ESTRO 37 Abstract book

S941

ESTRO 37

detector, with the couch retracted from the bore. To correlate the exit detector signal with the expected MLC fluence, the TQA® Daily QA procedure was run as a calibration file. To observe the effect of irradiation time on the detector, the TQA® Daily QA procedure was run once in the morning (before any treatment) and once in the evening (after all treatments) for three days on two TomoTherapy units. The measured MLC fluence for the selected IMRT plans was then analyzed and compared with the planned MLC fluence using the morning and evening Daily QA calibration files; specifically, the leaf opening time (LOT) was calculated for all leaf openings and projections. Lin’s correlation coefficients were calculated as a comparison between applying the day and night calibration files. Results Table 1 shows the percentage error between the planned and measured leaf opening times with different calibration files. The Lin’s correlation coefficients are found to be highly close to unity for all five plans, for all three days, and on both machines. One of the plans from Day 1 from TomoHD1, was chosen as an example in Figure 1. It is a scatterplot between the LOT percentage errors with a morning calibration file versus those with an evening calibration file.

Fig. 1. Gamma analysis tolerance maps (0.5 mm/0.5% criteria) of EPID-based in vivo dosimetry for a typical prostate VMAT plan. Errors were generated by changing the opening of the MLC into 3 mm, 2 mm and 1 mm and by shifting the position of the phantom in x-, y-, and z- directions by 1 cm. Table 1. Comparisons of gamma-index pass rates of the integrated and divided data.

Figure 1: LOT percentage errors with a morning calibration file versus those with an evening calibration file. Jaw opening: 5 cm, with dynamic jaw on TomoHD1. Table 1: Measured Mean Leaf Opening Time Percentage Error and Standard Deviation with Different Calibration Files

The difference between the reference and error patterns was evaluated using gamma analyses (0.5 mm/0.5% criteria). Conclusion The integrated data reduces the sensing ability of EPID- based in vivo dosimetry. However, it became clear that it can be improved by dividing the data. Furthermore, the present result suggested that it is possible to develop a real-time patient dose monitoring safety system using EPID base in vivo dosimetry. EP-1754 Robustness of the TomoTherapy Exit Detector for Verifying MLC Fluence Post-Irradiation T.Y. Lee 1 , H.H.F. Choi 1 , W.K.R. Wong 1 , Y.W. Ho 1 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Happy Valley, Hong Kong SAR China Purpose or Objective The exit detector has been proposed as a tool for verifying MLC fluence on TomoTherapy before and during treatments as an application to dosimetric verifications. This study investigates the consistency of the exit detector depending on the irradiation time between calibration and measurement. Material and Methods Five standard IMRT plans with various jaw settings were selected. MLC fluence was measured with the exit

Conclusion The overall performance of the exit detector is stable to a high extent. An average of 13 operational hours per day for irradiation did not cause any significant deviation for the LOT, despite using a different calibration file from