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.