Sha Wang¹*, Y Zheng¹, (1) ,Guangzhou Concord Cancer Center, 44, CN,

PO-GePV-T-231 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: The dose discrepancy for a commercial 2D detector during VMAT or IMRT QA could be high and lead to incorrect QA fails even calibration and operation procedures by vendor are closely followed. In this study we investigated the root cause of the discrepancy and developed a new procedure to more accurately account for the angular dependence.

Methods: A 2D array detector (Matrixx, IBA, Belgium) was used at our center for patient-specific QA. We measured profiles for a narrow beam with Matrixx located at various depths in increment of 0.5 mm from the top to bottom of the sensitive volume of the array detectors and at different gantry angles from 0 to 360 degrees. The optimal depth for QA measurement was determined based on the measured beam center shift at the depth where minimum shift was obtained.

Results: There is shift between the beam axis and measured dose profile by MatriXX for oblique beam incidence. This shift varies with beam angle, increasing from vertical direction to lateral direction, up to ~35mm at vendor suggested depth. The shift also varies with depth, with depth of ~2.5mm below the vendor suggested depth the minimum (almost 0 for most beams), which is chosen as the depth for all QA measurements. Using the new depth we determined, QA results (3%/2mm Gamma analysis) are largely improved with an average of 99.38% passing rate for 10 patients (none fails for 95% criteria) compared to 94.13% before.

Conclusion: MatriXX QA could fail incorrectly due to beam center shift for oblique beams at certain depths. We suggest that the measurement be made at the optimal depth based on measurements to improve the VMAT or IMRT QA accuracy with original beam angles and avoid unnecessary repeated QA or plan changes due to QA system errors.

Keywords

Not Applicable / None Entered.

Taxonomy

TH- External Beam- Photons: Quality Assurance - IMRT/VMAT

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