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

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 p rostate implants, was analyzed to determine whether potential EPE was adequ ately treated. The implants were peripherally loaded and planned with a 3-5 -mm dose margin at midgland. Seeds were not implanted outside of the capsul e 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 pr ostate edge and the prescription isodose line was measured at the left late ral, left posterolateral, posterior, right posterolateral, and right latera l positions on each prostate contour. Similar measurements were made of the preplan dose margins. Results: The mean postimplant dose margin was greater than or equal to4.5 m m at the midgland and apex of the prostate in agreement with the preplan. H owever, at the base, the mean margins at the five measurement locations wer e 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 p rescription dose 3-5 mm outside the capsule, which is believed to be suffic ient to treat 95-100% of EPE in favorable risk patients. However, dose cove rage of EPE, like dose coverage of the prostate, is operator-dependent. (C) 2001 Elsevier Science Inc.