International Journal of Radiation Oncology*Biology*Physics
Rapid communicationCan extraprostatic extension be treated by prostate brachytherapy? An analysis based on postimplant dosimetry☆
Introduction
Extraprostatic extension (EPE) is an unfavorable prognostic factor seen in 15% to 40% of prostate cancer patients with clinically organ-confined disease 1, 2. Two recent studies have reported the extent of EPE in surgical specimens. Davis et al. (3) reported that 11 of 107 patients who met the criteria for brachytherapy (prostate-specific antigen [PSA] < 10 ng/mL; Gleason < 7; volume < 60 cc) had EPE, with mean and maximum EPE of 0.03 and 0.6 mm, respectively. Sohayda et al. (4) reported that the EPE was within 3.3 mm of the capsule in 90% of favorable cases (clinical stage T1–2; PSA ≤ 10 ng/mL; Gleason ≤ 6). Based on these findings, it is postulated that a 3–5-mm postimplant dose margin would adequately treat EPE in 95–100% of favorable risk cases. The objective of the present work was to determine whether EPE could be adequately treated by a Pd-103 prostate implant. Pd-103 implants were studied instead of I-125 implants because limitations are more likely to occur with Pd-103 owing to its more rapid attenuation in tissue.
Section snippets
Methods and materials
The postimplant dosimetry of 22 consecutive Pd-103 seed (Theragenics, Atlanta, GA) implants performed between May 1997 and November 2000 was analyzed to determine whether potential EPE was adequately treated. The mean preimplant volume was 38 ± 12 cm3. These patients received a Pd-103 permanent implant either as monotherapy (120 Gy) or as a boost (80 Gy) after external beam radiotherapy. A preplan was generated based on a preimplant computed tomographic (CT) scan. The implants were planned with
Results
The mean postimplant extraprostatic margins measured at the base, midgland, and apex are listed in Table 1. These results show that the mean postimplant margins were ≥4.5 mm at all five measurement locations at the midgland and the apex. The mean margins at the base were notably smaller, typically between 2.5 and 3.5 mm. Of the five measurement locations on each contour, the margin was smallest at the posterior point in all three sectors of the gland. This reflects our attempt to minimize dose
Discussion
Figure 3 shows that, in this series of patients, potential EPE was adequately treated at the midgland and apex, but was not adequately treated at the base of the prostate in many of the cases. The suboptimal margin at the base is a result of source placement error; i.e., seeds planned for the base were not implanted there. In principle, this problem can be significantly improved or rectified by an improvement in technique; in this case, by using sagittal ultrasound imaging to localize the tip
Conclusion
Peripherally loaded Pd-103 prostate implants can deliver the prescription dose 3–5 mm outside the capsule, which is believed to be sufficient to treat 95–100% of EPE in favorable risk patients. However, dose coverage of EPE, like dose coverage of the prostate, is operator dependent.
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Supported in part by P30 CA 56036-03 (NCI) and through an unrestricted grant from Indigo Medical, Inc.