Design, Development and Operation of a Primary Standard Graphite Calorimeter for Proton Beam Dosimetry

The preferred method of calibration for any radiotherapy treatment facility is to calibrate the user's chamber in the beam that is to be used clinically or at least a beam similar in nature and beam structure. For photon treatments, a number of primary standards laboratories have linear accelerators which are very similar to those used for treatment thus offer calibrations for chambers across a range of beam qualities. This has shown to give a better uncertainty on the calibration coefficients of chambers, and hence improved reference dosimetry, than converting from a cobalt-60 calibration to linac MV photon energies. Proton beam characteristics are even further removed from those of a cobalt-60 beam and yet it is common practice in proton reference dosimetry that an ionization chamber is calibrated in a cobalt-60 beam and then converted using a protocol such as described in IAEA TRS-398. Consequently, the uncertainties in proton reference dosimetry are larger in comparison with photon beams. In the past few years, the number of proton centres worldwide has increased dramatically. It is no longer the case that ensuring consistency of dose in a single department is the priority but ensure consistency and accuracy of dose across all facilities in order to optimize our learning from clinical outcomes and maximize the benefits to patients.