse diameter) 2
Correlation of transrectal ultrasound volume determinations with prostate specimen weights Vol. Category Total Correlation Coefficient
Av. Error ± Standard Deviation (gm.)
<40 cc Av. Error ± Standard Deviation (gm.)
40-80 cc Av. Error ± Standard Deviation (gm.)
>80 cc Av. Error ± Standard Deviation (gm.)
0.93 0.90
9.4 ± 7.5 12.1 ± 8.8
8.6 ± 7.0 8.4 ± 5.0
10.9 ± 8.1 14.5 ± 7.2
17.6 ± 9.8 26.4 ± 13.8
0.88 0.75 0.61 0.90
17.6 26.6 17.1 11.7
0.85
18.1 ± 12.0
12.3 ± 6.5
22.2 ± 8.6
39.1 ± 20.2
0.84
11.0 ± 9.2
11.0 ± 9.1
10.1 ± 8.9
17.0 ± 9.3
0.91
10.8 ± 8.2
7.4 ± 4.8
13.0 ± 7.0
24.4 ± 13.6
0.94
8.8 ± 7.1
6.6 ± 4.8
9.8 ± 6.4
16.2 ± 9.2
0.88
18.9 ± 10.3
14.6 ± 5.5
21.8 ± 8.6
34.7 ± 19.1
0.87
10.8 ± 8.9
11.5 ± 9.3
10.1 ± 8.3
23.2 ± 13.8
0.86
22.3 ± 11.6
16.4 ± 5.9
26.3 ± 8.0
44.1 ± 19.3
0.76
12.7 ± 10.5
11.1 ± 8.8
11.7 ± 10.2
27.7 ± 11.0
0.80
16.6 ± 12.0
10.9 ± 6.4
20.2 ± 9.1
39.1 ± 19.0
Method of Measure
Step section ,r/6(transverse diameter x anteroposterior diameter X cephalocaudal diameter) ,r/6(transverse diameter)' ,r /6(anteroposterior diameter)' ,r/6(cephalocaudal diameter)' ,r/6[(transverse diameter+ anteroposterior diameter)/2] 3 ,r/6[(anteroposterior diameter+ cephalocaudal diameter)/2] 3 ,r/6[(transverse diameter+ cephalocaudal diameter)/2] 3 ,r/6[(transverse diameter+ anteroposterior diameter + cephalocaudal diameter)/3] 3 ,r/6(transverse diameter )2 (anteroposterior diameter) ,r /6(anteroposterior diameter )2( transverse diameter) ,r/6(transverse diameter )2 ( cephalocaudal diameter) ,r /6(anteroposterior diameter )2 ( cephalocaudal diameter) ,r/6(cephalocaudal diameter )2 ( transverse diameter) ,r /6(cephalocaudal diameter )2 ( anteroposterior diameter)
± ± ± ±
12.6 12.6 14.0 8.1
16.4 20.7 13.1 8.7
± ± ± ±
11.9 5.8 10.5 4.7
18.2 30.8 17.7 13.5
± ± ± ±
13.0 9.1 12.7 7.4
15.9 47.7 42.1 23.9
± 9.1 ± 25.0 ± 14.7
± 14.1
Average weight (range) was 44.9 gm. (14.0 to 149.3 gm.) for 150 total specimens, 30.8 gm. (14.0 to 39.5 gm.) for 83 specimens less than 40 cc, 51.5 gm. (40.7 to 76.6 gm.) for 56 specimens 40 to 80 cc and 110.2 gm. (87.0 to 149.3 gm.) for 11 specimens greater than 80 cc.
ULTRASOUND DETERMINATION OF PROSTATE WEIGHT
diameter). This formula also was optimal for prostates weighing less than 80 gm. In large glands, more than 80 gm., the spherical volume formula using the transverse diameter, 7r/6 (transverse diameter)'\ provided the best estimation of prostate weight. The anteroposterior and transverse dimensions are easily and rapidly obtained by ultrasound equipment with even the simplest of software. With the calculations presented, prostate volume can be quickly estimated. The importance of the transverse dimension in these calculations is interesting, since this is the dimension roughly measured by digital rectal examinations. Therefore, the fact that many experienced urologists can estimate volume well by digital examination is not surprising, especially in glands of more than 80 gm. Prostate gland volume estimations have proved to be useful in the clinical evaluation of benign hyperplasia, and response to hormonal and radiation therapy. 1-9 Other potential uses include the evaluation of patients with elevated prostate specific antigen levels, which may be explanable by the prostate size alone, and the pre-treatment determination of the volume of cancer within the prostate gland, which can provide important prognostic information and influence treatment decisions.21 Drs. Christos E. Constantinou, Iain Johnstone and Mark Mitchell assisted in the computations performed in this study. REFERENCES 1. Jakobsen, H., Torp-Pederson, S. and Juul, N.: Ultrasonic evaluation of age-related human prostatic growth and development of benign prostatic hyperplasia. Scand. J. UroL NephroL, suppl., 107: 26, 1988. 2. Roehrborn, C. G., Chinn, H. K. W., Fulgham, P. F., Simpkins, K. L. and Peters, P. C.: The role of transabdominal ultrasound in the preoperative evaluation of patients with benign prostatic hypertrophy. J. UroL, 135: 1190, 1986. 3. Clements, R., Griffiths, G. J., Peeling, W. B. and Edwards, A. M.: Transrectal ultrasound in monitoring response to treatment of prostate disease. Urol. Clin. N. Amer., 16: 735, 1989. 4. Stone, N. N.: Flutamide in treatment of benign prostatic hypertrophy. Urology, 34: 64, 1989. 5. Brandt, B., Menu, G., el Khansa, A. and Lardennois, B.: Monitoring of prostate volume by ultrasound in hormonally treated prostate cancer. Prog. Clin. Biol. Res., 243B: 13, 1987. 6. Peters, C. A. and Walsh, P. C.: The effect of nafarelin acetate, a luteinizing-hormone-releasing hormone agonist, on benign prostatic hyperplasia. New Engl. J. Med., 317: 599, 1987.
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