Effect of Energy and Field Size On Angular Dependence of a 2D Diode Array
H Jin*, V Keeling, S Ahmad, Oklahoma Univ. Health Science Ctr., Oklahoma City, OKSU-E-T-74 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To evaluate the effect of energy and field size on directional response (angular dependence) of 2D diode array detector, Sun Nuclear MapCHECK 2.
Methods: The MapCHECK 2 with MapPHAN-MC2 (a solid water block with water equivalent buildup of 5.0 cm above and below the detector plane) was irradiated using the Varian TrueBEAM STx for six different energies (6MV, 6MV-FFF (Flattening Filter Free), 8MV, 10MV, 10MV-FFF, and 15MV) and three field sizes (3cmx3cm, 5cmx5cm, and 10cmx10cm) for every 10 degree. The angular dependence was investigated by comparing these dose measurements with the corresponding dose calculations by the Eclipse treatment planning system (TPS). A mean dose difference ((Measurement-Plan)/Planx100%) at the comparison points (5 points (1cmx1cm) around the isocenter for 3cmx3cm; 13 points (2cmx2cm) for 5cmx5cm and 10cmx10cm fields) was computed. In addition, dependence of calibration on field size was investigated by calibrating MapCHECK 2 with the standard calibration field of 10cmx10cm and a small field size of 3cmx3cm.
Results: The angular dependence was substantial at 90+/-20° and 270+/-20° (mostly >5% for all energies except 15MV). The maximum differences decreased substantially with increasing energies (e.g., for 3cmx3cm at 270°, 53.3+/-3.1% (6MV-FFF), 37.8+/-2.3% (6MV), 23.2+/-2.0% (8MV), 19.2+/-2.0% (10MV-FFF), 9.7+/-2.3% (10MV), -0.1+/-2.0% (15MV)). The FFF beams showed higher angular dependence than conventional beams (~10%). The standard calibration using 10cmx10cm field underestimated dose for the 3cmx3cm field by ~2%. The calibration dependence on field size was approximately 1% smaller using FFF beams. The 15MV beam relatively did not show any substantial angular response. This is probably due to low response of diode to high energy scattered radiation.
Conclusion: The MapCHECK QA for IMAT (RapidArc or VMAT) with beam incident angles around 90° and 270° should be carefully examined because of the directional response and under-dosing of diode for small beam segments.
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