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, photon-based, 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 nuclei ions for the radiation treatment 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 radiobiological effectiveness between protons and photons. However, due to the dependence of the linear energy transfer (LET) with energy, the radiation quality of proton beams can vary within the depth of the irradiated volume. Experimental and theoretical studies have demonstrated that deviations from this constant RBE factor increases dramatically at the distal part of the proton range [2,3]. 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. A dedicated approach for measuring the energy imparted at the cell level is using microdosimetry. Microdosimetry consists of a systematic study of the spatial and temporal distributions of the 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 tissue-equivalent gas proportional counters (TEPC). TEPCs are the reference devices in experimental microdosimetry for characterizing the radiation quality in radiation protection and radiotherapy environments [4, 5, 6].
- Proton Therapy Coperative Group (PTCOG) http://www.ptcog.ch/index.php/ptcog-patient-statistics
- M. Biaggi, F. et al. Physical and biophysical characteristics of a fully modulated 72 MeV therapeutic proton beam: model predictions and experimental data. Nuclear Instruments and Methods in Physics Research Section B159, 89-100.
- Minna Wedenberg and Iuliana Toma-Dasu. Disregarding RBE variation in treatment plan comparison may lead to bias in favor of proton plans. Med. Phys. 41, 091706 (2014);
- Menzel, H. G., Pihet, P., and Wambersie, A. (1990). A microdosimetric specication of radiation quality in neutron radiation therapy. Int. J. Radiat. Biol, 57(4):865-883.
- De Nardo, L. et al. Microdosimetric investigation at the therapeutic proton beam facility of CATANA. Rad. Prot. Dosim 110 (1-4): 681-686 (2004).
- Chiriotti, S., Microdosimetry of hadron therapy beams using mini Tissue-Equivalent proportional counters. PhD thesis. Université catholique de Louvain (Belgium) (2015).