DE-CT in Lung Cancer Proton Therapy

In order to calculate the dose distribution of protons adequately, accurate estimations of the stopping power ratio (SPR) medium to water, are required. Using a conversion from single energy CT (SE-CT) images results in an uncertainty in the SPR of at least 3-4%. This uncertainty results in in the use of larger margins around the clinical target volume (CTV) and hence more dose to the organs at risk (OAR). It also effects in the conservative use of beam directions, which are often sub-optimal, to avoid irradiating normal tissues. Dual energy CT (DE-CT) improves the accuracy of the SPR and therefore the proton range estimation. An evaluation of the proton range for several tissues using SE-CT and DE-CT as input to Monte Carlo (MC) simulations showed on average improvements in range prediction from 0.1% to 2.1% when using DECT instead of SECT, but in several phantoms and also versus proton-CT, the errors on SE-CT based proton stopping power ratios are reported to be more than 7 %. A limitation of these studies is that most of them were performed in phantoms. In the first clinical data set on five patients with base of skull tumours, it was reported that although the SPR estimation was indeed better for DE-CT than for SE-CT, its clinical relevance was unclear. However, in the same study, phantom measurements showed a large uncertainty of the SPR in the lung. This is due to the large heterogeneity of the lungs and the huge difference in the density of the lungs compared to the mediastinum, the tumour and the chest cavity. It is therefore important to study the SPR differences of SE-CT compared to DE-CT in lung cancer patients and the impact on the dose distribution especially in the context of adaptive radiotherapy. As during the course of concurrent chemotherapy and radiotherapy, which is the standard treatment in the majority of stage III lung cancer patients, important anatomical changes may occur, it is also of clinical relevance to determine the influence of repeated dose calculations on DE-CT.