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SU‐E‐T‐91: The Effect of Phantom Setup Uncertainties On Gantry Angle Correction Factors for MatriXX Evolution
Author(s) -
Jacqmin D,
Vanek K,
McDonald D,
Peng J,
Ashenafi M,
Koch N,
Jenrette J
Publication year - 2013
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4814526
Subject(s) - imaging phantom , dosimetry , physics , rotation (mathematics) , optics , nuclear medicine , mathematics , geometry , medicine
Purpose: Prior research has shown that the MatriXX Evolution™ 2D ion chamber array has an angular‐dependent dose response. This can be corrected using user‐defined gantry angle correction factors (GACFs). The GACF for a given angle is defined as the ratio of the user's TPS‐calculated dose to the phantom‐measured dose at that angle. This work evaluates the effect of phantom setup uncertainties on GACF values. Methods: Measurements were made with the MatriXX in a MultiCube phantom (IBA Dosimetry). The GACFs were created using the vendor's recommended procedure. The phantom was irradiated with a 6 MV, 27×27 cm2 field at gantry angles 0° to 355° (IEC convention) in 5‐degree increments. Additional measurements were made in 1‐degree increments from 85° to 95° and 265° to 275°. The reference planar doses for each angle were computed using Philips Pinnacle3 TPS. Translation errors of 1 mm and 2 mm along each axis were simulated, along with rotational errors about the longitudinal axis of 0.5° and 1°. GACFs were computed for each shift and compared to the values for the unshifted phantom. Results: Vertical shifts produced changes in GACFs of up to 1.5% at a gantry angles near 180°. Vertical shifts produced changes of up to 8% near 90° and 270°, where the GACFs vary rapidly with angle. Lateral shifts produced changes of up to 1.7%. Longitudinal shifts, which occur parallel to the gantry rotation axis, caused negligible changes to GACFs. Rotational shifts of 1° produced up to 6% changes in GACFs near 90° and 270°. Conclusion: Shifts in phantom position and orientation of less than 2 mm or 1 degree generally produce changes in computed GACFs of less than 2%. Larger changes in GACFs at certain angles may be important for IMRT plan verification for plans with beam angles near the more sensitive angles.