Consequences of the spectral response of an a-Si EPID and implications for dosimetric calibration
The relative spectral distribution of photons detected at a plane corresponding to an EPID setup at for the open and attenuated beams. There is a substantial reduction in the overall number of photons detected, as well as an increase in the mean energy, indicating that the beam has been hardened.
The dose to the EPID phosphor in the conventional setup with of solid water on the EPID surface. The phosphor dose is assumed to be directly proportional to the detector response of Eq. (1). Above we note the very nearly linear rise of dose with incident photon energy. Below there is a sharp rise in the dose to the phosphor. This sensitive region is likely responsible for the discrepancy between open and attenuated beam dose calibration curves.
Mass energy-absorption coefficients plotted as a function of photon energy for (GOS) and air (Ref. 20). Air, the absorbing material of an ion chamber, has a reasonably uniform coefficient from . GOS, however, has a distinct rise below . The total attenuation coefficient for Cu has also been plotted to indicate the relative proportion of photons of a given energy that would interact in a Cu plate placed upstream from the phosphor.
The EPID dose response as a function of photon energy for different thicknesses of Cu plate placed in contact with the surface of the EPID. There are greater reductions in the region below than in the region above .
The EPID dose response as a function of photon energy for a Cu plate in the contact and elevated configurations. Linear fits to the copper plate data above are shown in gray. Below , the differences between the EPID dose response and the extrapolation of the respective linear fit are smaller for the elevated configuration.
Monte Carlo simulated calibration curves for a SDD attenuated beam scenario. Shown are curves for the conventional setup and for the contact and elevated configurations including a Cu plate. Results have been normalized to the open-field (no compensator) case. The lowest relative doses and ion chamber response correspond to the thickest compensator ( steel shot).
The maximum difference between an assumed ideal (linear) EPID calibration curve and simulated calibration curves for a beam attenuated by varying thicknesses of steel shot. The error bars derive from uncertainties in the fit coefficients obtained in the regression analysis.
Experimental calibration curves where the attenuated beam measurements were made at SDD (a) and SDD (b), respectively. Maximum differences between the attenuated beam data and the open-field trend lines (extrapolated in the SDD case) are 8.2% and 7.7%, respectively.
Calibration curves modified by the addition of of Cu in the contact (a) and elevated (b) configurations. The maximum deviation was reduced from 8.2% in the conventional setup to 6.4% in the contact and 3.2% in the elevated configurations.
The maximum discrepancy between the measured EPID response for an open field and an attenuated field at SDD as Cu thickness is increased in both the contact and elevated configurations. The error bars derive from uncertainties in the fit coefficients.
The relative contrast-to-noise ratio (CNR) as a function of external Cu plate thickness. CNR is reduced substantially in both configurations, but the drop is greatest for the elevated plate case. This is possibly due to a reduction in contrast otherwise present with more scattered photons from the Cu reaching the phosphor in the contact configuration.
Ratio of to for the conventional configuration vs Cu thickness measured using the QC-3V phantom for the contact (a) and elevated (b) configurations.
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