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

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.