SU-E-T-109: An Evaluation of a Newly Available Portal Dosimetry QA Tool for SRS Dynamic Arc and VMAT

In our institute, the MatriXX has been used for IMRT, VMAT and dynamic arc QA. However, the small fields (<3cm) shaped by HDMLC for SRS dynamic arc (DA) present QA measurement issues when using the MatriXX due to its limited resolution (7.6 mm). A recently available portal dosimetry (PD) software tool in Eclipse Version 10 is capable of performing QA for arc plans. This study is to evaluate this new tool for QA.

EPID calibration was performed using a Varian Novalis TX unit with HDMLCs for a 6X high dose rate SRS beam. The PDIP algorithm was configured using measured beam data. Brainlab iPlan was used to generate the DA patient treatment plans. The plans were then exported to Aria via DICOM RT. A verification plan was created in Eclipse and the predicted portal dose calculated. The comparison was done between predicted and measured portal dose distributions.

Compared with chamber array analysis, the measured portal dose shows significant improvement in correlation with predicted dose. To eliminate the uncertainty of dose normalization, absolute CU is chosen for analysis. Gamma factor was studied. Thecriterion was set at 3%/1mm using a 10% dose threshold such that 95% of the pixels had a gamma <1. QA was performed on 9 plans and the average gamma passing rate is 98.5%, with passing rates ranging from 95% to 100%.

With approximately 0.7mm resolution, it is shown that the newly available PD tool is capable of overcoming the limitations of chamber arrays for small field SRS QA and is also suitable for HDMLC QA to provide detailed dose comparison with highly modulated dose profiles within a small treatment field. In addition, the time needed to perform the QA procedure using the PD tool is significantly shortened compared to the MatriXX.