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The Effect of Phantom Setup Uncertainties On Gantry Angle Correction Factors for MatriXX Evolution

D Jacqmin

D Jacqmin*, K Vanek, D McDonald, J Peng, M Ashenafi, N Koch, J Jenrette, Medical University of South Carolina, Charleston, SC

SU-E-T-91 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

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, 27x27 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.

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