SU‐F‐J‐114: On‐Treatment Imagereconstruction Using Transit Images of Treatment Beams Through Patient and Thosethrough Planning CT Images

Abstract

Purpose:To reconstruct patient images at the time of radiation delivery using measured transit images of treatment beams through patient and calculated transit images through planning CT images.Methods:We hypothesize that the ratio of the measured transit images to the calculated images may provide changed amounts of the patient image between times of planning CT and treatment. To test, we have devised lung phantoms with a tumor object (3‐cm diameter) placed at iso‐center (simulating planning CT) and off‐center by 1 cm (simulating treatment). CT images of the two phantoms were acquired; the image of the off‐centered phantom, unavailable clinically, represents the reference on‐treatment image in the image quality of planning CT. Cine‐transit images through the two phantoms were also acquired in EPID from a non‐modulated 6 MV beam when the gantry was rotated 360 degrees; the image through the centered phantom simulates calculated image. While the current study is a feasibility study, in reality our computational EPID model can be applicable in providing accurate transit image from MC simulation. Changed MV HU values were reconstructed from the ratio between two EPID projection data, converted to KV HU values, and added to the planning CT, thereby reconstructing the on‐treatment image of the patient limited to the irradiated region of the phantom.Results:The reconstructed image was compared with the reference image. Except for local HU differences>200 as a maximum, excellent agreement was found. The average difference across the entire image was 16.2 HU.Conclusion:We have demonstrated the feasibility of a method of reconstructing on‐treatment images of a patient using EPID image and planning CT images. Further studies will include resolving the local HU differences and investigation on the dosimetry impact of the reconstructed image.