SU-E-T-206: IMRT/VMAT Quality Assurance Using Portal Dosimetry.
PURPOSE
The purpose of this work is to investigate the application of an amorphous silicon electronic portal imaging device (EPID) in IMRT/VMAT QA as a replacement for conventional IMRT/RapidArc QA tools.


METHODS
20 IMRT/RapidArc treatment plans for treatment site of head and neck, breast, lung, abdomen, and pelvis were selected randomly for the study. Verification plans on ArcCheck phantom (Sun Nuclear Inc.) (ArcCheck Verification Plan, AC_Veri_Plan) and for portal dose prediction (PD_Veri_Plan) were generated in Eclipse treatment planning system, and delivered in ArcCheck phantom measurement and portal dosimetry for each treatment plan, respectively. For AC_Veri_Plan, absolute dose was measured by ion chamber inserted in ArcCheck phantom and relative dose is evaluated by the gamma method (criterion %3/3mm) using SNC dosimetry software. For PD_Veri_Plan, absolute intensity was measured in an area of 1cmx1cm and relative dose by the gamma method in portal dosimetry. The results of absolute and relative dose difference using both of phantom measurements and portal dosimetry were compared.


RESULTS
For 20 AC_Veri_Plans, variation between the measured and calculated absolute dose is within 3% {range - 2.29% to 2.34%, average 0.46%}. The percentage of gamma<1 is above 90% {range 93.9% to 100%, average 98.85%}. For 20 PD_Veri_Plans, variation between measured and calculated intensity is within 4% {range -2.47% to 3.34%, average 1.26%}. The percentage of gamma<1 is also above 90% {range 95.9% to 100%, average 99.61%}. All of AC_Veri_Plans and PD_Veri_Plans pass the QA, although the variation of absolute intensity on average in PD_Veri_Plans has about 0.8% higher than that of AC_Veri_Plans. The tolerance is 5% for the absolute dose variation and 90% of gamma<1 for the relative dose.


CONCLUSION
Phantom measurement for IMRT/VMAT QA is a time-consuming task. Portal dosimetry can be used as a rapid and convenient plan verification tool for IMRT/VMAT.