Table 1.

Measurements taken to characterize the EPID and used to input into software to create the model.

Measurement	Comment	Equipment	Phantom ($cm\times cm\times cm$)	Field Size (cm2)
1. Calibrate ion chamber	For absolute dosimetry and to account for daily output variation	Farmer chamber used to calibrate Semiflex back to absolute dose	$30\times 30\times 20$ slabs	$10\times 10$
2. ${\mathrm{S}}_{\text{ij}}$ matrix (and offset)	To measure dose response over entire EPID.	a) Semiflex with a brass buildup in an empty PTW MP3 water tank b) Take 2 EPID images	–	$26\times 26$
3. Open‐field series	At 160 cm SSD	a) Semiflex with brass buildup b) EPID	–	Series
4. Phantom field series	Constant phantom thickness, varying field size	a) Semiflex at isocenter of phantom b) EPID	$30\times 30\times 20$ slabs	Series
5. Phantom thickness series	Constant field size, varying phantom thickness	a) Semiflex at isocenter of phantom b) EPID	Series	$10\times 10$