Pretreatment quality control of single isocenter half- beam treatment planning technique using an amorphous silicon electronic portal-imaging device (EPID)

Introduction: Electronic portal imaging devices (EPIDs) are fundamentally used for
instantaneous verification of the patient set‐up, block shape, and leaf positions during radiation therapy.
In radiotherapy, situations arise in which an inclined PTV must be treated mutually with adjacent nodal regions. This methodology is most widely used for matching tangential/lateral breast/head and neck fields to supraclavicular and axillary nodal fields. The single-isocenter half-beam technique (SIHB) is the commonly used technique for field matching in the three-dimensional radiation therapy (3DCRT) in which abutting fields asymmetrically with no divergence at the central axis are matched. Linear accelerator asymmetric jaw junctioning is used in single isocenter half-beam treatment planning (SIHB) technique. In practice, the major difficulty of the SIHB technique lies in creating the dose accuracy at the central axis and dose homogeneity in the abutting region of half beams with sufficient precision represents.
The aim of this work was to investigate the use of amorphous silicon EPIDs for pretreatment quality control of the SIHB-based treatment plans.
Materials and Methods: The zero jaw position was studied at the gantry angle of 0º. All measurements were made for 6, 10, and 15MV photon beams. The EPID used in this study was a CCD camera‐based iView EPID mounted to an Elekta synergy platform. The EPID has been calibrated to convert a grayscale EPID image into a two‐dimensional absolute dose distribution. In addition, calculations of the central field dose were performed at a fixed EPID distance of 157.5cm.
The double-exposure technique and EPID was used for measuring the junction doses for each asymmetric jaw. The junction dose was obtained as a function of jaw position.
Then, the EPID- based junction dose measurements were compared with radiographic film measurements of junction dose.
Results: The results, obtained for 6, 10, and 15MV photon beams, showed that there was an approximately linear relationship between doses and gap or overlap. Because of the non- water equivalent EPID scattering properties, there were a different in the junction doses measured with the EPID and film. A correction factor was obtained to convert the EPID measured junction dose to film measured equivalent.
Conclusions: The inaccurate abutting junction between the half beams results in under or overdose of the normal tissue between the two volumes. In the present study, we carried out a procedure for daily EPID-based quality assurance of the SIHB technique of treatment planning. The utilization of this method can be significant enough to warrant the accuracy and safety of the SIHB technique in clinical procedures.
Conclusion: our study showed that the aSi EPID can be used as a transit dosimeter. The technique to reconstruct the dose delivered in the patient is straightforward, reproducible, and rapid for repeated in vivo dose verification.