A novel 169 169 Yb‐based dynamic‐shield intensity modulated brachytherapy delivery system for prostate cancer

摘要

Purpose Intensity modulated brachytherapy (IMBT) is a novel high dose rate brachytherapy (HDR BT) technique which incorporates static or dynamic shielding to increase tumor coverage and/or spare healthy tissues. The purpose of this study is to present a novel delivery system (AIM‐Brachy) design that can enable dynamic‐shield IMBT for prostate cancer. Methods The AIM‐Brachy system dynamically controls the rotation of platinum shields, placed within interstitial catheters, which partially collimate the radiation emitted from an 169 Yb source. Conventional HDR BT (10?Ci 192 Ir) and IMBT (18?Ci 169 Yb) plans were generated for 12 patients using an in‐house column generation‐based optimizer, coupled to a Geant4‐based dose calculation engine, RapidBrachyMC. Treatment plans were normalized to match the same PTV D 90 coverage as the clinical plan. Intershield attenuation effects were taken into account. A sensitivity analysis was performed to evaluate the dosimetric impact of systematic longitudinal source positioning errors ( ± 1?mm, ± 2?mm, and ± 3?mm) and rotational errors ( ± 5 ° , ± 10 ° , and ± 15 ° ) on clinically relevant parameters (PTV D 90 and urethra D 10 ). Results The platinum shield reduced the dose rate on the shielded side at 1?cm to 18.1% of the dose rate on the unshielded side. For equal PTV D 90 coverage, the urethral D 10 was reduced by 13.3%? ± ?4.7%, without change to other plan quality indices (PTV V 100 , V 150, V 200 , bladder V 75 , rectum V 75 , HI, COIN). Delivery times for HDR BT and IMBT were 9.2?±?1.6?min and 18.6?±?4.0?min, respectively. In general, the PTV D 90 was more sensitive to source positioning errors than rotational errors, while the urethral D 10 was more sensitive to rotational errors than source positioning errors. For a typical range of positioning errors ( ± 1?mm, ± 5 ° ), the overall tolerance was 2%. Conclusions The AIM‐Brachy system was proposed to deliver dynamic‐shield IMBT for prostate cancer with the potential to create a low dose tunnel within the urethra. The urethra‐sparing properties are desirable to minimize the occurrence and severity of urethral strictures or, alternatively, to provide a method for dose escalation.