3D dose reconstruction for clinical evaluation of IMRT pretreatment verification with an EPID

Pretreatment verification with an electronic portal imaging device is an important part of our patient-specific quality assurance program for advanced treatment techniques. Up to now, this verification has been performed for over 400 IMRT patient plans. For every treatment field, a 2D portal dose image (PDI) is measured and compared with a predicted PDI. Often it is not straightforward to interpret dose deviations found in these 2D comparisons in terms of clinical implications for the patient. Therefore, a method to derive the 3D patient dose based on the measured PDIs was implemented.

For reconstruction of the 3D patient dose, the actual fluences delivered by the accelerator are derived from measured portal dose images using an iterative method. The derived fluence map for each beam direction is then used as input for the treatment planning system to generate an adapted 3D patient dose distribution. The accuracy of this method was assessed by measurements in a water phantom. Clinical evaluation of the 3D dose reconstruction was performed for 17 IMRT patients with different tumor sites. Dose differences with respect to the original treatment plan were evaluated in individual CT slices using dose difference maps and a 3D gamma analysis and by comparing dose-volume histograms (DVHs).

The measurements indicated that the accuracy of the 3D dose reconstruction was within 2%/2mm. For the patients observed dose differences with respect to the original plan were generally within 2%, except at the field edges and in the sharp dose gradients around the planning target volume (PTV). Gamma analysis showed that the dose differences were within 2%/2mm for more than 95% of the points in all cases. Differences in DVH parameters for the PTV and organs at risk were also within 2% in nearly all cases.

A method to derive actual delivered fluence maps from measured PDIs and to use them to reconstruct the 3D patient dose was implemented. The reconstruction eases the estimation of the clinical relevance of observed dose differences in the pretreatment measurements.