D. Franck

2D EPID dose calibration for pretreatment quality control of conformal and IMRT fields: A simple and fast convolution approach.

PURPOSE This work presents an original algorithm that converts the signal of an electronic portal imaging device (EPID) into absorbed dose in water at the depth of maximum. METHODS The model includes a first image pre-processing step that accounts for the non-uniformity of the detector response but also for the perturbation of the signal due to backscatter radiation. Secondly, the image is converted into absorbed dose to water through a linear conversion function associated with a dose redistribution kernel. These two computation parameters were modelled by correlating the on-axis EPID signal with absorbed dose measurements obtained on square fields by using an ionization chamber placed in water at the depth of maximum dose. The accuracy of the algorithm was assessed by comparing the dose determined from the EPID signal with the dose derived by the treatment planning system (TPS) using the ϒ-index. These comparisons were performed on 8 conformal radiotherapy treatment fields (3DCRT) and 18 modulated fields (IMRT). RESULTS For a dose difference and a distance-to-agreement set to 3% of the maximum dose and 2 mm respectively, the mean percentage of points with a ϒ-value less than or equal to 1 was 99.8% ± 0.1% for 3DCRT fields and 96.8% ± 2.7% for IMRT fields. Moreover, the mean gamma values were always less than 0.5 whatever the treatment technique. CONCLUSION These results confirm that our algorithm is an accurate and suitable tool for clinical use in a context of IMRT quality assurance programmes.

3D Absorbed Dose Reconstructed in the Patient from EPID Images for IMRT and VMAT Treatments

A back-projection method has been used in this study to reconstruct the 3D absorbed dose matrix in the patient from EPID images for IMRT and VMAT fields. Images were acquired with the Clinac 23iX aS-1000 imager (Varian) and a 6 MV beam. Then a calibration step was performed to transform the grey levels of the pixels into absorbed dose in water via a response function. Correction kernels were also used to correct for the scatter within the EPID. The dose was then back-projected into the patient for all EPID parallel planes for each gantry angle. Finally the total dose was obtained by summing the 3D dose associated for each gantry angulation. First, the 3D dose reconstruction algorithm was tested for 20 IMRT and VMAT prostate and head and neck treatments in a homogeneous cylindrical phantom. Then the algorithm was used for IMRT brain cancer plans on 20 real patients. The EPID reconstructed 3D dose distributions were then compared to the planned dose from TPS (Treatment Planning System, Eclipse Varian) with a 3D global gamma index of 3% and 3 mm criteria. The percentage of points of which the gamma index was larger than unity was greater than 97% for all IMRT treatments both in the phantom and in the patients and over 96% for all VMAT treatments checked in the cylindrical phantom. Our 3D reconstruction algorithm, validated for homogeneous medium, can be used to verify the dose distribution for IMRT and VMAT fields using in vivo EPID images.