Table 1.

Summary of commercially available EPID units used in this study.

Model type	Active area (pixels)	Spatial resolution (mm/pixel)
Varian aS500	$512\times 384$	0.784
Varian aS1000	$1024\times 768$	0.392
Siemens OptiVue 500	$512\times 512$	0.4
Siemens OptiVue 1000	$1024\times 1024$	0.4