Contrast Enhanced Proton Radiography for In-room Soft Tissue-based Setup

Abstract

Purpose/Objective(s): The current clinical protocol is to align the patient based on bony anatomy using X-ray KV images. No soft-tissue alignment is used, mainly because the tumor is usually not visible in the beams eye view (BEV) of the KV image. The aim of this study was to investigate setup based on soft tissue registration for lung cancer patients treated with particle therapy, by using proton radiography and CT prior knowledge. Materials/Methods: The planning CTs of 5 patients were pre-processed by masking out the GTV volume plus 3 cm isotropic margin, if included inside the lung, to account for tumor motion. A DRR along planned BEV was generated from this dataset, for a total of 6 BEV angles. Proton radiographs were also computed for the same BEV relying on Monte Carlo simulations. The DRR was subtracted from the radiograph, thus obtaining a high contrast image around the tumor region. The GTV was automatically located on the new image and compared to the ground truth extracted from the CT plan. Results: Contrast enhanced proton radiography permitted a 400% increase of the contrast between tumor and background. The table below reports the mismatch between the estimated and the actual center of mass of the tumor for the 6 planned proton BEVs: the median quartile was 0.91 1.59 mm. As expected, lowest mismatches were