4D cone beam computed tomography as a novel tool for accurate time-resolved dose calculation in particle therapy

点击翻译按钮获取中文摘要

Cone beam computed tomography (CBCT) scanners are increasingly installed in state-of-the-art gantry-equipped proton therapy facilities. These scanners employ flat panel detector technology and allow the acquisition of a three dimensional (3D) volume in a single gantry rotation. They are currently used in image guided photon and proton therapy to correct interfractional patient positioning errors.In the past three years, several international research groups, including our group, have developed the ability to compute proton dose distributions on intensity corrected CBCT images. This task is non-trivial due to the lower image quality of CBCT compared to CT. CBCT based dose calculations can be used to detect errors in dose delivery caused by interfractional changes such as weight loss. This supports the implementation of adaptive radiation therapy (ART) where corrective measures are applied during the treatment course to restore the initial plan quality. Proton therapy is now beginning to tackle the additional challenges raised by tumours with respiratory intrafractional movement located in the abdominothoracic region (lung or liver tumours). A four dimensional CT scan (4D-CT), which captures the patient’s average breathing cycle, is used for treatment planning. The accuracy of the resulting treatment plan for moving targets can easily be degraded by changes in the breathing pattern, amplitude or baseline. Especially the high sensitivity of protons to such changes suggests that 4D imaging at the treatment site would be desirable, since it would allow ART for moving targets. With their slow rotation speed, clinical CBCT scanners intrinsically allow 4D image reconstruction by tracking the motion of the patient’s diaphragm. However, without intensity correction, the image quality of 4D-CBCT is insufficient for proton dose calculation.There is thus a clear potential for extending 3D-CBCT image correction methods to 4D-CBCT images. For this purpose we will develop a 4D-CBCT correction protocol including optimised CBCT acquisition, image registration and iterative reconstruction algorithms. The methods will be evaluated in a phantom study and retrospectively applied to patient datasets. Corrected 4D-CBCT datasets would allow the careful evaluation of the various strategies proposed to handle moving targets – e.g. different scanning schemes and margin concepts – and their robustness to changes during the course of fractionated proton therapy.The aim of the proposed project is to develop methods for generating corrected 4D-CBCT datasets that allow time-resolved proton dose calculation based on the current breathing pattern and anatomy of the day, and to evaluate ART based upon these methods. The results are expected to enable further research and developments on other ART methods for moving tumours, with the ultimate goal to introduce the best possible high precision proton therapy of moving tumours into the clinic.