Choosing Radiologic Examinations for the Patient With Prostate Cancer
Written by Jeffrey H. Newhouse, M.D.
Prostate cancer is extremely prevalent, and screening for the disease is widespread. Radiologic examinations play a crucial role in the evaluation of prostate cancer patients, but indiscriminate use of imaging modalities would be exceedingly expensive, and so it is of great importance that use of radiologic examinations be limited to situations in which the likelihood that they will provide clinically important information is high enough to warrant their cost.
The major potential uses for radiologic imaging in prostate cancer are in (1) screening, (2) pre-therapeutic staging, (3) follow-up after initial therapy, and (4) guiding therapy for recurrent disease.
Screening
Reliable diagnosis of prostate cancer virtually always requires biopsy and pathologic examination. No clinical imaging technique is sufficiently accurate in initial diagnosis to have become accepted as a replacement for biopsy; screening tests are therefore used to determine who should undergo biopsy. Although there is still some controversy about the utility of screening for prostate cancer (data which might firmly establish that screening programs prolong life are still being accumulated), many organizations recommend annual screening for all males over 50, and at an earlier age for males with familial or racial risk factors.
Methods for screening include digital rectal examination (DRE), serum prostatežspecific antigen (PSA) measurement and transrectal ultrasound (TRUS) of the prostate. The likelihood that each of these, if positive, will lead to a positive biopsy varies from study to study, due to the subjectivity of palpation and imaging findings and to varying thresholds for a "positive" PSA value. However, approximate figures are available: A positive DRE alone predicts a positive biopsy in 10%-21% of patients; a positive PSA alone predicts a positive biopsy in 12%-32% of patients. If both are positive, 42%-72% of patients will have a positive biopsy.
In current practice, either a positive DRE or PSA usually leads to a biopsy no matter what the TRUS findings may be. And although a positive TRUS in a patient with a negative DRE and PSA may yield a positive biopsy in about 13% of patients, the increased yield of positive biopsies for TRUS is only slightly greater than the total yield of biopsies performed on patients whose screening exams are all negative. Therefore, TRUS is not in common use as an independent screening test. However, most biopsies are currently performed with TRUS as a guide, both to indicate suspicious regions in the prostate and to be sure that random sextant biopsies remove tissue from all appropriate parts of the organ.
Pre-Therapy Staging
For patients who are to be treated, staging requires assessment of the local extent of the tumor and of possible metastatic disease. Most metastases from prostate cancer appear in pelvic or abdominal nodes or in bones. Screening for skeletal metastases remains the province of the radionuclide bone scan. Several studies have shown that the yield of such scans is negligible for patients whose PSA is below 10, so that a strong argument can be made to limit use of these scans to patients with higher PSA levels.
Both PSA and the Gleason score, which is the usual histologic assessment of degree of malignancy, indicate the likelihood of enlarged (presumably tumor-containing) nodes as observed by CT or body-coil MRI. The Gleason score appears to be a better discriminator, so that a useful rule of thumb might be to limit nodal imaging to those patients with a Gleason score of 7 or higher; as those with a Gleason score of 2 through 6 have very rarely have positive nodes. It should be noted that microscopic disease in nodes is common, so that even patients with non-enlarged nodes who undergo radical prostatectomy often have a preliminary node dissection and histologic examination so that patients with micrometastatic disease can be spared a prostatectomy.
Choice of initial therapy for prostate carcinoma is a complex issue. Some patients, especially elderly men with low Gleason scores and small tumors, may merely be observed. Men over 70 years of age, and/or who are poor operative risks, often have localized disease treated with radiotherapy. Younger men with localized disease frequently undergo radical prostatectomy, and patients with known metastases usually have orchiectomies or receive anti-androgen pharmacologic therapy. Treatment may often be life-prolonging or curative, but the effects of surgery, pelvic irradiation, and castration can severely impair quality of life, so that thoughtful selection of therapy is crucial.
Local staging is important since most surgeons prefer to limit prostatectomies to those patients whose carcinoma is entirely within the gland, or who have only minimal transcapsular tumor invasion. Neither TRUS nor CT have proven to be sufficiently reliable for this assessment to enjoy widespread use in guiding therapy. Only MRI using a rectal coil is sufficiently accurate to be reliably employed. Its accuracy is far from perfect, however, and it is relatively expensive, so that it, too, is not in universal use as a pre-prostatectomy staging maneuver. An approach to selecting patients for MRI might be to first limit the study to patients who might be surgical candidates, then to limit the group further so that it includes neither those for whom surgery will almost certainly be selected (very young patients, for example) nor those in whom PSA and Gleason scores predict that the likelihood of extracapsular disease is very low. The remaining patients (i.e., those for whom the choice between prostatectomy and radiation is problematic and depends specifically on local staging) should be sufficiently narrowly-defined that MRI will ultimately be used only on a small fraction of all patients with prostate cancer.
Post-Therapy Follow-up
After prostatectomy or definitive local radiotherapy, patients are usually followed with serial DRE and PSA for most histologic types of prostate cancer. PSA is so much more sensitive and so much cheaper that no imaging modality should be employed to search for a recurrence as long as the PSA remains low. But if the PSA rises, and assessment of the anatomic extent of disease is felt to be necessary, bone scan, or abdominal/pelvic CT, or both, may be employed. Metastatic disease may appear first either in bones or nodes, so that it is not clear which one of these two examinations should be selected first.
In some patients who have had local therapy (prostatectomy or irradiation) and in whom disease recurs, salvage local therapy may be indicated. In other words, prostatectomy or cryotherapy may be indicated in patients whose PSA rises after radiation therapy, or radiation or cryotherapy in the prostate bed may be indicated after prostatectomy has failed. In these patients, it is particularly important that metastatic disease be excluded as reliably as possible since local salvage therapy is useless in cases with disseminated disease. For patients whose bone scans and CT's are negative, radionuclide scans with monoclonal antibodies targeted to prostate cancer are sometimes employed, since these exams may be more sensitive than morphologic images in detecting soft-tissue metastases. The sensitivity, specificity and accuracy of these scans have not yet been thoroughly evaluated, however, and research on these issues continues.
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