Microdosimetry of therapeutic proton beams

Abstract

All kinds of radiotherapy treatments aim to deliver the maximum dose of radiation to the tumour cells while sparing the surrounding healthy tissues as much as possible. However, there are many cases of conventional, photon­based, radiotherapy where it is not possible to avoid the irradiation of critical organs surrounding the tumour. In order to overcome these physical limitations, the use of charged particles like proton or light ions for treatments with ionizing radiation fields has increased enormously in the last few decades due to their more selective dose deposition (Bragg peak) and lesser lateral spread. For clinical applications using proton beams, a fixed relative biological effectiveness (RBE) of 1.1 is used to account for differences in the efficiency in inducing lethal lesions to cells between protons and photons. However, due to the dependence of RBE on the linear energy transfer (LET) which increases with decreasing energy of the primary particles, the radiation quality of proton beams can vary within the depth of the irradiated volume. Experimental and theoretical studies have demonstrated that the highest deviations from this constant RBE factor are observed in the last few millimetres of the proton range. Therefore, a complete characterization of the proton beam radiation quality in terms of measurable physical properties at the subcellular scale is necessary information for improving treatment plans. An indicated approach to measure the energy imparted at the cell level is microdosimetry. Microdosimetry consists of a systematic study of the spatial and temporal distributions of the single energy deposition events at the microscopic (i.e. cellular) level. Basically, it records the frequency distribution of probability quantities such as the specific energy and the lineal energy. Variations of the radiation quality can be quantified with microdosimetric measurements performed with tissue­equivalent gas proportional counters (TEPC). TEPCs are the reference devices in experimental microdosimetry for characterizing the radiation quality in radiation protection and radiotherapy environments. A novel microdosimeter optimized for the clinical environment will be constructed in collaboration with other laboratories. This new mini TEPC will be developed with the aim that it can be practically used in hospitals to characterize the radiation quality of proton beams.