ESTRO 36 Abstract Book

S94 ESTRO 36 2017 _______________________________________________________________________________________________

interest, only 0.2% of voxels differ by >5%, showing good agreement throughout. Results from measured delivery errors, such as those in figure 1, will also be presented.

Fig. 1 demonstrated the capability of this QA system by showing a source transit process and Fig. 2 indicated that measured dwell time was affected by source separation. Table 1(a) tabulated the measured dwell time for 3 different assigned dwell times with 5 mm separation between source dwell positions. In all three scenarios, the dwell time at starting position was close to the assigned value. Dwell time at next dwell position experienced a larger discrepancy up to 40% for 0.1 s dwell time. This discrepancy in dwell time was due to the transit time for which control computer could not fully account. Hence, dwell time would be shorter than the assigned value except at the starting position. Table 1(b) tabulated measured dwell times at 3 different source separations with 0.5 s assigned dwell time to assess the compensation method stated. Discrepancy could be up to 0.33 s in 6 cm separation. Transit time occupied a larger portion of the dwell time for longer source separation.

Figure 1: Dosimetric comparison between planned and delivered doses for a HDR brachytherapy treatment in a phantom with an introduced error. Conclusion Real-time dosimetric treatment verification is possible with our source tracking system combined with MaxiCalc. Fast dose calculation based on measured source dwell positions is achieved and overcomes the limitation of current TPSs. References 1. Smith, R L., et al. Medical physics 43.5 (2016): 2435-2442. 2. Daskalov, G M., et al. Medical physics 25.11 (1998): 2200-2208. PV-0187 Source dwell time and transit time measurement for a HDR afterloading unit T.L. Chiu 1 , B. Yang 1 , H. Geng 1 , W.W. Lam 1 , C.W. Kong 1 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics & Research Department, Happy Valley, Hong Kong SAR China Purpose or Objective To evaluate dwell time and transit time of HDR brachytherapy treatment by an in-house fluorescent screen based QA system. Since dosimetric effect would be directly affected by source dwell time, an accurate QA method on temporal accuracy is essential. Material and Methods The system included a fluorescent screen (Kodak, Lanex regular screen) which converts the radiation signal to optical signal and a high-speed camera with frame rate up to 500 fps and pixel resolution of 1280X720. The temporal resolution was 2 ms. A catheter in which an Ir-192 source would be loaded was fixed on the fluorescent screen and the camera was placed 30 cm away from the screen. The whole system was light-shielded. When the source travelled inside the catheter, the camera would capture images on the fluorescent screen sequentially. Source position was traced out by locating the centroid of the captured image. The accuracy of dwell time was assessed by measuring 3 different dwell times, namely, 1 s, 0.5 s & 0.1 s. According to a white paper from vendor, transit time for separations below 35 mm would occupied part of the next dwell time and those for separations above 35 mm would have 0.1 s compensation. Thus, the influence of transit time on dwell time was studied by measuring 0.5 s dwell time under 3 different dwell separations, namely, 6 cm, 4 cm & 0.5 cm. Dwell time was assessed by counting the number of images in which source positions were unchanged to the subsequent image. Transit time was the time between two dwell positions. Results

Conclusion Dwell time and transit time could be measured using the fluorescent QA system with uncertainty down to 2 ms. High temporal resolution in this system helped measure the transit time accurately which could hardly be achieved in commonly used QA systems. The effect of transit time on actual source dwell time could be significant and was not fully accounted for by treatment computer. Clinically possible combinations, like 0.5 s dwell time and 5 mm separation, could have a dosimetric error of 8%. PV-0188 Improved class solutions for prostate brachytherapy planning via evolutionary machine learning S.C. Maree 1 , P.A.N. Bosman 2 , Y. Niatsetski 3 , C. Koedood er 1 , N. Van Wieringen 1 , A. Bel 1 , B.R. Pieters 1 , T. Alderliesten 1 1 A cademic Medical Center, Radiation oncology, Amsterdam, The Netherlands 2 Centrum Wiskunde & Informatica, Amsterdam, The