Microdosimetry of therapeutic proton beams

Bianchi Anna


Reniers Brigitte, brigitte.reniers@uhasselt.be

SCK•CEN Mentor

Vanhavere Filip
+32 14 33 28 59

SCK•CEN Co-mentor

Chiriotti Alvarez Sabina
+32 14 33 21 53

Expert group

RP Dosimetry and Calibration

PhD started


Short project description

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 [1].

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].



  1. Proton Therapy Coperative Group (PTCOG) http://www.ptcog.ch/index.php/ptcog-patient-statistics
  2. 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.
  3. 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);
  4. 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.
  5. De Nardo, L. et al. Microdosimetric investigation at the therapeutic proton beam facility of CATANA. Rad. Prot. Dosim 110 (1-4): 681-686 (2004).
  6. Chiriotti, S., Microdosimetry of hadron therapy beams using mini Tissue-Equivalent proportional counters. PhD thesis. Université catholique de Louvain (Belgium) (2015).



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.

In this PhD proposal we propose to characterize this new mini TEPC in reference fields by performing measurements at the Laboratory for Nuclear calibrations of SCK•CEN. The main objective will be to perform in-field measurements with this new microdosimeter in clinical beams such as in the new research proton beam line in Leuven and/or other available facilities, such as LNS (Catania, Italy), GSI (Darmstadt, Germany) or CNAO (Pavia, Italy). In addition, calculations of proton microdosimetric spectra using Monte Carlo methods will be carried out to model the microdosimeter device and will be validated with experimental data. Monte Carlo codes such as MCNPX, FLUKA or GEANT4 can be used for this end.

Out-of-field measurements are also very important in order to quantify the so-called peripheral absorbed doses, in particular originating from secondary neutrons. SCK•CEN has already experience in these kind of measurements for different radiotherapy techniques including proton fields. For this purpose, we will perform peripheral doses measurements with a standard TEPC (present at SCK•CEN).