Characterization of the neutron field in proton therapy treatments.

2019 edition

Miguel Ángel Caballero-Pacheco

Since the first moment it was thought that hadrons could be used as projectile to kill tumours (a pioneer technique different from the usual radiotherapy treatments where photons and electrons are used, a brief comparison between the two techniques will be explained to the audience) it was clear that side effects could show up since in case we use protons as projectiles (proton therapy) at energies of 200 MeV it is possible that neutrons (among other particles as photons) can appear from the nuclear reactions not only between the protons and the elements that form all the accelerator machinery used to treat the patients but also due to the interaction between this beam of protons with the atoms of the patient itself. These neutrons generated can deposit their energy in healthy tissues and therefore they can contribute to an undesired radiation exposure that in the future could end up in developing secondary tumours. 

Despite of this knowledge, at the first moment it was stated that the contribution of these neutrons to the absorbed dose by the patient was negligible but lately it has been shown that a better study and characterization of this neutron field should be performed in all facilities where proton therapy is already used in order to avoid as much as possible the side effects that we commented previously.

It is in that point when my Phd thesis enters in the topic, in particular it will be developed, among others topics, under the one presented above, that is to say, Study and Characterization of the neutron fields that appear in any facility where an initial beam produces neutrons.

In order to study these neutrons fields we dispose, essentially, of two devices, the Bonner Sphere Spectrometer and the CR-39 based dosimeter, whose performance, experimental set up and some easy features will be explained to the audience. We can say that with these two devices we can study the neutron field by leaving the devices in the radiation field (for example when a proton therapy treatment is being carried out) and later it is required to do an additional process in our lab to identify how this neutron field is. Also during the Phd it will be studied the feasibility of developing and constructing new detectors that enable us to study this neutron field in a real time without the necessity of performing the additional process that we commented before. It must be said that in addition to using these two devices presented before (this can be said that is the experimental part), we study any neutron field by doing Monte Carlo Simulations of the Radiation Transport, where we have to simulate all the room details and machinery details that we find in the particular facility where a proton therapy is used.

To finish this summary, an important final outcome of these studies should be to modify the radiotherapy planner that radiophysicists use in a Hospital in order to take into account the undesired radiation in the healthy tissues.