The Validation of a Quick Three Dimensional Dose Verification System for Pre-treatment IMRT QA

Purpose/Objective(s):Recent complicated radiotherapy such as intensity modulated radiotherapy (IMRT) requires quick and reliable comprehensive quality assurance (QA) systems for the safety of patients who continue increasing.In this study, we have evaluated the performance of a three dimensional (3D) dose verification system. The system, COMPASS version 3 (IBA Dosimetry, Germany), has a dedicated beam model and dose calculate engine. It is able to reconstruct 3D dose distributions on the patient anatomy based on measured fluences using the MatriXX 2D array. The MatriXX is mounted on the linear accelerator head to acquire the fluences.Materials/Methods:Fist, the COMPASS system was modeled by dosimteric data (percent depth dose, off-axis ratio, and output factor) for a 6 MV photon beam. The dose distributions were reconstructed from fluence maps measured by the system and CT data. The dosimetric properties for simple square fields and various MLC test patterns were checked by comparison with those of EDR2(Eastman Kodak, Rochester, NY) films. The Monte Carlo simulations (MC) were also used as needed. The system was also used for dose verification in clinical IMRT plans of head and neck cases. The dose distributions were compared with those measured by 3DVH (Sun Nuclear, Melbourne, FL) and MC. Most of our analyses are performed using the dose profiles and gamma method (criterion:3% local dose difference and 3 mm distance to agreement as reference values). Furthermore, we compared QA time between COMPASS, 3DVH and EDR2.Results:For MLC test patterns, the COMPASS dose profiles agreed within 3% with those of EDR2 films and MC. MC agreed within 2% with EDR2 in the region more than 30 cGy. The physical resolution of the COMPASS detectors is lower than those of film. However, the dose resolution for MLC patterns is comparable to that of film. This is because COMPASS makes 3D dose distributions from fluences using beam modeling.In clinical plans, the pass rates of gamma method were equal by all systems. In all cases, the COMPASS dose profiles agreed within 3% with those of 3DVH.The average QA times of COMPASS, 3DVH and EDR2 film in comparison were 42, 58 and 120 minutes respectively. The COMPASS provided the quickest QA because the COMPASS does not require the phantom plans for QA.Conclusions:The COMPASS system provides fast and reliable 3D dose verification for clinical IMRT QA. The system can use with the similar accuracy to traditional film measurements and other 3D detector. Besides, the COMPASS QA process which does not require phantom plans makes simple QA workflow.

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The Validation of a Quick Three Dimensional Dose Verification System for Pre-treatment IMRT QA