RADIOTHERAPY TREATMENT VERIFICATION

During a radiotherapy treatment, a dosimetric verification or a geomet ric localization can be done, in order to assess the quality of the tr eatment. The dosimetric verification is generally performed measuring the dose at some points inside (natural cavities) or outside the patie nt, and comparing it to the dose at the same points calculated and pre dicted by the treatment planning system. This can be done either with thermoluminescent or diodes dosimeters or with ionization chambers. Th e geometric localization can be done acquiring a portal image of the p atient. Portal imaging can be performed either with films placed betwe en metallic screens, or with an electronic portal Imaging device such as fluoroscopic systems, solid state devices or matrix ionization cham ber systems. In order to assess possible field placement errors, the p ortal images have to be compared with Images obtained with the simulat or in the same geometric conditions and/or with the digitally reconstr ucted radiograph (DRR) obtained with the treatment planning system. In particular, when using matrix ionization chamber systems, the portal images contain also information regarding the exit dose. This means th at this kind of imaging device can be used both for geometric localiza tion and for dosimetric verification. In this case, the exit dose meas ured by the portal image can be compared with the exit dose calculated and predicted by the treatment planning system, Some ''in-vivo'' appl ications of this methodology are presented.