3176 - Sensitivity of In-Transit Integrated EPID Gamma Criteria to Phantom Thickness Changes

Purpose/Objective(s):

The accurate detection of changes in patient thickness is an important factor for identifying patient- or setup-related errors that affect treatment delivery. To identify a change in patient anatomy like thickness the limits of the integrated in-transit Electronic Portal Imaging Device (EPID) using gamma analysis needs investigation. The purpose of this study was to evaluate the sensitivity of integrated in-transit EPID gamma analysis criteria by controlling phantom thickness changes and determine detection thresholds relevant to adaptive treatment workflows.

Materials/Methods:

A cubic solid water phantom of 30cm in thickness was used to simulate a patient reduction in thickness. The phantom thickness was reduced in 2mm increments starting with a 30cm block of solid water and an EPID image taken following each reduction. These reductions were meant to simulate progressive anatomical thickness changes.

Gamma analysis was performed by comparing each measurement to the reference measurement obtained at the original 30 cm thickness. Common pre-treatment QA passing gamma analysis criteria 5%/3 mm and 3%/2 mm, and dose difference between 0.5-5% and distance-to-agreement between 0.5-5mm were evaluated. A failure was defined as <90% of evaluated points passing gamma criteria. Measurements were obtained for three beam geometries: anterior–posterior, lateral, and 45° oblique to quantify the effect on EPID sensitivity from beam orientation.

Results:

Conventional gamma analysis criteria of 5%/3 mm and 3%/2 mm showed limited ability to detect the small changes in thickness. For the anterior-posterior and 45° oblique geometries beam failure occurred at 1.2cm, 1cm, and 8mm using 5%/3mm, 3%/2mm, and 1%/1mm criteria, respectively, corresponding to change in the overall thickness of 4.0%, 2.6%, and 1.3%. For the lateral beam geometry, the 5%/3mm and 3%/2mm did not detect thickness changes until reductions 2mm inside the field edge, while tighter criteria like 1%/1mm detected small changes 5mm outside the field edge.

Conclusion:

Standard EPID QA gamma analysis criteria demonstrated limited sensitivity necessary to detect small thickness, requiring reductions of approximately 8–12 mm before failure was detected. While stricter gamma criteria improved sensitivity, the detection thresholds remain relatively large compared to clinically relevant anatomical variations. These findings highlight the need for more sensitive monitoring approaches such as optical surface mapping to identify patient setup from anatomical changes to better support adaptive radiotherapy workflows.