National Physical Laboratory

Proton and ion beam dosimetry

Proton and ion beams exhibit better dose characteristics than x-rays for radiotherapy. Their superiority lies in the fact that the radiation dose can be confined largely to the tumour with much lower doses to the surrounding healthy tissue than can be achieved with x-rays. This allows for dose escalation in the tumour itself and therefore better control of the treatment outcome or for more sparing of critical organs. Proton and ion beam therapy is not new but has become much cheaper in the last decade and proton therapy can now be delivered at about twice the cost of x-ray therapy. Emerging new accelerator technologies such as laser induced beams, dielectric wall accelerators and fixed field alternating gradient accelerators promise lowering the price tag even further. The NHS has increased support for proton therapy by sending patients abroad as a transitory solution and by committing to support two or three modern large-scale proton therapy facilities to start treatments in 2015. In order to make the most out of this modality (highest treatment outcome with minimal side effects) dosimetric accuracy similar as in x-ray therapy is required and improved reference dosimetry is needed.

The energy deposition mechanisms for protons and ions are described by the same physical models as for electrons (and thus indirectly also for photons) and many of the same technologies are used for their dosimetry. There are, however, a few important differences which require or allow different approaches. While the general consensus is that calorimeters are also for proton and ion beams the primary instruments of choice and ionisation chambers the clinical reference instruments of choice, acceptably accurate reference dosimetry can also be established by a simple measurement of the particle fluence thanks to the ballistics of protons and ions. Another important difference is due to the effect of nuclear interactions which remove a substantial amount of particles from the beam and contribute substantially to the dose via nuclear fragments. Nuclear interactions play an important role in dose conversion procedures between graphite to water in graphite calorimeters, between phantom materials and water, between tissues and water and between detector materials and water. For ionisation chamber dosimetry a great concern is also that the mean energy required to produce an ion pair in air is different for clinical proton and ion as compared to electrons (and thus indirectly also for photons).

Specifically for beams of ions heavier than protons, an additional complication is that the biological effect is not proportional to the SI quantity absorbed dose but is in addition dependent on the density of ionisation clusters which is vastly increased compared to x-ray beams. It has therefore been proposed to define a new physical quantity that takes the geometrical distributions of ionisation into account and is thus better represents the biological outcome of radiotherapy. NPL contributes in the effort to establish this quantity by the development of a micrometer-sized calorimeter which is based on SQUID technology.

NPL's research efforts in proton and ion beam dosimetry focus on:

  • the development of a primary standard for protons and ions based on a graphite calorimeter, similar to the one for photon and electron beams but smaller in size in order to be portable, with major emphasis on the conversion procedure from dose to graphite to dose to water in proton, alpha and carbon ion beams;
  • improved measurements of the mean energy required to produce an ion pair in air for ionisation chamber dosimetry in proton and ion beams;
  • improved measurement and simulation using Monte Carlo methods of ionisation chamber perturbation factors;
  • characterisation of the energy dependent absorbed dose response of relative dosimeters such as alanine and radiochromic film;
  • establishment of accurate dose conversion procedures from different materials to water;
  • development of a micrometer-sized calorimeter based on SQUID technology to measure microdosimetric spectra in ion beams in order to underpin the definition of a new, more biologically relevant quantity for radiotherapy.

Contact: Hugo Palmans

Figure 1

Proton Dosimetry Figure 1

Design of the graphite calorimeter for primary dosimetry in proton and ion beams

Figure 2

Proton Dosimetry Figure 2

Left: Experimental set up for measurements in graphite (upper drawing) and for measurements in water (lower drawing).
Right: fluence correction factors for the conversion of dose to graphite to dose to water from experiment (symbols) and from three Monte Carlo codes (full lines).

Figure 3

Proton Dosimetry Figure 3

Ratio of wall perturbation factors as a function of proton energy for a Farmer type chamber with a graphite and A150 wall obtained by experiment (green hollow squares; the green box represents the experimental standard uncertainty of about 0.2%) and by Monte Carlo simulations (pink diamonds). The full line is an exponential fit through the simulated data points.

Last Updated: 4 Feb 2014
Created: 3 May 2011

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