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Impact of Detector Element-Specific Angular Correction Factors for the MaxtriXX Ion Chamber Array for Patient-Specific Dose Validation

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O Casares

O Casares1*, S Kim2, J Armas1, N Papanikolaou2, A Gutierrez2, (1) Hospital Universitario de Canarias, La Laguna, Santa Cruz de Tenerife, (2)University of Texas HSC SA, San Antonio, TX

SU-E-T-200 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The purpose of the study was to create detector element-specific angular correction factors (DESACF) for each detector element of the MatriXX planar ion chamber array and compare them to the default ACF provided by the vendor. Additionally, the impact of these correction factors on the Gamma Index (GI) was quantified.

Methods: The ACF for each detector was determined using the MatriXX manual guidelines. By doing so, the ACF of a specific ion chamber element is determined by the ratio of the calculated dose to the MatriXX measured dose as a function of gantry angle. Dose calculations were performed in Pinnacle TPS (Ver. 9.2) using 6 and 10MV energies. Open field beams were calculated every 5° (0° to 180°) with a 1° sampling (85° - 95°). With this, a DESACF table was created. To evaluate the impact of the DESACF on patient-specific dose validations, eleven (n=11) previously irradiated patient-specific dose validations using 6MV VMAT beams were re-analyzed. GI values were recorded using the criteria: 3%/3mm, 2%/2mm, 1%/1mm.

Results: The DESACFs for 6MV were found to be different than the default ACF by up to 7% for given angles, significant differences existed for angles (85° to 95°). For 10MV, differences (5%) existed when DESACFs were created using solely the mean CF values of a 10x10cm2 ROI versus a 21x21cm2. Differences between the mean patient GI values using the DESACF and default ACF for 3%/3mm, 2%/2mm, & 1%/1mm were 0.6 (p>0.01), 2.3 (p=0.02), & 0.1% (p>0.01), respectively. Statistically significant differences might be noted with a larger sample, especially at 2%/2mm.

Conclusion: Large differences in the default ACF and DESACF were noted for 6MV. For 10MV, the DESACF showed dependence on ROI size to derive mean CF value. For evaluated cases, no significant difference was noted in the GI due to DESACF.

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