Characterization of the phantom material Virtual Water™ in high‐energy photon and electron beams

Abstract

The material Virtual Water has been characterized in photon and electron beams. Range-scaling factors and fluence correction factors were obtained, the latter with an uncertainty of around 0.2%. This level of uncertainty means that it may be possible to perform dosimetry in a solid phantom with an accuracy approaching that of measurements in water. Two formulations of Virtual Water were investigated with nominally the same elemental composition but differing densities. For photon beams neither formulation showed exact water equivalence-the water/Virtual Water dose ratio varied with the depth of measurement with a difference of over 1% at 10 cm depth. However, by using a density (range) scaling factor very good agreement (<0.2%) between water and Virtual Water at all depths was obtained. In the case of electron beams a range-scaling factor was also required to match the shapes of the depth dose curves in water and Virtual Water. However, there remained a difference in the measured fluence in the two phantoms after this scaling factor had been applied. For measurements around the peak of the depth-dose curve and the reference depth this difference showed some small energy dependence but was in the range 0.1%-0.4%. Perturbation measurements have indicated that small slabs of material upstream of a detector have a small (<0.1% effect) on the chamber reading but material behind the detector can have a larger effect. This has consequences for the design of experiments and in the comparison of measurements and Monte Carlo-derived values.

Evaluation of backscatter dose from internal lead shielding in clinical electron beams using EGSnrc Monte Carlo simulations

“…The resulting mean range scaling correction factor was 1.02. This compares well with the range scaling factor measured by McEwen and Niven (19) of 1.01.…”

“…Utilizing virtual water decreases the positioning uncertainty when compared to performing measurements in water. The energy dependence of virtual water has been shown to be in the range of 0.1% to 0.4% over an energy range of 4 to 22 MeV at and around the depth of dose maximum (${\mathrm{d}}_{\text{max}}$) (19) . This is an acceptable uncertainty.…”

Accurate dosimetry with GafChromic™ EBT film of a 6MV photon beam in water: What level is achievable?

“…͑a͒. At first, as reported in literature, 19 most plastic phantom materials ͑e.g., solid water, virtual water, 20 polystyrene, etc.͒ have relative electron densities comparable to water. However, the mass energy absorption coefficient and electron scattering relative to water depends on the energy and is not a constant value.…”

Monte Carlo study of correction factors for the use of plastic phantoms in clinical electron dosimetry

“…So far there have been many studies on calculated pl w values and measured h pl values for various plastic phantoms, 6,7,[9][10][11][12][13][14][15] …”