A comparison of CT scan to transrectal ultrasound-measured prostate volume in untreated prostate cancer

Purpose: To compare CT and transrectal ultrasound (TRUS)-measured prostate volumes in patients with untreated prostate cancer. Methods and Materials: Between 1995 and 1999, 48 consecutive patients at the Portland Veterans Affairs Medical Center were treated with external beam radiotherapy. In 36 of these patients, TRUS and CT measurements of the prostate volume were obtained before treatment and <6 months apart. The TRUS volume was calculated using the prolate ellipsoid formula. The CT volume was calculated from the contours of the prostate drawn by one physician, who was unaware of the TRUS volume calculation, on axial CT images. Results: The TRUS and CT prostate volume measurements correlated strongly (Pearson's correlation coefficient = 0.925, 95% confidence interval 0.856-0.961, p < 0.0001). The CT volume was consistently larger than the TRUS volume by a factor of approximately 1.5. In men with a TRUS prostate volume less than the median (<28 cm³), the CT/TRUS volume ratio was 1.7, and it was 1.4 for men whose volume was greater than the median. The CT volumes were correlated similarly with the TRUS volumes regardless of the CT slice interval. Conclusion: A strong correlation was found between CT scan and TRUS measurement of the prostate volume; however, CT consistently overestimated the prostate volume by approximately 50% compared with TRUS.