Establishing implantation uncertainties for focal brachytherapy with I-125 seeds for the treatment of localized prostate cancer

<div><p></p><p><b>Background.</b> The efficacy of focal continuous low dose-rate brachytherapy (CLDR-BT) for prostate cancer requires that appropriate margins are applied to ensure robust target coverage. In this study we propose a method to establish such margins by emulating a focal treatment in patients treated with CLDR-BT to the entire gland.</p><p><b>Material and methods.</b> In 15 patients with localized prostate cancer, prostate volumes and dominant intra-prostatic lesions were delineated on pre-treatment magnetic resonance imaging (MRI). Delineations and MRI were registered to trans-rectal ultrasound images in the operating theater. The patients received CLDR-BT treatment to the total prostate volume. The implantation consisted of two parts: an experimental focal plan covering the dominant intra-prostatic lesion (F-GTV), followed by a plan containing additional seeds to achieve entire prostate coverage. Isodose surfaces were reconstructed using follow-up computed tomography (CT). The focal dose was emulated by reconstructing seeds from the focal plan only. The distance to agreement between planned and delivered isodose surfaces and F-GTV coverage was determined to calculate the margin required for robust treatment.</p><p><b>Results.</b> If patients had been treated only focally, the target volume would have been reduced from an average of 40.9 cm<sup>3</sup> for the entire prostate to 5.8 cm<sup>3</sup> for the focal plan. The D<sub>90</sub> for the F-GTV in the focal plan was 195 ± 60 Gy, the V<sub>100</sub> was 94% [range 71–100%]. The maximum distance (cd<sub>95</sub>) between the planned and delivered isodose contours was 0.48 cm.</p><p><b>Conclusions.</b> This study provides an estimate of 0.5 cm for the margin required for robust coverage of a focal target volume prior to actually implementing a focal treatment protocol.</p></div>