Investigation of applicability of alanine pellets and films for dosimetry of proton clinical beams

Laser-driven proton has recently gained a great interest as an alternative to conventional and more expensive acceleration techniques. These ion beams have desirable qualities such as small source size, high luminosity and small emittance to be used in different physics fields. This is very promising specially for the future perspective of a new concept of hadrontherapy based on laser-based devices could be developed, replacing traditional accelerating machines. ELIMED (Medical Applications at Extreme Light Infrastructure) is a task-force originally born by an idea of ELI-Beams (Prague) and INFN-LNS (Italian Institute for Nuclear Physics of Catania) researchers. ELIMED main goal is to perform proof-of-principle experiments aimed to demonstrate that laser-accelerated high-energy proton beams (up to 70 MeV in the first phase) can be potentially used for the specific case of ocular proton therapy. In this work we report the investigation of the dosimetric features of alanine pellets exposed to protons produced by means of the CATANA (Centro di AdroTerapia e Applicazioni Nucleari Avanzate) proton therapy facility. This analysis is preliminary for the application of the alanine dosimeters in laser-driven proton beams. ESR spectrometry with alanine is now widely recognized as the most accurate method of transfer dosimetry in the industrial (kGy) dose range. It is well established for calibrating industrial radiation sources against national standards (NIST, IAEA, NPL) and for comparisons between national laboratories. The accuracy of the method is generally very high, largely due to the low sensitivity of the alanine response to irradiation variables (energy, dose rate, temperature, etc.), and the ability of ESR spectrometers to measure dosimeter signals very precisely. As a matter of fact, the main requirements for a suitable system such as: linear response to dose, sensitivity, tissue equivalence, absence of energy dependence, absence of fading, small dimensions, ruggedness, and nondestructive readout, to a large extent are met by the alanine/ESR dosimetry. Here we analyzed the performances of two different alanine/ESR systems (3 mm pellets and 0.1 mm films) irradiated with therapy proton beams (62 MeV). The LET dependence of the response was obtained from the analysis of pellet irradiated with a modulated beam. The energy dependence of the response was derived from the analysis of film stacks irradiated with pristine beams. Use of thin films allowed for a high resolution sampling of the proton slowing down mechanisms. Alanine measurements are compared with Markus parallel plate ionization chamber and are aided by Monte Carlo calculations for medical physics using GEANT4 code