Abstract
Range verification is one of the most relevant tasks in ion beam therapy. In the case of carbon ion therapy, positron emission tomography (PET) is the most widely used method for this purpose, which images the -activation following nuclear interactions of the ions with the tissue nuclei. Since the positron emitter activity profile is not directly proportional to the dose distribution, until today only its comparison to a prediction of the PET profile allows for treatment verification. Usually, this prediction is obtained from time-consuming Monte Carlo simulations of high computational effort, which impacts the clinical workflow. To solve this issue in proton therapy, a convolution approach was suggested to predict positron emitter activity profiles from depth dose distributions analytically. In this work, we introduce an approach to predict positron emitter distributions from depth dose profiles in carbon ion therapy. While the distal fall-off position of the positron emitter profile is predicted from a convolution approach similar to the one suggested for protons, additional analytical functions are introduced to describe the characteristics of the positron emitter distribution in tissue. The feasibility of this approach is demonstrated with monoenergetic depth dose profiles and spread out Bragg peaks in homogeneous and heterogeneous phantoms. In all cases, the positron emitter profile is predicted with high precision and the distal fall-off position is reproduced with millimeter accuracy.
Highlights
The physical advantage of carbon ion therapy lies in its high ballistic precision in longitudinal and transversal direction, but it makes it more sensitive to deviations from the treatment plan
Since the positron emitter activity profile is not directly proportional to the dose distribution, until today only its comparison to a prediction of the positron emission tomography (PET) profile allows for treatment verification
While the distal fall-off position of the positron emitter profile is predicted from a convolution approach similar to the one suggested for protons, additional analytical functions are introduced to describe the characteristics of the positron emitter distribution in tissue
Summary
The physical advantage of carbon ion therapy lies in its high ballistic precision in longitudinal and transversal direction, but it makes it more sensitive to deviations from the treatment plan. Since carbon ions undergo nuclear reactions in tissue and create positron emitters like 11C (T1/2 = 20.39 min) and 15O (T1/2 = 2.03 min) (Tomitami et al 1993, Parodi et al 2002), positron emission tomography (PET) has been introduced as an in vivo treatment verification tool (Enghardt et al 1999, Shakirin et al 2011, Bauer et al 2013a). Due to different underlying physical processes, dose deposition and positron emitter activation are not directly comparable to each other, which prevents direct treatment verification. Differences between the two distributions indicate deviations between the delivered and planned treatment and the consequential need for a revision and refinement of the treatment plan or patient setup
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