In the last decades, proton therapy has gained great interest in the medical community thanks to its excellent clinical results. The bases of this success are: (1) the physical advantages of protons with respect to conventional radiation therapy with photons, due to their more selective energy deposition in depth; and (2) their increased radiobiological effectiveness as compared with photon radiotherapy for a same level of absorbed dose, property usually referred to as Relative Biological Effectiveness or RBE. Even if it is agreed that proton RBE varies towards the distal Bragg peak region, increasing with LET, nowadays in clinical proton therapy treatments, a uniform RBE value of 1.1 is generally used. Therefore, studies of RBE at low proton energies are necessary in order to reach a consensus on the RBE variations near the Bragg peak, which could be significant in the optimization of proton therapy treatment plans. With the purpose of providing a monochromatic beam for RBE measurements at low energies (below 18 MeV in our case), we are preparing an experimental setup at the external beam line of the 18MeV proton cyclotron facility installed at the CNA (Seville, Spain). In this work, we present our first feasibility studies. Two are the main constraints when dealing with the irradiation of biological samples in our setup: low beam intensity, of the order of some pA, to control properly the fluence during irradiation time, and broad irradiation field, of the order of 3cm side, uniform in both energy and space. To improve the homogeneity and decrease the beam intensity, we have decided to use a completely defocused beam and to scatter the beam downstream the exit window, both placing tungsten foils of different thicknesses and changing the amount of air between the window and the position of the samples. So far, we have measured the properties of wide beams produced with tungsten scattering foils of various thicknesses. With a 150 m thick tungsten foil, we could produce a 10 MeV proton beam with almost homogeneous intensity, having deviations of the order of ~10% in the central 35 mm and at ~50 cm distance from the exit window. Furthermore, we have performed preliminary dosimetric studies, using EBT3 radiochromic films and a transmission ionization chamber for dose and proton fluence evaluation. Studies of this nature are of great interest, since radiochromic films would be a handy and easy to use dosimeter solution for proton RBE studies. Finally, we are developing with the Geant4 toolkit a Monte Carlo tool, which reproduces accurately the cyclotron beam properties with the aim of simulating future improvements proposed on the final optimization of the setup for the irradiation of biological samples.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 675265, OMA - Optimization of Medical Accelerators, and from the Spanish Ministry of Economy and Competitiveness under grant No FPA2016-77689-C2-1-R. The Monte Carlo simulations were carried out at the FIS-ATOM cluster hosted at CICA (Seville, Spain).

Trabajo presentado en las II Jornadas RSEF / IFIMED de Física Médica, celebradas en Madrid (España), del 14 al 15 de junio de 2018

Peer reviewed