Volume of interest cone beam CT with a 2.35 MV beam generated with a carbon linear accelerator target

Purpose: This is a proof-of-concept study of volume of interest (VOI) cone beam CT (CBCT) imaging using an x-ray beam produced by 2.35 MeV electrons incident on a carbon linear accelerator target. Methodology regarding image acquisition and reconstruction is presented and exemplary image data are shown. The dose distribution in a head phantom resulting from the low-Z target VOI method is shown to demonstrate the potential of the technique to minimize dose to peripheral tissues. Methods: A workflow is presented whereby a VOI is designed in the planning system and a dynamic MLC sequence is established to track that volume during imaging. Missing data in truncated CBCT projection images are completed using a priori DRR information generated from the planning CT set. The utility of this approach in reducing truncation artefacts is shown through imaging of the head-and-neck section of a lamb. TLD and Gafchromic film measurements are used to compare the imaging dose distributions for the 2.35 MV/Carbon beam, from VOI and full-field techniques. Results: The feasibility of the low-Z target VOI CBCT is demonstrated. Marked boundary ring truncation artefacts surrounding the imaged VOI in three dimensions are resolved by the DRR filling technique. Dosimetric measurements in the anthropomorphic head phantom demonstrate that the dose to peripheral tissues is reduced to approximately 25% of that inside the VOI. Conclusion: VOI CBCT is viable using a 2.35 MV/Carbon beam. The technique couples the desirable imaging characteristics of the low-Z target beam with the flexibility to localize the imaging dose to the anatomy relevant to the image guidance task.