(Tuesday, 7/31/2018) 3:45 PM - 4:15 PM
Room: Exhibit Hall | Forum 3
Purpose: To evaluate the use of a new QA phantom with embedded OSLDs as an independent audit of linac beam properties. This phantom was designed to be compact so it can easily be distributed to clinics across the world to assess beam properties beyond output.
Methods: A T-shaped phantom was designed with pairs of OSLDs located along the central-axis at 1.5, 5, 10, 15, and 20cm depths, and symmetric off-axis distances of 5 and 10cm from the central axis at a depth of 1.5cm. The phantom was tested with a Varian Halcyon linac 6MV FFF beam. Measurements were taken at the nominal energy and with intentional energy changes of -10.0%, -5.0%, +2.5%, and +5.0%. An IC Profiler was used to measure the off-axis-ratios (OAR) at 5 and 10cm from the central axis at a depth of 1.5cm. A 1-D water tank/CC04 ion chamber system was used to measure the percent depth doses (PDDs) for each energy setting. OARs and PDD(20cm)/PDD(10cm) collected with the IC Profiler and 1-D water tank/CC04 ion chamber system, respectively, were compared against those collected with the T-shaped phantom submerged in the water tank in the same geometry as the conventional measurements.
Results: For the nominal 6MV FFF beam, the OAR(5cm), OAR(10cm), and PDD(20cm)/PDD(10cm) were 0.93, 0.79, and 0.56, respectively. Agreement of OAR(5cm) and OAR(10cm) measured with the T-shaped phantom and IC Profiler was strong with differences of -0.018 ± 0.003 and -0.02 ± 0.01, respectively across all energy changes. Agreement of PDD(20cm)/PDD(10cm) measured with the T-shaped phantom and 1-D water tank/CC04 ion chamber system was also strong with a difference of 0.02 ± 0.03 across all energy changes.
Conclusion: A T-shaped phantom with embedded OSLDs showed good agreement with conventional measurements; therefore, this can be used as a convenient remote audit to assess linac beam characteristics.
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