ESTRO 36 Abstract Book

S115 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective Recently the 0.35T CO 60 MRIdian system (Viewray Inc., Cleveland) is clinically implemented at our institution. IMRT online adaptive patient treatment is daily routine. Quality assurance (QA) on dose delivery (both absolute and position), and end to end test (per fraction and overall) are prerequisites for safety and quality. Hereto, a reliable dosimeter is required. EBT GafChromic film might appropriate. However, a magnetic field can cause perturbations to absolute and relative dosimetry measurements. In addition, the amount of radiation induced dipole polymers might be influenced by the orientation of the film in the B 0 -field, i.e. an angular dependency in absolute dose response might be present. The purpose of this study is to investigate suitability of EBT3 GafChromic film for MRIdian QA purposes. Material and Methods GafChromic film sheets were irradiated in a water phantom using the MRIdian and a Linac (Varian 2300C/D, 6MV) as comparison system. The film sheets are placed in a rotatable device positioned in the water phantom such that the B 0 field is frontal or sagittal to the film surface in initial position (figure 1). The film was rotated over 7 angles from 0° to 90° and irradiated with 3Gy. On each machine 4 measurements were performed for each orientation. The irradiation setup is 5cm depth, 100cm SSD and 10.5x10.5cm 2 /10x10cm 2 (MRIdian/Linac) field size. Optical density (OD) to dose (D) calibration measurements (4x) were performed up to 8Gy both on MRIdian and Linac, with the film perpendicular to the beam axis. The film sheets were scanned in portrait mode with the A4 Epson 1680 Expression Pro flatbed scanner and at the film rotation point OD and D values were analyzed for the red, green and blue channel. The following comparisons were made: a. Reproducibility of MRIdian and Linac film measurements b. Angle dependent film response with (MRIdian) and without B 0 (Linac) influence c. Angle dependent film response in frontal or sagittal setting

averaged over all angles for frontal and sagittal planes. The blue channel shows the widest range, which is i.a. due to the fact that the blue channel is barely dependent on dose and is sensitive to the dye homogeneity in the film. Using triple color correction on the results in (c) gives better mutual correspondence.

Conclusion All data indicates that there is no dose deviation between MRIdian and Linac measurements neither in frontal or sagittal setting. In addition, within experimental uncertainty there is no B 0 influence on absolute film dose response. This means that EBT3 GafChromic film can be used for absolute dosimetry regardless its orientation in the field. This makes it a suitable dosimeter for QA measurements and end to end testing in a 0.35T CO 60 machine. OC-0232 Development of a novel ‘end to end’ dosimetry audit of motion management in radiotherapy A.L. Palmer 1 , D. Nash 1 , S. Jafari 1 , S. Muscat 1 1 Portsmouth Hospital NHS trust, Medical Physics Department, Portsmouth Hampshire, United Kingdom Purpose or Objective External dosimetry audit is valuable to assure quality, safety, and enable improvements. However, motion management in radiotherapy has not previously been subject to rigorous audit. 4DCT allows assessment of motion at treatment planning, but verifying the accuracy of mitigation strategies at treatment planning and delivery is poorly reported. This includes any detrimental effect of MLC, gantry and target motion interplay. We have developed an end-to-end dosimetry audit system to objectively assess the success of motion management strategies. The audit system may be used for interdepartmental dosimetry audit as well as to provide improved understanding and accuracy of motion managed radiotherapy. Material and Methods The system uses a respiratory motion lun g-phantom (Quasar), custom designed insert with targe t (~40 mm diameter irregular structure) and lung OAR. Dose plane measurement is made using triple-channel film dosimetry (EBT3 and FilmQAPro) with uncertainty reduction strategies. Measurements are performed with the phantom static to provide a conventional end-to-end evaluation of accuracy and then moving (+/- 7 mm sinusoid) to add separately the effect of motion. A novel dose-area-histogram, via Matlab, was used to assess dose coverage of the moving target (GTV and ITV) in the film plane. Agreement between treatment planning system (TPS) calculation and measured dose was performed using

Results Table 1 represents (a) the reproducibility of film OD values for both machines, (b) the influence of B 0 on film dose response over all angles and (c) the mean dose values