A study of the spectral response of portal imaging detectors

Abstract

There is significant current interest into the use of electronic portal imaging systems for radiotherapy dosimetry. Most work presented to date assumes idealized models for the spectral sensitivity of imaging systems, such as Compton, photopeak or photon counting. In this paper it is shown that a typical portal imaging detector, comprised of thin metal converter plate (1 mm Cu) with a 134 mg/cm2 Gd202S:Tb phosphorescent screen, exhibits highly non-linear energy response with a sharp rise in sensitivity at low energies.The presence of scattered radiation significantly reduces the mean energy of a radiotherapy beam, leading to overestimation of the fluence measured by the imaging device. Two simple methods are studied for reducing this low-energy contamination effect: (i) increasing the air gap between the patient and detector, and (ii) increasing the metal converter plate thickness in order to absorb the lower energies. Monte Carlo (MC) simulations are presented showing both the spectral response of detectors with varying converter plate thickness and the photon spectra expected at varying distances from a scattering object. The effects of low-energy multiplyscattered photons are also investigated.Experimental results are presented showing the effects of air gap, converter plate thickness and multiple-scattering.Results are compared to the MC predictions for an ideal Compton detector model, showing fluence over-estimates of more than 30% for the worst case. Excellent agreement is seen between the experimental and MC results for the variable air gap, variable converter plate thickness and multiplescattering studies.

A simple backprojection algorithm for 3D in vivo EPID dosimetry of IMRT treatments

“…the EPID, is, in principle, energy dependent. 4,[34][35][36] The latter effect has, however, been reduced by the additional 2.5 mm thick copper plate. The build-up correction parameter describes an empirical correction of the depth-dose curve ͓see Eq.…”

Pretreatment verification of IMRT absolute dose distributions using a commercial EPID

“…24,25 The parameters which have been considered are the phantom thickness, field size, and the air gap between the phantom and the EPID. The aim was to evaluate the opportunity of introducing in the measurement system a filter to prevent low-energy scattered radiation from reaching the EPID, thus drastically reducing the dependence on field size of the system response.…”

On Radiotherapy Dose Verification With a Flat-Panel Imager

“…51,52,73,76,92,138 The distribution of lowenergy photons reaching the EPID is in principle field size and position dependent (when there is a patient or phantom in the beam, it also depends on the exact patient or phantom geometry). Due to the shape of the flattening filter, low-energy photons contribute relatively more to the energy spectrum offaxis.…”

“…The latter is especially relevant at small patient-detector air gaps (< 40 cm), in which case the scatter contribution from the irradiated volume (of a phantom or patient) to the EPID dose may be considerable. 92,138,138 Yeboah and Pistorius 138 investigated the dependence of the EPID dose-response on scatter from a phantom using Monte Carlo calculations for a copper-phosphor portal imaging screen. They found a broad peak in the photon spectra at low energies (50-100 keV) and observed a high sensitivity of the imager for low energy photons.…”

“…They found a broad peak in the photon spectra at low energies (50-100 keV) and observed a high sensitivity of the imager for low energy photons. Previous authors have reported on the need for additional build-up layer material such as polystyrene (PS), 12,73,73 stainless steel 96 and Cu 90,90,92 for EPID dosimetry applications. As suggested by Partridge et al, 92 a sharp rise in the mass attenuation coefficient of Cu below 500 keV indicates that this material should preferentially filter out low energy photons.…”