Feasibility of graphite and water calorimetry in low-energy clinical proton beams. (2003)

Proton therapy offers an attractive alternative to conventional high-energy photon and
electron therapy due to the characteristic dose distributions for a proton beam, exhibiting a
typical Bragg peak (BP). Calorimetry is generally recommended as primary dosimetry
method for clinical proton beams and given the growing number of proton therapy centres
worldwide, the development of a set of standard calorimeters for protons would be very
valuable. Another reason for developing calorimeters for proton beams is that in a recent
code of practice, IAEA TRS-398, preference is given to using absorbed dose calibration
factors for ionisation chambers obtained in proton beams or, equivalently, to using measured
beam quality correction factors kQ. Despite all this, no long-term primary calorimetry based
standards for protons exist at present.
The feasibility of calorimetry for dosimetry of clinical high-energy proton beams as well as for relatively low-energy beams has been demonstrated and a few number of experimental comparisons between calorimetry and ionization chamber dosimetry has yielded indispensable information on the average energy required to produce an ion pair in dry air by protons. For low-energy proton beams used for the treatment of ocular melanoma, on the other hand, the feasibility of calorimetry has not been explored mainly because of the short range of these beams (lower then 3.5 cm) and their limited lateral extension (a few cm in diameter).
At NPL, graphite and water calorimeters have been developed for photon and electron beam dosimetry. Among the graphite calorimeters, a portable device was developed which is used in clinical photon and electron beams. In this work, the feasibility of adapting the NPL calorimeters for dosimetry of low-energy proton beams is investigated. The focus is on low-energy protons, but where the results have interesting consequences for high-energy proton dosimetry they will be discussed as well. The major aim of this work is to propose designs for graphite and water calorimeters dedicated to low-energy proton beam dosimetry.
For graphite calorimetry, the equivalence of graphite and water for proton beam dosimetry is investigated. The basic interaction data of protons with graphite and water are compared. It is shown that the contribution to absorbed dose from electromagnetic interactions can be accurately scaled to water. Differences in non-elastic nuclear interaction cross sections, however, result in fluence perturbation corrections. Monte Carlo calculations for a 60 MeV beam indicate that they are smaller than 1% at the reference depth. By comparing experimental depth dose curves in water and graphite using plane-parallel ionization chambers this result is confirmed. Gap effects and volume averaging effects are calculated using Monte Carlo simulations. For non-modulated beams, the gap corrections amount to 2- 5% in the entrance region (at depths lower than half the csda range), whereas compensated gap corrections are smaller than 0.1% except in the BP region. Volume averaging corrections for a core with a thickness of 2mm amount to 3-10% in the entrance region and impose restrictions to the applicability of graphite calorimetry in non-modulated proton beams. In modulated proton beams, both gap and volume averaging corrections can be kept smaller than a few tenths of a percent at the reference depth, demonstrating that accurate dosimetry is possible for those beams. Preliminary measurements with the portable calorimeter in a 60 MeV proton beam will be presented and discussed. Heat transport phenomena associated with this experiment will be discussed in another talk at this meeting.
For water calorimetry, dose distributions, scatter perturbations and dose perturbations are calculated using Monte Carlo simulations. Calculated dose distributions are used as input for heat flow simulations of profile heat losses using FEMLab. The same program is used for evaluating excess heat arriving from vial walls. Knowledge of the magnitude of these effects will allow a proposal for a water calorimeter design for dosimetry in a 60MeV proton beam.