Verification of dose distributions of intensity-modulated radiotherapy beams using a portal imaging device

Abstract

We have developed a method for using a portal imaging device based on amorphous silicon (EPID) for the dosimetric verification of intensity modulated radiotherapy (IMRT) treatments using 6 MV beams. The images obtained using the imaging system are deconvolved with a kernel for radiation and scattering of light generated in the EPID. This kernel has an analytical form and has been experimentally determined. The fluence thus obtained is convolved with a dose deposition kernel, also modeled analytically in a empirical way. The result is an absorbed dose distribution in water at 5 cm depth. The difference between the measured dose using portal imaging device and ionization chamber is –0.3 ± 0.9 % in open fields, with a maximum deviation of 1.5% in a 20 cm × 20 cm field with 60° virtual wedge. The degree of agreement between dose distributions measured with the EPID and ionization chamber array is satisfactory, exceeding all cases evaluated 95% of points with γ < 1 using 3%, 3 mm criteria for gamma analysis.