Electronic portal imaging for dosimetric purposes

Dosimetric aspects of Electronic Portal Imaging Devices (EPIDs) have been explored. Three models to extract the primary signal from EPIDs have been studied. The first simple model removes the scatter component using a method based on the scatter-to-primary ratio (SPR). This model has several limitations. Two more complex models based on convolution of Pencil Beam Spread Kernels (PBSK) have been studied, one analytical approximation and one iterative solution. Both models based on PBSK were tested on a large set of Monte Carlo generated EPID signals. The primary signal was extracted to within 2% of the true primary signal, using either model, even when SPR was 0.3 in the original image. On the basis of the primary signal of the EPID, generated from any of the above models, two new applications were developed and studied: Transit dosimetry,1 this is verification of the dose delivered using an EPID; and the design of intensity modulated beams (IMB) for tangential breast fields. The IMBs were implemented using either a physical compensator or a set of fields shaped with a multileaf collimator. Dose measurements in an anatomical phantom showed that both implementations gave better dose uniformity than the standard treatment. 1V. N. Hansen, P. M. Evans, and W. Swindell, Med. Phys. 23, 713–721 (1996). © 1997, American Association of Physicists in Medicine. All rights reserved.