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.