TU‐C‐T‐6E‐01: Accurate IMRT Verification with Fluoroscopic EPIDs: A Generally Applicable Method Based On Direct Imaging of Cross‐Talk Kernels

Purpose: CCD camera‐based electronic portal imaging devices (CEPIDs) are well suited for absolute dosimetric measurements and are applied routinely for IMRT verification in our institute. CEPID dosimetry is mainly hampered by cross‐talk, which originates predominantly from optical photons reflected within the EPID structure. Previously, this cross‐talk was corrected for based on specific assumptions. However, the sensitive cameras in our present CEPIDs allow for direct imaging of the low‐amplitude, spatially extended cross‐talk kernels. Thus complete, position dependent characterization of the cross‐talk has become feasible. Here, we investigate the accuracy with which portal dose images (PDIs) can be derived from CEPID images (EPIs), using this cross‐talk characterization.
Method and Materials: Cross‐talk kernels were obtained from EPIs of 2×2 cm2 ‘pencil’ beams, centered on a 30‐position grid covering the EPID sensitive area. In the experimental set‐up and post‐processing, care was taken to exclude contributions of low‐magnitude long‐range incident MeV‐radiation to the kernels. By interpolation of the measured kernels, a high resolution kernel‐database was generated. Using this database, PDIs can be derived from EPIs by iterative deconvolution of the cross‐talk contribution. Absolute PDI calibration was based on EPIs and corresponding PDIs, obtained by scanning a linear diode array (LDA), for two reference fields. To test the accuracy of this method, we compared EPI‐derived PDIs with LDA‐based PDIs for a series of symmetric and asymmetric fields, including Alderson‐phantom thorax and IMRT fields.
Results: The measured cross‐talk kernels were asymmetrical and position‐dependent. PDIs derived from EPIs with these cross‐talk kernels matched with LDA‐based PDIs to within 1% (1 SD) for all investigated fields (including IMRT), both inside and outside the field penumbras.
Conclusion: We have developed a generally applicable method to determine position‐dependent cross‐talk kernels via direct imaging. With these kernels, we can derive accurate PDIs from EPIs that are well‐suited for dosimetric IMRT verification.