1 Department of Physics and Astronomy, Science and Technology, Aarhus University2 Department of Clinical Medicine - Department of Medical Physics, Department of Clinical Medicine, Health, Aarhus University3 Department of Clinical Medicine - Department of Experimental Clinical Oncology, Department of Clinical Medicine, Health, Aarhus University4 Department of Clinical Medicine - The Department of Oncology, Department of Clinical Medicine, Health, Aarhus University5 Dept. of Physics and Astronomy, Aarhus University6 Institute for Nuclear Research of the Russian Academy of Sciences7 Deutsches Krebsforschungszentrum (DKFZ) Heidelberg8 Department of Physics and Astronomy, Science and Technology, Aarhus University9 Department of Clinical Medicine - Department of Medical Physics, Department of Clinical Medicine, Health, Aarhus University10 Department of Clinical Medicine - The Department of Oncology, Department of Clinical Medicine, Health, Aarhus University11 Department of Clinical Medicine - Department of Experimental Clinical Oncology, Department of Clinical Medicine, Health, Aarhus University
Particle therapy with fast ions is increasingly applied as a treatment option for localized inoperable tumour sites. One of the reasons for the increased complications of understanding heavy ion dosimetry and radiobiology stems from the mixed particle spectrum which occurs due to nuclear fragmentation of the primary ions. Even if patient treatment with heavy ions has been established, the influence of nuclear fragmentation is yet to be fully quantified. The fragmentation spectrum of ions is relevant for particle therapy in numerous ways: 1. Dose distribution: A distinct tail of secondary particles is formed beyond the Bragg-peak. This tail may deliver unwanted dose to normal tissue, however the magnitude of the tail is directly depending on the inelastic nuclear reaction cross sections. 2. Dosimetry a. Stopping power ratios: Routine dosimetry is performed with air-filled ionization chambers. However, the dose is by convention expressed in dose relative to water. The link between these different ways of calculating dose is primarily provided by stopping power ratios, i.e. atomic physics, however since the average stopping powers are weighted by fluence, one might expect a weak dependence on the secondary particle spectrum from fragmentation. b. In addition hereto, fluence correction factors can be calculated which take this effect into account, which are directly a result of nuclear fragmentation in the medium. 3. Radiobiology: Physical dose is not sufficient to describe the outcome of a treatment. The concept of relative biological effectiveness (RBE) translates the physical dose to a biological effective dose which is iso-effective to photon radiation. Radiobiological models based on amorphous track structure such as the Local Effect Model, but also microdosimetry based models both rely on a full description of the mixed particle spectrum found in a point. To date only little has been published about how much the secondary particle spectrum influences the radiobiology. Other effects which have not been investigated by us include the impact on the occurrence of secondary cancer arising from the secondary neutron spectrum in surrounding tissue, and the production of secondary radiation for retrospective (or online) treatment plan verification. We change essential parameters in the underlying nuclear models of the Monte Carlo particle transport code SHIELD-HIT10A, in order to quantify the sensitivity on the three fields mentioned above, including: turning off nuclear fragmentation entirely, changing all ineleastic cross sections +/- 20%, changing key parameters in the Fermi-Breakup (FB) model. Results show nuclear effects have their largest impact on the dose distribution. Stopping power ratios are entirely unaffected, fluence correction factors are merely in the order of 5% or less. Impact on the RBE is next to negligible, and is rather associated with the dependency of dose.