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Comparing ArcCHECK and Portal Dosimetry Gamma Index Scores for VMAT TBI

J Rembish*, S Taneja, K Osterman, N Bice, J Teruel, D Barbee, NYU Langone Health, New York, NY

Presentations

MO-H345-IePD-F3-4 (Monday, 7/11/2022) 3:45 PM - 4:15 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 3

Purpose: To compare the measured gamma index scores obtained through ArcCHECK and Portal Dosimetry measurements for newly implemented volumetric arc therapy (VMAT) total body irradiation (TBI) patient-specific quality assurance (PSQA).

Methods: Pre-treatment patient-specific IMRT QA was performed for 15 separate TBI cases, each using two
methods: measurements on an ArcCHECK phantom and Portal Dosimetry using the Varian aS1200 EPID detectors. Each plan consisted of 8 VMAT fields (field size 15cm x 40cm) which were measured and compared with the treatment planning system using a global gamma analysis, with dose difference (%) and distance-to-agreement (DTA) criteria set to 3% and 2mm, respectively. Due to the large field size, the ArcCHECK is measuring only a portion of the delivered field while Portal Dosimetry is capable of capturing the entire field.

Results: The ArcCHECK gamma score measurements were negatively skewed with a mean of 95.47 and a median of 99.15, with a significantly lower median pass rate among the thorax fields. The Portal Dosimetry gamma score measurements were normally distributed with a mean of 98.58 and a median of 98.55, with a significantly higher mean pass rate among the head fields. Using a Wilcoxon signed-rank test, a significant difference in median gamma index scores was observed between the two measurement methods (p=0.035). A Spearman correlation test suggests there is no correlation between the scores from each method (p=0.949).

Conclusion: The discrepancy in gamma index scores between the two measurement methods must be further investigated before any recommendations can be made on which method to use for VMAT TBI pre-treatment PSQA. It is likely the decreased ArcCHECK scores among the thorax fields are a result of the large field sizes not being fully accounted for on the device, and the fluence outside of the measurement area being larger than the fluence inside.

Keywords

TBI, Quality Assurance

Taxonomy

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

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