Rectal Wall Invasion of Locally Advanced Prostate Cancer: Comparison of Magnetic Resonance Imaging with Transrectal Ultrasound

Locally advanced prostate cancer (LAPC) is a complex clinical problem, even in the absence of distant metastasis. Locally advanced prostate cancer generally consists of a bulky tumor invading pelvic organs, such as the seminal vesicles, bladder, ureters, rectum, and pelvic side wall. Traditional treatment modalities used for LAPC include radiation therapy, androgen ablation, transurethral prostatectomy (channeling), and chemotherapy. None of these treatments provides effective palliation because they fail to eliminate tumor bulk in the pelvis. Both transrectal ultrasound (TRUS) and MRI can be useful in examining the rectal wall. A study compared the accuracy of these two modalities in a series of 16 patients who underwent TPE and 24 who underwent cystoprostatectomy for palliation of symptomatic LAPC. The results showed that TRUS was highly specific and sensitive in detecting rectal wall invasion by LAPC, whereas MRI demonstrated 100% specificity but poor sensitivity. These results imply that MRI can reliably rule out rectal wall invasion when the two imaging examinations are performed preoperatively; MRI cannot reliably rule out rectal wall invasion if the TRUS is positive. Conversely, when the TRUS is negative and the MRI is positive, the likelihood of actual rectal wall invasion is 5%. Thus TRUS is the modality of choice for examining the rectal wall for evidence of tumor invasion, and, in most cases, MRI is noncontributory.