The role of imaging in prostate cancer.

For patients with prostate cancer, diagnostic imaging can play three roles: screening, staging, and monitoring. Bayesian analysis dictates that if the prior probability of cancer is relatively low or if the consequences of a false-positive result are unacceptable, the test must be optimally specific. If the prior probability of cancer is high or if the consequences of missing it are unacceptable, the test must be optimally sensitive. For screening, the consequences of a miss are slight, and the consequences of labeling an insignificant cancer significant are serious. Thus, a very specific test is required. No current imaging modality fulfills this criterion. For staging, the prior probability of significant disease is relatively high, and the consequences of a miss serious, so a very sensitive test is required. Transrectal sonography, plus biopsy under sonographic control, fulfills this criterion for local disease, as does a bone scan for bone metastases. For monitoring, the prior probability is high, and the consequences of a miss serious, so a very sensitive test is needed. The bone scan is sensitive for bone metastases. Although CT is not sensitive for detecting lymph node metastases, it has practical clinical advantages over other imaging modalities for monitoring purposes in that it can detect disease in multiple structures at once. It is the only test that can monitor prostate size, the size of the lymph nodes, and whether hydronephrosis or liver metastases are present all in the scope of one examination.