A study of the spectral response of portal imaging detectors

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 Gd₂O ₂S:Tb phosphorescent screen, exhibits highly nonlinear 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 the 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 multiply-scattered 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 overestimates 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 multiple-scattering studies