Online phantom-less quality assurance (QA) software (EPIbeam) has been utilized to perform and analyze patient pretreatment verification using an electronic portal image device (EPID) for 160 volumetric modulated arc therapy (VMAT) cases. This study aims to propose an action level based patient planning target volumes from the gamma analysis pretreatment QA results.

The EPIbeam software utilized the facility’s Linac MV panel to measure and calculate the gamma analysis index (%GAI) pass rates of delivered VMAT arcs. A total of 320 VMAT arc data points were grouped based on treatment site locations/cancer cases and retrospectively evaluated. Two different gamma calculation parameters were employed by the facility to achieve the required gamma acceptable criteria based on measured planning target volumes of the cases.

The mean %GAI pass rates for the EPIbeam evaluations using 2.0%/2.0 mm and 3.0%/3.0 mm gamma calculation parameters were 98.55 ± 1.12% and 98.72 ± 1.19%, respectively. Approximately 43% of all treatment cases evaluated had larger planning volumes of mean volume of 679.50 ± 54 cm3.

A less stringent gamma calculation parameter of 3.0%/3.0 mm was used by the facility to achieve gamma acceptable criterion of at least 95.0% for large planning volumes. However, a stricter gamma calculation parameter of 2.0%/2.0 mm when used for small planning volumes (of mean PTV of less than 400 cm3), met the same facility’s gamma acceptable criteria of at least 95.0%.