PHSOR11 Presentation Time: 1:20 PM: Verify the Transit Time Correction Accuracy of the Bravos Afterloader System Using a Novel Scintillation-Based Brachytherapy Quality Assurance Device

Abstract

<h3>Purpose</h3> Compared to the older generations, the new Bravos high dose rate (HDR) afterloader offers the function to automatically correct dwell time to compensate the transit dose effect. To our best knowledge, there is no QA attempt to verify the accuracy of this correction. To address this issue, a new transit time measurement module has been developed and included in the OriQA HDR QA device, an autonomous phantom for HDR brachytherapy quality assurance. In this work, we employ the OriQA system to verify the transit time correction accuracy of the Bravos afterloader system. <h3>Method</h3> The OriQA device utilizes radioluminescence imaging technique to accurately measure HDR source strength, dwell time, and dwell position. It consists of a 20 cm needle applicator insert, a radioluminescence phosphor sheet placed directly atop the applicator, a digital camera, and software for signal processing and displaying results. Radiation-induced luminescence photons are emitted from the phosphor sheet and captured by the camera. The signal intensity and location are processed to yield relative source strength and dwell information. For the verification test, one of the Bravos channels was connected to the OriQA applicator using a one-meter transfer tube. After specific plan delivery, the raw data of OriQA measurement including source position and time was exported to Matlab for further analysis. The real dwell time at each dwell position and the transit time between dwells during cable extension and retraction were extracted. The transit time correction method provided by the vendor was applied to the measurement, yielding the delivered dwell time. The percentage difference between the delivered and planned dwell time was calculated and used as an indicator of the transit time correction accuracy. Several verification plans were created including seven with fixed dwell steps, one with variable dwell steps, and one with variable dwell time. Plans with fixed dwell steps were delivered three times. The rest were delivered once. Plan details are listed in table 1. For each plan, the mean and standard deviation of the difference between planned and delivered dwell time were calculated from all dwell positions and deliveries. <h3>Results</h3> The deviations between the planned and delivered dwell time are listed in table 1. For plans with less than 4 cm dwell step, the accuracy of transit time correction is, on average, less than 1.2% of the dwell time. For plans with 4 cm and 9 cm dwell step, the accuracy is 1.5% and 2.85%, on average, respectively. Dwell time variation does not affect the correction accuracy, as shown in plan 9. Standard deviations of plan 3, 4, and 9 are greater than 1% due to the relatively large difference at the proximal dwell. <h3>Conclusion</h3> This is the first study that successfully verifies the transit time correction accuracy of the Bravos system. Less than 2% accuracy is observed for clinically relevant dwell steps (0.3 cm to 4 cm). Variations in dwell step, dwell position, and dwell time do not worsen the accuracy.