Two-dimensional transmitted dose measurements using a scanning liquid ionization chamber EPID.

The use of a scanning liquid ionization chamber electronic portal imaging device (SLIC-EPID) for two-dimensional transmitted dosimetry was investigated and a calibration method was developed using extended dose range (EDR2) film. In order to convert pixel value to dose, the acquired SLIC-EPID pixel values were calibrated using an ionization chamber on the central axis. The relationship between pixel values, dose rate and absorbed dose was identified for various linac output repetition rates. To correct EPIs for dosimetric purposes, the off-axis ratio of dose profiles measured by EPIDs and EDR2 film was used to derive correction factor matrices (CFMs) for a range of source-to-EPID distances (SEDs). The corrected relative dose maps acquired for different conditions, including open and wedged fields, measured using a SLIC-EPID were compared with EDR2 film images using a gamma function algorithm with distance to agreement (DTA) = 2.5 mm and dose difference (DeltaDmax) = 1% criteria. The results showed that (a) for two-dimensional dosimetric purposes, EPIDs must be calibrated using appropriate two-dimensional correction factors and (b) SLIC-EPIDs can be used to measure the transmitted dose with good accuracy.