A Novel Brachytherapy Needle Design to Reduce Urethra Dose in HDR Prostate Brachytherapy.

Abstract

Ultrasound-guided high-dose-rate (HDR) prostate brachytherapy is a safe and effective treatment option for prostate cancer patients; however, some patients still experience acute and late genitourinary (GU) toxicity. It has been reported that urethra dose is associated with the incidence and severity of GU toxicity. Therefore, a technique that can further spare the urethra while ensuring adequate target coverage is highly desirable. Intensity modulated brachytherapy (IMBT) designs, such as rotating shield brachytherapy (RSBT), offer ideal dosimetry on-paper but are challenging to implement clinically due to the need for high precision in moving mechanisms and the use of non-conventional HDR sources, such as 153Gd, electronic, and 169Yb. In this study, we propose a novel clinically implementable solution based on the direction modulated brachytherapy (DMBT) design concept, which has no moving parts and works effectively with the commercially available 192Ir sources. The widely used GammaMedPlus (GMP) 192Ir sources, with outer diameters of 0.9 mm, was simulated using the GEANT4 Monte Carlo (MC) simulation code. The novel DMBT needle concept consists of a 14-gauge nitinol needle with an outer diameter of 2.1 mm and a crust thickness of 0.1 mm, which housed a platinum shield inside. A single groove, consistent with the outer diameter of each source, was incorporated inside the platinum shield to accommodate the HDR source. The maximum thickness of the shield was 0.8 mm. To evaluate the effectiveness of the DMBT needle concept in reducing urethra dose, two proof-of-concept in silico DMBT plans were created from the standard clinical plan by replacing two needles close to the urethra with the DMBT needles. The dosimetrical comparisons between the two DMBT plans, and the clinical BT plan were done by assessing the target coverage, and urethral, bladder and rectum dose. The MC results showed that the use of the novel DMBT needle could reduce the radiation dose by approximately 35% at 1 cm distance from the needle behind the platinum shield, as compared to the unshielded side. Additionally, when using the same dose volume histogram (DVH) planning criteria as the original plan, the DMBT plan reduced the maximum urethral dose at the pre-apical region by 16% and 28% for 0 mm and 2 mm planning organ at risk (PRV) margins, respectively, while maintaining equivalent V100 and D90 target coverage. The novel DMBT needle design offers a promising solution for reducing the dose to the pre-apical region of the urethra without compromising target coverage, increasing the dose to other organs-at-risk, or increasing the treatment time.