Transit Dose Measurement of a HDR Afterloading Brachytherapy Source by a Fluorescent Screen Based QA System

Abstract

To assess the dosimetric discrepancy between treatment delivery and TPS in HDR brachytherapy due to effect of transit time and step size. The source positions during treatment were detected by a fluorescent screen system which converted the radiation signal to optical signal and utilized a high-speed camera with frame rate of 500 fps and pixel resolution of 1280X720 to capture the luminence. A catheter in which an Ir-192 source would be loaded was fixed on the fluorescent screen and the camera was placed about 30 cm away from the screen. When the source was in transit, the camera would capture images on the fluorescent screen sequentially. Source position was traced out by locating the centroid of the captured image. After capturing source positions, dose distribution of an Ir-192 source calculated by a brachytherapy TPS was assigned to each location and superpositioned to obtain a final dose distribution including transit dose. A series of treatment plans each with two source dwell positions of equal dwell times were created. Two different step sizes were tested, namely, 5 mm & 60 mm. Two different dwell times were assigned, namely, 0.5 s & 1 s. The measured dose distribution and TPS generated dose distribution were both normalized at the first dwell position while the second dwell position and the mid-point between dwell points were selected for dose comparison. From Table 1, at second dwell point, cases with 5 mm step size have a dose increase at treatment distance below 10 mm due to contribution from transit dose and this effect diminished when treatment distance increased. At 60 mm step size, second dwell position has a significant dose deficiency due to an decrease in dwell time. All step sizes have an increase in transit dose for treatment distance below 10 mm. Such an increase is larger when 60 mm step size was used. In case of shorter dwell time, transit dose has a greater change. For 60 mm step size, transit dose has a maximum discrepancy at around 10 mm distance. This was due to the higher normalization dose at 5 mm distance so that the transit dose effect is less significant. The dose discrepancy at different treatment distances was below 6% for 5 mm step size and can go up to 58% for 60 mm step size.Abstract 3547; Table 1Difference in percentage between measured and TPS calculated dose, both are normalized at the first dwell positionTreatment distances (mm)5101520Step size (mm) / Dwell time (s)% DifferenceSecond dwell point5 / 0.55.31.5-0.5-1.15 / 13.10.5-0.6-0.960 / 0.5-58.1-49.5-46.0-43.560 / 1-31.9-30.8-30.2-29.0Mid-point between dwell points5 / 0.52.90.4-0.5-0.85 / 12.60.2-0.4-0.560 / 0.511.914.611.55.660 / 17.19.07.44.0 Open table in a new tab Larger source step size and shorter dwell time would lead to a greater dose discrepancy. Transit dose could have a contribution up to 14.6% of prescribed dose which was not accounted for by brachytherapy TPS. This method may help to evaluate the transit dose contribution which could lead to dosimetric impact.