PO19: Quantifying the Ir-192 Source Position Accuracy QA Test for HDR Brachytherapy Afterloaders

Abstract

<h3>Purpose</h3> The source positioning test is a recommended part of HDR Brachytherapy QA with an accuracy of ±1 mm per AAPM TG-40, and ±2 mm per AAPM TG-56 and ESTRO Report No. 8. Our institutional QA tolerance is ±1 mm. Typically, the test is performed using radiochromic film and a Perma-Doc jig. At a minimum, visual inspection to verify the accuracy of radioactive source position with respect to their expected dwell position should be performed as part of daily QA prior to patient treatment and at every source exchange. However, an automated digital analysis of the accuracy of the source position QA film is not routinely done. The purpose of this study is to develop a method to quantitatively measure the offset of the Ir-192 source position and to analyze factors that may impact the offset based on different afterloaders, different radioactive sources, different dwell positions, and over time. <h3>Materials and Methods</h3> In this study, one-year data for the source position accuracy tests from two GammaMedplus™ iX (GMPiX) HDR afterloaders (B and C) were analyzed. A method was developed to scan the radiochromic QA films, straighten the films, and quantitatively calculate the source position offsets to the of expected nine dwell positions per film using MATLAB R2021b. We analyzed the offset of the radioactive source position as a function of time, different radioactive source, and different afterloaders. A reproducibility test of the method was done by performing the procedure end-to-end seven times for six arbitrarily chosen QA films. <h3>Results</h3> The Ir-192 source is exchanged quarterly to account for radioactive decay, thus four Ir-192 sources and five Ir-192 sources for the B and C afterloaders were analyzed for a total of 171 QA films. Each QA film has nine expected dwell positions that are analyzed for a total of 1539 dwell positions. One hundred percent of the offsets of 1539 radioactive source positions in all the films are less than ± 1 mm and 99% of offsets were less than ± 0.5 mm. Differences of the source position offset between the B and C afterloader were observed: 60% of all source position offsets to the expected dwell positions for both afterloaders were found to be more proximal (away from afterloader, shifted towards patient). The average offset for the B and C afterloader for all source positions were -0.05 mm and -0.01 mm, respectively. Analysis of each dwell position offset showed variability in offset: when comparing between the B and C afterloader: (1) 0.04 vs 0.01 mm, (2) -0.02 vs 0.01 mm, (3) -0.04 vs -0.01 mm, (4) -0.07 vs -0.01 mm, (5) -0.05 vs -0.02 mm, (6) -0.05 vs -0.05 mm, (7) -0.08 vs -0.03 mm, (8) -0.10 vs -0.04 mm, (9) -0.07 -0.01 mm, respectively. The source position offsets for the seventh, eighth, ninth positions were worse than average for both afterloaders. No correlation between source offset over time was found. Result of reproducibility test shows that the offsets of the location of radioactive sources in the daily QA the reported method is feasible, with a standard deviation less than 0.2 mm. <h3>Conclusions</h3> This study has the potential to be used in daily QA for quantitative analysis of the source position accuracy test. We demonstrated the variability of the source position to the expected dwell position with respect to different afterloaders, different sources, and different dwell positions. Despite the upcoming new, currently unpublished, guidelines by the AAPM that increase the tolerance for the source positioning accuracy test to be equal to or less than ± 2 mm, our results show that our QA methodology can detect differences within ± 1 mm.