Abstract ID: 169 Monte Carlo calculated correction factors for a proton calorimeter in clinical proton beams

Abstract

Calorimetry is the only fundamental method for measuring the absorbed dose according to its definition. A calorimeter measures the temperature rise resulting from irradiation in an absorber, assuming all the energy deposited in a material appears as a heat. Unlike other detectors, e.g. ionization chambers, a calorimeter inherently provides a method to measure the energy deposited by radiation directly. In particular, the National Physical Laboratory (NPL) has developed and is now commissioning a graphite calorimeter as a primary standard of absorbed dose for clinical proton beams. To determine the absorbed dose to graphite from calorimeter measurements, different correction factors have to be calculated, including the gap correction factor ( k gap ) and a volume averaging correction factor ( k vol ) [1] . The former is related to the effect due to the presence of vacuum gaps within the calorimeter, the latter converts the mean absorbed dose in the graphite core to the absorbed dose in a point located at the centre of the core. A separate conversion factor is then applied to convert the absorbed dose to graphite in the calorimeter to the absorbed dose at the reference depth in water. Monte Carlo simulations are currently the most precise tool to calculate these corrections in complex experimental configurations. A Monte Carlo application has been developed with the TOPAS platform, based on the Geant4 toolkit to calculate k gap and k vol for monoenergetic proton beams with energies ranging between 60 and 230 MeV [2] . Initial results indicate that k gap ranges from 0.06% above unity at 60 MeV to 0.36% above unity at 230 MeV with k vol being of a similar magnitude at these energies. Moreover, to obtain the corrections in a more realistic environment close to clinical conditions, a typical passive proton therapy beam line for ocular melanoma treatment has been simulated in detail and similar results are being obtained for active scanning systems. The results of the presented work will significantly contribute to the establishment of the NPL calorimeter as a primary standard in proton therapy.