ESTRO 37 Abstract book

S938

ESTRO 37

semi-automated and consist of a series of beam constancy and mechanical geometric tests and the procedure acquires MV and kV images at various gantry, collimator and couch positions. Material and Methods The MPC check was performed daily on a Truebeam (SN: H192584, v 2.5) for a period of 12 months (n=279). Beam constancy for 6MV and 10MV photons was compared with daily beam output checks using a Sun Nuclear Daily QA3 device as well as outputs measured with a Farmer ionisation chamber in solid water. For the MPC beam output constancy check, both gantry and collimator were set to 0 degrees, the EPID is moved to a source-detector distance of 150 cm and 40 monitor units (MU) were delivered using an 18 x 18 cm 2 field size. As the electronic portal imaging device (EPID) forms an integral part of the MPC procedure, it was re-calibrated whenever its output differed by >1% compared with the Farmer chamber measurements. Beam data was exported in .csv format. The QA3 was set up on the couch, aligned to the crosshairs, with an FSD of 100 cm using a 20 x 20 cm 2 field size, with 100 MU delivered per measurement. The Farmer chamber was placed in a solid water phantom at a depth of 5 cm with 10 cm of backscatter, an FSD of 100 cm and 100 MU delivered using a 10 x 10 cm 2 field. The EPID was also used to measure the 6MV beam output using a 10 x 10 cm 2 field and 100 MU per field. The histogram tool within the Varian Portal Dosimetry software used to determine the output. A 30 x 30 pixel region of interest was drawn and the mean CU recorded. Results

Electronic Poster: Physics track: Dose measurement and dose calculation

EP-1748 The need to cross-check control tools to validate tps model calculation for dynamic arctherapy G. Rucka 1 , N. Asquier 1 , P. Budillon 1 , J.C. Mouttet 1 1 Croix rouge Française, Centre de radiothérapie, Toulon, France Purpose or Objective Volumetric modulated arc-therapy (VMAT) is a fast treatment technique allowing the delivery of good quality radiotherapy. TPS are used for treatment planning and need to perform precise calculations. It is thus important to affine and validate the models created before starting treatments. Our study demonstrates the need to do quality assurance adapted to the model. It is necessary to cross-check the measurements from at least two detectors before proceeding to pre-treatment controls. Material and Methods Photon beams of 6MV, delivered by an Elekta Synergy linac equipped with an Agility multi-leaf collimator, were modeled in Pinnacle TPS V9.8. Two models were created and compared: M1 and M2. M1 focuses on border and out- of-field parameters while M2 focuses on output factors agreement between models and measurements. Both models were evaluated using several tests found in the literature used as quality assurance. On the other hand, 20 treatment cases were controlled with the ArcCHECK device, the PTW TM31010 ionization chamber as well as EBT3 gafchromic films. Results M1 exhibits good agreement with measurements when performing tests from the literature, while M2 performs poorly in the "strip pattern" test. However, M1 calculation/measurement ratio goes up to 4.0% when comparing the dose in atypical fields, while M2 ratio stays below 1.3%. ArcCHECK gamma-test pass rates are similar between both models with 98.2% and 97.6% for M1 and M2, respectively (3%, 3 mm, local). Single point calculations differences with ionization chamber measurements are 2.5% and 1.0% in average, and reach a maximum of 3.6% and 2.0% for the M1 and M2 models, respectively. Gafchromic films analysis confirm the significant differences found with the ionization chamber for the M1 model and exhibit much better agreement for the M2 model. Conclusion The results demonstrate the importance to cross-check control tools. The significant gaps found with the M1 model were neither detected with quality assurance tests, neither with ArcCHECK measurements. It was necessary to implement home-made ionization chamber and gafchromic films test. This gap between calculated and measured dose concerns target volume area where ionization chamber is located but not the surrounding tissues where arccheck diodes are located. EP-1749 Longterm stability of the Varian Truebeam Machine Performance Check (MPC) K. Armoogum 1 , G. Cornish 1 1 Derby Hospitals NHS Trust, Department of Radiotherapy, Derby, United Kingdom Purpose or Objective A longterm comparison was performed to assess the reliability and accuracy of the Varian MPC check compared with more established methods of beam output measurement. The Machine Performance Check (MPC) is a Varian application intended to be used on a daily basis to verify the mechanical and dosimetric performance of Truebeam linear accelerators. MPC measurements are

The results show that over a period of 12 months, the outputs of the 6 and 10 MV beams as measured by the MPC tracked the outputs measured with the QA3 but with an average difference between the two of approximately 1%. The MPC consistently measures the output lower (- 1.08%) than the QA3 device. Farmer chamber output measurements aligned more closely with the QA measurements and the mean MPC-Farmer measurements was -1.26%. The MPC-EPID difference was -0.89%. There were 3 output adjustments during the 12 month period with the 6MV output adjusted from 102.62% to 100.02%, 101.36% to 99.94% and 101.24% to 99.54% respectively. The 10MV output showed a similar trend with a mean difference of -1.34% again with the MPC reading lower than QA3. Conclusion The small variation in MPC output indicates that a monthly intercomparison with a calibrated Farmer chamber is needed if the MPC is to replace the QA3 as a daily check. A weekly Farmer chamber output