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

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.

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 D₉₀ 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 D₉₀ and urethra D₁₀ ).

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 D₉₀ coverage, the urethral D₁₀ was reduced by 13.3%  $$  4.7%, without change to other plan quality indices (PTV V₁₀₀ , V_150, V₂₀₀ , bladder V₇₅ , rectum V₇₅ , HI, COIN). Delivery times for HDR BT and IMBT were 9.2 ± 1.6 min and 18.6 ± 4.0 min, respectively. In general, the PTV D₉₀ was more sensitive to source positioning errors than rotational errors, while the urethral D₁₀ was more sensitive to rotational errors than source positioning errors. For a typical range of positioning errors ( $$ 1 mm, $$ 5 $$ ), the overall tolerance was <2%.

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.