THE APPLICATION OF TRANSIT DOSIMETRY TO PRECISION RADIOTHERAPY

A method of using electronic portal imaging (EPI) for transit dosimetr y is described. In this method, a portal image of the treatment field is first aligned with a digitally reconstructed radiograph (DRR) to ge ometrically relate the computed tomography (CT) scan, used to generate the DRR, with the EPI. Then the EPI is corrected for scatter within t he patient to yield a map of primary fluence striking the detector. Th is is backprojected through the planning CT data set to yield a distri bution of primary fluence within the patient. This distribution is the n convolved with dose deposition kernels to yield a map of dose delive ry within the patient. Such a distribution may be compared with the do se distribution resulting from the original treatment plan in order to evaluate the adequacy of the treatment. This method has been evaluate d using a humanoid phantom. We find the transit dosimetry relative dos e distribution when compared with film and thermoluminescent dosimeter (TLD) measurements and compared with our planning system to agree wit hin 2% in the pelvic region of a humanoid phantom. (C) 1996 American A ssociation of Physicists in Medicine.