Sensitivity of QA testing to beam drifts in LINACs: a MC Study

Quality assurance guidelines for medical accelerators ensure patients get the dosage as established in their treatment plan. A portion of dose discrepancies seen in photon therapy are caused by changes in the initial electron beam, before they hit the target in the treatment head. We started by validating a Monte Carlo model of the Elekta Agility radiation therapy treatment head at three photon treatment energies, 6MV, 10MV and 18MV using GATE/Geant4. This model was then used to quantify the how electron beam deviations at the target manifest themselves in the resultant photon beams. Using deviated beams as the source from our validated model, standard quality assurance tests were simulated to determine if they were sensitive enough to detect and categorize these deviations. At all three treatment energies, simulated beam deviations of average beam energy, beam spot position and the incident angle were conducted and analyzed using the quality assurance setups for profile constancy, tissue phantom ratio and overall beam symmetry. The results of this research will be used to observe small beam drifts clinically and correct for errors before they become an issue that affects patient treatments and throughput.