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Development of Correction Factors for a 2D Ion Chamber Array Inserted in a Cubic and Homogeneous Phantom for VMAT Patient-Specific QA

F Clemente*, C Perez-Vara, Hospital Central de la Defensa Gomez Ulla, Madrid, Madrid

SU-E-T-211 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Volumetric modulated arc therapy (VMAT) requires, as another kind of intensity-modulated radiation therapy (IMRT), patient-specific QA procedures. 2D ion chamber arrays are used to analyze and compare dose distributions. When these detectors are used in a stationary phantom, the measured dose is dependent on the angle of the beam. This work shows the development of correction factors (CFs) needed to obtain good agreement between calculated and measured dose distribution.

Methods: VMAT treatments are delivered in our institution with an Elekta Synergy linac. 2D ion chamber array (MatriXX, IBA Dosimetry) is used to perform dose distribution comparisons for patient-specific QA, inserted in a cubic and homogeneous special phantom (MultiCube, IBA Dosimetry). CFs are determined for each individual chamber in MatriXX detector by comparing measurements taken with MatriXX/MultiCube set to dose reference values calculated by our TPS (Monaco 3.1, Elekta) with the same setup. The detector has been set up to avoid irradiation through the treatment couch. Measurements consist on a set of open fields of 27x27 cm² with gantry angles covering the range from 0° to 180° (interval of 1° between 85° y 95° and 5° for any other angle). Assuming a symmetric response, values from 180° to 360° are the same. An external gantry angle sensor has been used to match the measurements with their gantry angle.

Results: CFs have been evaluated for each chamber in all gantry positions. Deviations of up to 18% and 7% are observed for gantry angles at 90° and 180°. These values must be used to correct dose measurements performed with MatriXX/MultiCube set.

Conclusion: Angular dose dependency of MatriXX detector inserted on MultiCube phantom has been studied and calibrated. This calibration should be used to evaluate actual treatments as a part of patient-specific QA procedures.

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