|CANCER & TREATMENTS FOR CANCER CENTER PATIENTS PREVENTION & RISK ASSESSMENT CLINICAL TRIALS & RESEARCH LIVING WITH CANCER|
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Michigan Oncology Journal Fall 98
Advances in Radiotherapy for Prostate Cancer
---Howard M. Sandler, M.D. and P. William McLaughlin, M.D.
The use of prostate specific antigen (PSA) for screening and the associated huge increase in newly diagnosed cases has led to significant changes in prostate cancer diagnosis and treatment. Radiotherapy for prostate cancer also has evolved dramatically and remains one of the most important treatment modalities for this common neoplasm.
External Beam Treatment
Data from University of Michigan reported this year at the American Society of Clinical Oncology meeting on the role of 3D conformal external beam therapy show that for early prostate cancer (T-stage T1 or T2, pre-RT PSA ² 10.0 ng/ml, Gleason score ² 7), the freedom from biochemical recurrence at five and eight years was 85 percent (SE 5 percent) at each time point. These data indicate that, at least out to eight years, freedom from biochemical recurrence is excellent and continues to be at least as favorable as reports from selected surgical series. The stability of the results between five and eight years also is encouraging regarding the durability of the response to RT, although longer follow-up is desirable.
Prostate implants changed significantly in the early 1980s. Ultrasound evaluation of the prostate by transrectal probe allowed detailed evaluation of the prostate and provided a technique to direct needles into specific locations for biopsy. Once this technology was developed, the concept of placing radioactive seeds using needles placed with ultrasound guidance emerged. The initial clinical results of ultrasound-guided permanent implants have been positive (5). The treatment results of early prostate cancer by implant compare favorably to both prostatectomy and external-beam radiotherapy. Some of the perceived advantages of the procedure are that it can be performed in one day as an outpatient and that there may be fewer severe complications than radical prostatectomy.
Although permanent implants show great promise, limitations and pitfalls remain. One limitation is that only the prostate is fully treated. The volume immediately adjacent to the prostate, which is encompassed during 3DCRT and may contain microscopic tumor, receives a limited dose of radiation. Therefore, it is critical that implants be offered to patients with a high probability of disease confined to the prostate, i.e. those with PSA ² 10, Gleason score ² 6, and unilaterally palpable disease, at most. Implants cannot be adequately performed if the prostate is too large (> 40-50 cc) and side effects may be increased if a previous TURP was performed. The most serious limitation, though, is the possibility of inadequate dose delivery to the prostate itself, despite ultrasound guidance.
University of Michigan studies have addressed the issue of inadequate dose delivery to the prostate (6, 7), and currently, additional seeds are placed in areas where lower dosing might occur and post-implant dose calculations are performed, using CT and 3D technology to locate each seed (up to 50-100) and to calculate the dose to the prostate. The prostate may swell due to the trauma of the implant, therefore a post-implant CT is performed two to four weeks later to allow for the prostate size to stabilize. If the actual dose as observed on the post-implant CT appears to be subtherapeutic, additional seeds are placed to correct the under-dosed region.