Two-dimensional dose reconstruction using scatter correction of portal images

Electronic portal imaging devices (EPIDs) could potentially be useful for patient setup verification and are also increasingly used for dosimetric verification. The accuracy of EPID for dose verification is dependent on the dose-response characteristics, and without a comprehensive evaluation of dose-response characteristics, EPIDs should not be used clinically.

A scatter correction method is presented which is based on experimental data of a two-dimensional (2D) ion chamber array. An accurate algorithm for 2D dose reconstruction at midplane using portal images for in vivo dose verification has been developed.

The procedure of scatter correction and dose reconstruction was based on the application of several corrections for beam attenuation, and off-axis factors, measured using a 2D ion chamber array. 2D dose was reconstructed in slab phantom, OCTAVIUS 4D system, and patient, by back projection of transit dose map at EPID-sensitive layer using percentage depth dose data and inverse square. Verification of the developed algorithm was performed by comparing dose values reconstructed in OCTAVIUS 4D system and with that provided by a treatment planning system.

The gamma analysis for dose planes within the OCTAVIUS 4D system showed 98% ±1% passing rate, using a 3%/3 mm pass criteria. Applying the algorithm for dose reconstruction in patient pelvic plans showed gamma passing rate of 96% ±2% using the same pass criteria.

An accurate empirical algorithm for 2D patient dose reconstruction has been developed. The algorithm was applied to phantom and patient data sets and is able to calculate dose in the midplane. Results indicate that the EPID dose reconstruction algorithm presented in this work is suitable for clinical implementation.