"Plastic materials have been developed and optimized for their water-equivalence in photon and electron beams. But though their water-equivalence in proton beams is way inferior, they are nevertheless widely used also for proton beam dosimetry, introducing larger dosimetric uncertainties," explained Ana Lourenco from University College London and the National Physical Laboratory. "For reference dosimetry, those materials are even not recommended since material- and energy-dependent corrections factors are not known."

With this in mind, Lourenco and colleagues have designed and evaluated three new trial plastics for dosimetry in clinical proton beams. They determined the water-equivalence of each material by computing a plastic-to-water conversion factor, which they developed previously for carbon-ion beams. They characterized the trial materials experimentally in low- and high-energy proton beams, and compared the results with Monte Carlo simulations (Phys. Med. Biol. 62 3883).

Proton experiments

The researchers examined three plastics specifically formulated for light-ion beam dosimetry, based on an epoxy resin mixed with different low-Z compounds. They measured the plastic-to-water conversion factor, Hpl,w, of the materials using the 60 MeV proton cyclotron at the Clatterbridge Cancer Centre, UK, and the 226 MeV proton cyclotron at the Trento Proton Therapy Center in Italy. Experiments were performed in a water phantom, using laterally integrated depth-dose ionization chamber measurements, with and without plastic slabs of various thicknesses placed in front of the phantom.

The team also calculated Hpl,w using Monte Carlo simulations. For the low-energy beam, they performed simulations with the FLUKA-2011.2c.4 code, while for the high-energy beam, they used the Geant4-9.6.p01 code as well. They also computed fluence correction factors, kfl, between water and the various materials using the Monte Carlo method.

In the 60 MeV proton beam, experimental and FLUKA-simulated Hpl,w values for the three plastics agreed to within experimental uncertainties. For this low-energy beam, maximum deviations of Hpl,w and kfl from unity were within 1% for all trial plastics. As such, the results indicated no preference for a preferable plastic.

Calculated and experimental Hpl,w factors also agreed well for the 226 MeV proton beam. The experimental data did not rule out a most-water-equivalent plastic, with Hpl,w correction factors (deviation from unity, expressed as a percentage) of up to 1% for the three trial plastics.

Commercial comparisons

Lourenco and colleagues also compared the water-equivalence of the three trial plastics (in the high-energy beam) with a series of commercial plastics (A-150 tissue equivalent plastic, PMMA, polyethylene, polystyrene, Rando soft tissue, Gammex 457-CTG and WT1), using the FLUKA and Geant4 Monte Carlo codes.

Using FLUKA, plastic #1 gave the largest correction, increasing from 0% to 2% at a depth near the Bragg peak. For polystyrene, the correction increased towards 1.7%, while for polyethylene and PMMA, maximum corrections did not exceed 0.5%. Plastic #3 was the most water-equivalent of the three trial materials.

The authors note that Geant4 and FLUKA also gave different Hpl,w values for the commercial materials. The largest correction was found for A-150 and polyethylene, where corrections increased from 0% toward 1–1.5%. All other materials gave corrections within ±1%.

They also calculated kfl values using FLUKA and Geant4. Plastic #1 showed the largest variation of kfl with depth, with deviations from unity ranging from –0.8% to 3% in FLUKA and from –0.1% to 2% in Geant4. When using FLUKA, polyethylene was the plastic with the smallest variation (–0.5–0.5%), whilst for Geant4, plastic #3 gave the smallest range (–0.1–0.5%).

Considering results from both Monte Carlo codes, plastic #3 and PMMA had the smallest Hpl,w, with maximum values of about 1%, suggesting that these are the most suitable water-substitutes for measuring dose-to-water using ionization chambers in clinical proton beams. However, PMMA range differed by 16% from that of water.

"For reference dosimetry, range is not a crucial issue and PMMA would be as a good substitute as plastic #3, according to our findings," said Lourenco. "But for other applications, such as dosimetry in anatomic phantoms, it is essential that the range is the same as in water or tissue and thus plastic #3 is superior."

Lourenco noted that as plastic #3 is based on an epoxy resin, gas-filled spheres can be added to adjust the density – and thus the range – of the final compound. "We have an ongoing project that aims to refine the composition of plastic #3 in terms of range to be within 1% as well," she said.

Related articles in PMB
Evaluation of the water-equivalence of plastic materials in low- and high-energy clinical proton beams
A Lourenco et al Phys. Med. Biol. 62 3883
Theoretical and experimental characterization of novel water-equivalent plastics in clinical high-energy carbon-ion beams
A Lourenco et al Phys. Med. Biol. 61 7623
Characterization and validation of a Monte Carlo code for independent dose calculation in proton therapy treatments with pencil beam scanning
F Fracchiolla et al Phys. Med. Biol. 60 8601

Related stories

• Reference dosimetry for hadron therapy
• Dosimeter simulations: caution advised
• A new take on particle therapy dosimetry
• Monte Carlo codes under close scrutiny