Advanced photon and proton imaging for proton therapy planning
The agreement between the actual and planned dose distributions in proton therapy of tumours critically depends on accurate proton stopping powers in the tissues traversed by the protons. These stopping powers are determined from the electron densities with corrections due to the elemental composition and molecular structure.
Currently, proton therapy treatment plans are based on CT-images using one X-ray spectrum (Single Energy CT, SECT). A SECT image quantifies the photon attenuation of a tissue as a CT number which combines information on the tissue’s electron density and elemental composition. Using a general correlation between CT number and proton stopping power, the stopping powers for the treatment planning are determined.
The uncertainty in the proton range caused by the uncertainty of the stopping powers deduced from SECT-images amounts, depending on the complexity of the tumour surroundings, up to about 3%, i.e. 3 – 6 mm for typical depths of tumours. This uncertainty imposes significant constraints on the treatment planning process (e.g. limitation of possible fields due to critical organs, sub-optimal reduction of dose in healthy tissue). In particular, large uncertainties are associated with situations where the protons pass through materials with significant differences in density or elemental composition. Typical examples are the treatment of tumours in the head and neck region, in the lungs and in the lower abdomen. Image reconstruction artefacts for the growing group of patients with prostheses containing ceramic and/or metallic parts complicate the situation even more. In the head and neck region, metallic inlays in teeth, present in most patients, pose serious problems.
We are studying the potential of combining advanced X-ray imaging techniques such as Dual Energy CT (DECT) and spectral CT with proton imaging (proton radiography and proton CT) for stopping power prediction. Being charged particles, protons, in contrast to X-ray photons, gradually lose energy by interacting with electrons and nuclei in the matter, leading to multiple Coulomb interactions. Both proton energy loss and scattering can be used for radiographic imaging. Proton imaging has a poorer spatial resolution than X-ray imaging but a higher contrast between different soft tissues. Moreover, proton-based images contain direct information on proton stopping power and do not need a conversion procedure as is the case for photon-based images.
The project encompasses extensive simulations of and experiments using phantoms of precisely known geometry and composition that in the final stage of the project will be representative for the complex geometries encountered in practice. CT-data of the phantoms are generated experimentally with single and dual energy and possibly experimental spectral CT-scanners. From these data the stopping power of the charged particles is deduced using different algorithms for the conversion of the CT-data into electron density and effective atomic number and subsequently stopping power. The stopping powers are also obtained directly from the known composition and geometry of the phantoms. Experimental stopping powers are determined from proton radiography, proton CT or direct proton stopping power measurements on the phantoms performed at the KVI-CART accelerator facility. Comparison with the calculated values obtained via the different routes make it possible to identify the origin of discrepancies and thus to improve the modelling.
The ultimate goal of this project is to generate stopping powers with sub-percent accuracy so that the stopping power data is no longer a limiting factor for the quality of treatment plans in proton therapy.
|Last modified:||28 June 2017 10.17 a.m.|