Accuracy is Key: Transrectal Ultrasound Versus Magnetic Resonance Imaging to Determine Prostatic Volume

A multitude of options can be used to image the prostate, but transrectal ultrasound and magnetic resonance imaging are the mainstays. Accuracy in prostate measurements is key to the preoperative planning of any surgical intervention planned on the prostate such as tissue resection or prostatectomy. Some evidence exists that magnetic resonance imaging is the most accurate modality, but given its cost, the clinical significance remains uncertain. This study aimed to compare transrectal ultrasound and magnetic resonance imaging prostate volumes with prostate gross specimens in men having tissue-confirmed prostate cancer who underwent robotic-assisted radical prostatectomy to determine accuracy in sizing.

This study was a retrospective analysis conducted on men who underwent robotic prostatectomy for prostate cancer and had a preoperative transrectal ultrasound and magnetic resonance imaging prostate size measurement (147 men in total) between 2012-2024. Patients were required to undergo robotic prostatectomy ≤2 months from the time of transrectal ultrasound and magnetic resonance imaging prostate-size determination.

Mean transrectal ultrasound was significantly smaller on average than magnetic resonance imaging prostate volume (-4.8 mL; p < 0.001). Transrectal ultrasound (-14.4 mL) and magnetic resonance imaging (-9.5 mL) were significantly smaller than actual prostate weight (p < 0.001). The mean difference between transrectal ultrasound volume and prostate weight was significantly greater from the mean difference between magnetic resonance imaging volume and prostate weight (p < 0.001). Participants with high-grade (Gleason grade ≥8) and non-high-grade cancer (Gleason grade <8) did not differ in actual prostate weight after robotic radical prostatectomy or in estimated prostate volume (p > 0.05).

Although transrectal ultrasound appears to be less accurate than magnetic resonance imaging in estimating prostate size, the difference is small, and it remains an adequate imaging modality in patients with prostate cancer.