One Shot Absorption Technique an Efficient Method to Evaluate the X-Ray Beam in a Quality Assurance Program

One Shot Absorption TechniqueAn Efficient Method to Evaluate the X-Ray Beam in a Quality Assurance ProgramMaoz Ben Ayun, MSc, Dror Alezra, PhD, Alex Tsechanski, PhDIntroduction/Purpose:Measurement of linear accelerator X-ray beam quality with an ion chamber in a water phantom is time consuming and may not detect small changes in the MV X-ray beam spectrum. We propose a new method to measure the quality of the X-ray beam using an Electronic Portal Imaging Device (EPID), a specially arranged set of lead absorbers and Eclipse portal dosimitry software.Methods:We constructed an absorber using 12 small lead foils ranging in thickness from 0.25mm to 3mm in increments of 0.25mm and a 0.5mm thick polystyrene plate. The foils were fixed to the plate in a circumferential pattern 72mm from the central axis of the beam. An EPID located 100cm from the source (SSD = 100cm) and 35cm from the polystyrene plate was used as the detector for the absorption measurements. The linear accelerator was used to deliver a single 100 monitor units (MU) irradiation through the 12 foil absorber and onto the EPID using a 28cm x 28cm field. The single output was measured by the EPID. Portal dosimetry software on the Varian Eclipse treatment planning system was used to measure output in calibration units (CU) under each lead absorber from the single image. Beam quality was altered by placing full lead absorbers above the polystyrene plate or by adjusting the bending magnet current. Five different beam energies were studied (6MV, 6MV-FFF, 10MV, 10MV-FFF, 15MV) and compared with conventional methods for assessing beam quality.Results:Output decreases with increasing foil thickness in a linear relationship when CU measurements are plotted as a function of lead absorber thickness. Straight line fitting of the absorption curves of different X-ray beams results in substantially different slopes (tangent). Each measured X-ray beam was characterized by a specific slope. The slope of the absorption curve was sensitive to beam quality and small changes in beam quality caused small changes in slope.We predicted that increasing beam hardness would decrease the slope and decreasing beam hardness increase the slope. To test this hypothesis, we initiated variations in beam quality and found that change in slope was up to 50% more sensitive and better able to detectthe predicted small variations in X-ray spectrum than ion chamber/water phantom measurements. The average time to collect a full set of measurements was substantially less than other methods.Conclusion:The one shot absorption method is reproducible, fast, efficient and easy to use in a clinical setting. It is more sensitive to small changes in X-Ray beam quality than other methods which use an ion chamber and water phantom.

SSG13-05

One Shot Absorption Technique an Efficient Method to Evaluate the X-Ray Beam in a Quality Assurance Program