Purpose: To evaluate the accuracy of the conventional transrectal ultrasonographic (US) prostate volume formula and determine whether a more accurate volume formula, calculated on the basis of prostate shape as observed at planimetry, can be described for the majority of prostate glands.
Materials and methods: This retrospective study was institutional review board approved, with waiver of informed consent. A total of 138 consecutive patients who underwent prostate brachytherapy were included for analysis. Prostate volume was estimated by using the conventional prolate ellipsoid formula (length x height x width x [pi/6]). A reference standard for prostate volume was calculated by using planimetry. The mean prostate shape was assessed by using three-dimensional volume-rendering of the planimetric images. The prostate shapes were evaluated to determine the best-fit mathematic formula for accurate volume estimation. Statistical analyses were performed by using Pearson correlation, paired Student t test, Bland-Altman plots, and concordance correlation coefficient.
Results: Planimetric data showed the majority of prostate glands to be more bullet-shaped than ellipsoid. Only 13.3% of volumes determined by using the conventional prolate ellipsoid formula were within 10% of the planimetric volume. The prolate ellipsoid formula underestimated volume by 17% on average (95% confidence interval: 14%, 19%). A mathematic formula representing a bullet shape (length x height x width x [pi/4.8]) was determined to best represent the majority of prostate glands presented for brachytherapy; 75% of volumes were within 10% of planimetric volume by using this formula. Concordance correlation coefficient increased from 0.87 to 0.95. Formula accuracy was particularly improved in prostate glands smaller than 55 cm(3) (P = .14).
Conclusion: A modified prostate volume formula that closely represents the shape of the prostate smaller than 55 cm(3) demonstrated improved volume measurement accuracy compared with the prolate ellipsoid formula used in men presenting for brachytherapy.
(c) RSNA, 2009.