Effect of mean exicitation energy of water on proton range uncertainties

One of the challenges in proton therapy planning is the range uncertainty. This uncertainty comes from two essential factors. The CT-calibration in one side and the semi-empirical model of deriving the stopping power (SP) in the other side. The latter is dominant by the intrinsic basic physics uncertainty in the so called the mean excitation energy. Range uncertainty due to the CT-calibration is generally taken as 3.5 % of proton to generate the Planning Target Volume PTV, which translating to adding a distal margin of 7-10 mm. The current study is focus on studying the effect of mean excitation energy of water on proton range uncertainties. To do this, we study two different model of calculating the I-values, the published and used I-values in the clinical treatment as it is stated in ICRU report 49 namely 75eV , and the suggested Janni's I values which is equal to 81.75 eV . The preliminary result of the comparison between the mass stopping power shows that the suggested Janni's I-values differed in so far as 1-1.5 % from the ICRU (49) values which used in the treatment planning for both the soft and bony tissues.