Two-dimensional dosimetric calibration of an electronic portal imaging device (EPID) for transit in vivo dose verification in external radiotherapy

Introduction An algorithm to convert EPID signal (SEPID) into dose in water (DWATER) in transit conditions (i.e. with patient) is proposed and evaluated. This work is a necessary first step to perform patient dose reconstruction during his treatment. Materials and methods Conversion of EPID signal into dose is performed according the 4 following steps: 1. Conversion of (SEPID) into relative dose (DREL) thanks to a relative dose response function (fRD) defined for a reference field size (cREF). 2. Deconvolution of DREL with a field size dependance kernel (Kc). 3. Convolution of the array with a dose to water redistribution kernel (Kw). 4. Use of a transmission function to take into account EPID response variation induced by the modification of the beam spectrum energy in case of transit dosimetry. Development and evaluation of our algorithm were perform using in-house MatlabTMsoftware. On a first part, the dose determined from SEPID was compared to ionization chamber measurements on beam axis for several fields. Then, dose profiles were reconstructed from SEPID and compared with those calculated by our treatment planning system for field size ranging from 3 × 3 cm 2 to 20 × 20 cm 2 . Results The relative dose response function established for cREF = 10 × 10 cm 2 was found to be linear. Indeed, fRD can be written as SEPID = a * DWATER + b. Kernels were both modeled as 2D functions dependent on pixel coordinates ( x,y ). O Kc ( x,y ) is an exponential function. O Kw ( x,y ) is a Gaussian function. For all fields, the deviation between the calculated dose and the one measured with the ionization chamber was within ±0.5%. Concerning dose profiles, more than 90% of studied points have a gamma index below than 1 (3%-3 mm). Conclusion All results obtained from our model are in line with experimental measurements. It also confirms that the proposed method will allow a 2D and 3D in vivo dose reconstruction from EPID signal.