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Accurate relative stopping power prediction from dual energy CT for proton therapy

Methodology and experimental validation
PhD ceremony:Ms J.K. (Joanne) van Abbema
When:November 24, 2017
Supervisor:prof. dr. S. (Sytze) Brandenburg
Co-supervisors:dr. M.J.W. (Marcel) Greuter, dr. E.R. (Emiel) van der Graaf
Where:Academy building RUG
Accurate relative stopping power prediction from dual energy CT for
proton therapy

Proton therapy is part of radiotherapy and increasingly applied in treatment of cancer, especially for children and patients with tumours in the head and neck region. With proton therapy the tumour can be irradiated with less damage to the surrounding healthy tissues and critical structures compared to irradiation with photons. To optimally exploit this benefit of protons, the energy transferred by the protons to the tissues (the dose) must be calculated very accurately. For this, the specific energy loss of the protons for each tissue is determined based on imaging through x-ray computed tomography (CT). In clinical practice, a phenomenological model is used based on an image obtained with a single x-ray spectrum (single energy CT, SECT). The predictions of this model are not patient specific and very inaccurate for materials which differ in composition and density from the materials used for determination of the model parameters.

We have developed a method using two x-ray spectra (dual energy CT, DECT). With this method the electron densities and effective atomic numbers, which determine the specific energy loss of protons in a material, are derived from two images on basis of fundamental theory of the interactions of x-rays. This method provides patient specific predictions with an accuracy better than 2%. This is a large improvement in accuracy and stability of the method with respect to the clinically applied SECT method and can lead to clinical benefit for proton therapy.