Introduction: Electronic portal imaging devices (EPIDs) could be used to dose verification of radiotherapy treatment plans. In vivo dose verification is performed to reduce differences found between dose delivered to the patient and the prescribed dose. The aim of this study was to perform a fast and efficient technique for the verification of delivered dose to the patient using an a-Si EPID. To this end, the transit dose was measured using the EPID. Then, back projection of the portal dose images obtained during the treatment was performed to reconstruct the in vivo dose.
Materials and Methods: This study was performed using an amorphous silicon flat panel- type imager (a-Si EPIDs) (Elekta iViewGT) on an Elekta linear accelerator. EPID transit images were acquired for different high-energy open photon beams (i.e., 6, 10, and 15 MV). Then, the following procedure was used to derive the parameters of a formalism to reconstruct the dose in a single plane, intersecting the isocenter, perpendicular to the beam axis and parallel with the EPID detector plane.
The formalism is based on the conversion of the aSi EPID response to dose at the maximum depth in water. Moreover, the doses at the maximum depth in the absence of phantom as well as dose in the phantom at the point of interest are obtained by using the transit Tissue Maximum Ratio (TMR).
The ionization chamber located at the center of the EPID plane was used to measure the dose. Then, a comparison between the central axis doses of the EPID signal and ionization chamber measurements was performed.
In order to check the clinical implementation of the technique as well as the verification of patient plans, the protocol has been performed on 10 patients with prostate cancer treated with three dimensional conformal beams. For every patient, EPID images acquisition and measurement were made during the radiotherapy for seven successive fractions.
Results: The deviation between the ionization chamber and EPID signals in this study was in acceptable range (1 SD) which is consistent with the tolerance of the established studies. Furthermore, ratios between the prescribed dose planned by TPS and reconstructed dose from EPID measurements agreed well (mean±SD: 0.995±0.033).
Conclusion: our study showed that the aSi EPID can be used as a transit dosimeter. Furthermore, the applied method to reconstruct the dose delivered in the patient is straightforward, reproducible, and rapid for repeated in vivo dose verification.