Is it time to change the way we calculate prostate volume?

Several novel imaging methods came on the scene in the last few years (multiparametric MRI, PSMA-PET, among others) to evaluate prostatic diseases. However, we are still taking into account ultrasound measurements to estimate prostate volume. This piece of information is extremely valuable for clinicians seeing patients with potential prostatic diseases, helping them in both diagnoses and treatment.1 Numerous therapeutic decisions, particularly those related to benign prostatic hyperplasia (BPH) and prostate cancer are heavily influenced by prostate volume. Researchers from McGill University (Montreal, Canada) raised the question that the mathematical method we are routinely using to predict prostate volume (width x height x length x 0.52, the ellipsoid formula) is frequently inaccurate and are now proposing a new way to calculate this volume.2

Thus far, geometrical models were used to describe prostate shape, assuming the gland to be a prolate ellipsoid and more recently, a bullet. Also, pathology reports present prostate dimensions acquired after a period of formaldehyde fixation, which causes significant tissue shrinkage. In this study, the authors, for the first time, prospectively used fresh prostate specimens3 as a reference to define a new coefficient of 0.66, which performed better than the universally adopted coefficient of 0.52 from the ellipsoid formula to estimate prostate volumes.

The comparison of the 0.66 (new) coefficient with the ellipsoid and bullet coefficients showed superiority of the prostate-derived coefficient, performing better than the universally adopted 0.52 value and being closer to the bullet formula using 0.65 as the coefficient.4 As expected, very small or very large prostates will sometimes present erratic shapes and will not fit any standard formula. Another interesting experiment (using Archimedes’ Principle) was performed to properly define prostate density as 1.02 ± 0.01 g/cc, considering the fact that clinicians consistently believe prostate volume (cc) and prostate weight (g) are interchangeable measurements.

The study presents strong evidence and comprehensive mathematical analysis supporting its findings. This new coefficient not only provides a more accurate estimation of true prostate volumes than the universally established ellipsoid coefficient but also supports the suggestion that the prostate shape is closer to that of a bullet.

Written by: Murilo Luz, MD, Urologic Oncology Research Group, McGill University, Montreal, Canada; Robotic Surgery Program Director, Rede D’Or São Luiz, São Paulo,  Brazil

1. Nickel JC. Benign prostatic hyperplasia: does prostate size matter? Rev Urol. 2003;5(Suppl 4):S12–7.
2. Rodriguez E Jr, Skarecky D, Narula N, Ahlering TE. Prostate volume estimation using the ellipsoid formula consistently underestimates actual gland size. J Urol. 2008;179(2):501–3.
3. Schned AR, Wheeler KJ, Hodorowski CA, Heaney JA, Ernstoff MS, Amdur RJ, Harris RD. Tissue-shrinkage correction factor in the calculation of prostate cancer volume. Am J Surg Pathol. 1996;20(12):1501–6.
4. MacMahon PJ, Kennedy AM, Murphy DT, Maher M, McNicholas MM. Modified prostate volume algorithm improves transrectal US volume estimation in men presenting for prostate brachytherapy. Radiology. 2009;250(1):273–80.