Advanced treatment planning: robust optimization and proton RBE

This thesis aims to improve the way uncertainties are incorporated in the treatment planning process of radiotherapy treatments.

By using robust optimization instead of the more commonly used PTV-optimization the effects of movement on the radiation dose to the target can be calculated more accurately. This thesis describes the expected benefits of robust optimization are for head and neck cancer patients treated with photons. Additionally, an estimate is made of the magnitude of shifts that need to be accounted for during robust optimization of head and neck cancer patients treated with proton therapy.

The biological effect of proton therapy dose is higher than that of equal photon therapy dose. This is caused by the higher linear energy transfer (LET) of protons compared to photons. In clinical practice this is compensated by multiplying proton therapy dose with a constant relative biological effectiveness (RBE) factor of 1.1 when it needs to be compared to photon therapy dose. In reality, the LET is variable which is expected to result in a variable RBE. In this thesis, measurements are described of LET calculations with which the RBE can be calculated for clinical patients. Additionally, two RBE models are compared to simplified models to better understand them. Lastly, a simulation is used to show that the effects of variable RBE for patients cannot be observed directly using patient toxicities, even though it is expected to have an effect on it.