University of Seville / Centro Nacional de Aceleradores
Radiobiological effectiveness of protons
Trainee: Anna Baratto Roldán
Supervisor: Joaquín Gómez Camacho
Particle therapy is one of the most promising techniques in the fight against cancer, with a growing number of dedicated accelerators being constructed and installed in hospitalbased clinical centers. Proton therapy is the most widely used particle therapy technique and is gaining an increasing interest in the medical community.
Proton beams are generally assumed to be 10% more biologically effective than photon beams in clinical practice, and a constant Relative Biological Effectiveness (RBE) value of 1.1 is currently used in proton therapy treatment planning. Research suggests that proton RBE varies towards the distal edge of the Bragg curve, increasing with proton Linear Energy Transfer (LET), leading to possible toxicity in the healthy tissue. Therefore, studies of proton RBE at low energies, typically found at the Bragg peak region of clinical beams are highly relevant.
The beam lines available at two of the accelerator facilities at the National Centre of Accelerators (CNA) in Seville, the 3 MV tandem and the 18 MeV proton cyclotron, are suitable to perform such studies. These beamlines offer the advantage of providing proton beams with nominal energies in the region of interest, minimizing straggling due to passive degradation. A setup for the irradiation of biological samples was designed and mounted at the 3 MV tandem facility.
This project aimed to create a similar system at the 18 MeV proton cyclotron facility, to extend the energy range available for proton RBE studies. A comprehensive characterization of the beam line and beam properties has been completed, both with direct measurements of the dose profiles and with Monte Carlo simulations. Multiple solutions have been proposed to reach a good degree of dose homogeneity at the position of the biological samples and within the whole area of the samples. The set up proposed will allow irradiation of mono-layer cell cultures. The first experiments involving the irradiation of cell samples at the cyclotron facility are foreseen in the next future.
Preliminary measurements with 3D-microdetectors and Monte Carlo simulations have also been used to characterize radiation quality from a microdosimetric point of view. A Monte Carlo application, based on Geant4-DNA, has been developed for the computation of microdosimetric quantities in spherical sites for protons in liquid water which is intended to be included among the official examples of Geant4.