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Can extraprostatic extension be treated by prostate brachytherapy? An analysis based on postimplant dosimetry

Presented at the 42nd Annual Meeting of ASTRO, October 2000, Boston, MA.
https://doi.org/10.1016/S0360-3016(01)02652-9Get rights and content

Abstract

Purpose: To determine whether extraprostatic extension (EPE) can be treated by Pd-103 prostate implants.

Methods and Materials: The postimplant dosimetry of 22 consecutive Pd-103 prostate implants was analyzed to determine whether potential EPE was adequately treated. The implants were peripherally loaded and planned with a 3–5-mm dose margin at midgland. Seeds were not implanted outside of the capsule except at the base and apex. The postimplant dosimetry was based on a CT scan obtained 32 ± 8 days postimplant. The radial distance between the prostate edge and the prescription isodose line was measured at the left lateral, left posterolateral, posterior, right posterolateral, and right lateral positions on each prostate contour. Similar measurements were made of the preplan dose margins.

Results: The mean postimplant dose margin was ≥4.5 mm at the midgland and apex of the prostate in agreement with the preplan. However, at the base, the mean margins at the five measurement locations were less than planned, typically ranging from 2.5 to 3.5 mm. The postimplant margin at the base was smaller than expected due to source placement errors, a correctable problem.

Conclusions: 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.

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.

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