Abstract Book

S917

ESTRO 37

Material and Methods The a-Si 500 EPID of a Varian Clinac 2100 CD was investigated against to an ionometric system (ion chamber PTW 31002 plus dosimeter PTW Unidos). The EPID was placed at 100 cm from the linac focus. The EPID was calibrated in dosimetric mode according to the procedure given by Varian. Photon beams of 6 MV and dose-rate of 600 MU/min were selected, as it is the clinical setting used in our clinical practice. The ion chamber with its build-up cap was attached to the EPID cover, lying within the flat area of a 10x10 cm 2 field size. Integrated images were taken for 5, 10, 25, 50, 100, 300, 600 and 900 MUs, with gantry at 0 degree (IEC scale). Simultaneous readings were aken with the ion chamber. Images analysis was performed using the Portal Dosimetry application of Varian to get the EPID reading at the center of the image (area of 5x5 mm 2 ). Photon output vs. gantry angle was evaluated using 100 MUs and angles of 0, 90, 270 and 180 degrees (IEC scale). Linearity was computed as the reading (EPID or ion chamber) to MU ratio, normalized at 100 MU. Measurements were taken in three different sessions. Output vs. gantry angle was evaluated as the deviation of the reading obtained for each angle against to the zero gantry angles. Results Figure 1 shows the linearity results for both kind of detectors. Differences between both methods within ±1% were noted for MU ≥ 25. At 5 MU, difference of 2.5% was observed, probably due to dead time effect of the EPID. Negligible differences (within ± 0.5%) were observed between the EPID and ion chamber-based method for the output vs. gantry test. Conclusion The recent AAPM-RSS Medical Physics Practice Guideline 9.a. for SRS-SBRT (J Appl Clin Med Phys. 2017 Sep;18(5):10-21) gives a tolerance of ±2% for MU linearity for MU ≥ 5. According to our data, EPID can be used to perform this check by replacing the ion chamber-based method. EPID definitively allows us to eliminate the use of holder to place the ion chamber at the linac gantry when photon output vs. gantry measurements are performed. EP-1717 Optimal response function for radiochromic film dosimetry system S. Devic 1 , S. Aldelaijan 1 1 Jewish General Hospital, Oncology, Montreal, Canada Purpose or Objective Different dose response functions of EBT3 GafChromic TM film model have been tested in terms of sensitivity. We also made an assessment of the necessity of scanning film pieces before and after irradiation. Material and Methods Pieces of EBT3 film model were irradiated to different doses in a Solid Water phantom. Based on images scanned in both reflection and transmission mode before and after irradiation twelve different response functions were calculated using optical density, raw pixel data and normalized pixel data. For every response function, reference radiochromic film dosimetry system was established by generating calibration curve and by performing error vs. uncertainty analysis. Results Figure 1 shows dose sensitivity curves for various response functions obtained from the transmission (left) and reflection (right) scanned images. Solid symbols correspond to a double scan method (using film images scanned before and after irradiation) while open symbols correspond to the single-scan method (using film images scanned after irradiation only). The highest sensitivity is associated with the use of normalized pixel values, and as expected, the response in reflection mode shows higher sensitivity when compared to transmission mode

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Conclusion The fluorescent screen based system has an excellent spatial resolution which can provide fast field profile verification. MLC leakage test is an essential test in Cyberknife InCise2 ® MLC acceptance. In the current system, due to its excellent sensitivity, MU and required time were reduced by a factor of 10 and 3, respectively as compared with QA procedures using EBT3 film. EP-1716 Using Portal Dosimetry To Check The Photon Mu Linearity And Output Vs. Gantry Angle Of A Linac I. Lorenzo 1 , J.F. Calvo-Ortega 1 , S. Moragues-Femenia 1 , C. Laosa-Bello 1 , J. Casals 1 1 hospital Quironsalud, Radiotherapy, Barcelona, Spain Purpose or Objective To investigate the use of the EPID of the linac to check the photon MU linearity and output constancy with gantry rotation.