Prostatic volume may be measured by transrectal or transabdominal ultrasound. Traditionally, the prostate has been assumed to be an ellipsoid with V = ⁴⁄₃π(^a⁄₂)(^b⁄₂)(^c⁄₂) where a, b and c are the dimensions that you would measure on ultrasound. This can be simplified to 0.52(abc) because 0.52 = (⁴⁄₃π)/2³.

It has been shown that the ellipsoid formula underestimates prostate size in transrectal sonography^1,2. For transrectal ultrasonography, McMahon et al demonstrated that a bullet shape may better reflect the shape of the prostate and the formula used for caculation of volume was ^π⁄_4.8(lwh) or 0.65(lwh) where l is length, w is width, and h is height. This study was performed on a population of patients presenting for brachytherapy.

There have been variable reports of the correlation between transrectal and transabdominal ultrasound^3-7, and most demonstrate good correlation, but is it possible that the degree of prostatic hypertropy affects accuracy⁸ and that transabdominal ultrasound overestimates the volume³. The bladder should probably be filled with <400 ml of urine⁶.

In addition to volume measurements, the ratio of width to height may be used to estimate the degree of prostatic obstruction⁹.

References

1. MacMahon, P. J., Kennedy, A.-M., Murphy, D. T., Maher, M. & McNicholas, M. M. Modified prostate volume algorithm improves transrectal US volume estimation in men presenting for prostate brachytherapy. Radiology 250, 273–280 (2009).
2. Rodriguez, E., Skarecky, D., Narula, N. & Ahlering, T. E. Prostate volume estimation using the ellipsoid formula consistently underestimates actual gland size. J. Urol. 179, 501–503 (2008).
3. M Blanc, A. S. Prostatic volume: suprapubic versus transrectal ultrasonography in the control of benign prostatic hyperplasia. Radiol. Med. (Torino) 95, 182–7 (1998).
4. Kim, S. H. & Kim, S. H. Correlations between the Various Methods of Estimating Prostate Volume: Transabdominal, Transrectal, and Three-Dimensional US. Korean J. Radiol. 9, 134–139 (2008).
5. Huang Foen Chung, J. W. N. C. et al. Prostate Volume Ultrasonography: The Influence of Transabdominal versus Transrectal Approach, Device Type and Operator. Eur. Urol. 46, 352–356 (2004).
6. John Shyi Peng Yuen, J. T. K. N. Effects of bladder volume on transabdominal ultrasound measurements of intravesical prostatic protrusion and volume. Int. J. Urol. Off. J. Jpn. Urol. Assoc. 9, 225–9 (2002).
7. Malemo, K. et al. Validation of supra-pubic ultrasonography for preoperative prostate volume measurement in sub-Saharan Africa. Int. Urol. Nephrol. 43, 283–288 (2010).
Yang, C.-H., Wang, S.-J., Lin, A. T.-L., Jen, Y.-M. & Lin, C.-A. Evaluation of Prostate Volume by Transabdominal Ultrasonography With Modified Ellipsoid Formula at Different Stages of Benign Prostatic Hyperplasia. Ultrasound Med. Biol. 37, 331–337 (2011).
9. Watanabe, T. & Miyagawa, I. New simple method of transabdominal ultrasound to assess the degree of benign prostatic obstruction: size and horizontal shape of the prostate. Int. J. Urol. Off. J. Jpn. Urol. Assoc. 9, 204–209 (2002).