Modified prostate volume algorithm improves transrectal US volume estimation in men presenting for prostate brachytherapy

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 · height · width · [π/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 • height · width · [π/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 cm³ (P = .14). Conclusion: A modified prostate volume formula that closely represents the shape of the prostate smaller than 55 cm³ demonstrated improved volume measurement accuracy compared with the prolate ellipsoid formula used in men presenting for brachytherapy.