EPID-based in vivo dose reconstruction for intensity-modulated radiation therapy treatment

In vivo dosimetry (IVD) is still a complex procedure for intensity-modulated radiation therapy (IMRT). The use of conventional point detectors does not give a good representation of the actual dose delivered to the target volume. On the other hand, EPID-based in vivo dosimetry methods appears to be an interesting and efficient option for carrying out such measurements provided that accurate algorithms allowing to convert EPID signal into patient absorbed dose are available. The present work consists in developing clinically applicable calculation methods to perform EPID-based IVD on intensity-modulated fields. As a starting point, the imaging device used throughout this work is described before considering its response with respect to several irradiation parameters. An IVD method enabling the calculation of the on-axis patient point dose from EPID signal was studied and evaluated for 92 pelvic cancer patients treated with IMRT. Then, a 2D back-projection in vivo dose reconstruction algorithm was developed. In this model, the EPID signal was first converted into absorbed dose in water by convolving the EPID image with dose redistribution kernels. The 2D dose distribution was then back-projected applying correction factors calculated from the patient morphology and transmission factor. The validity of the method was checked for 6 treatment plans (26 fields) in pretreatment situation (i.e. without patient) with homogeneous phantom and finally in vivo. The last part of this work deals with the introduction of mathematical attenuation functions in order to devise a complete in vivo 3D dose reconstruction algorithm. (author)