Evaluation of the water equivalence of solid phantoms using gamma ray transmission measurements

Department of Radiation Oncology, Royal Prince Alfred Hospital, Sydney, Australia
Radiation Measurements (Impact Factor: 1.21). 08/2008; 43(7):1258-1264. DOI: 10.1016/j.radmeas.2008.01.019


Gamma ray transmission measurements have been used to evaluate the water equivalence of solid phantoms. Technetium-99m was used in narrow beam geometry and the transmission of photons measured, using a gamma camera, through varying thickness of the solid phantom material and water. Measured transmission values were compared with Monte Carlo calculated transmission data using the EGSnrc Monte Carlo code to score fluence in a geometry similar to that of the measurements. The results indicate that the RMI457 Solid Water, CMNC Plastic Water and PTW RW3 solid phantoms had similar transmission values as compared to water to within ±1.5%. However, Perspex had a greater deviation in the transmission values up to ±4%. The agreement between the measured and EGSnrc calculated transmission values agreed to within ±1% over the range of phantom thickness studied. The linear attenuation coefficients at the gamma ray energy of 140.5 keV were determined from the measured and EGSnrc calculated transmission data and compared with predicted values derived from data provided by the National Institute of Standards and Technology (NIST) using the XCOM program. The coefficients derived from the measured data were up to 6% lower than those predicted by the XCOM program, while the coefficients determined from the Monte Carlo calculations were between measured and XCOM values. The results indicate that a similar process can be followed to determine the water equivalency of other solid phantoms and at other photon energies.

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    • "For gel calibration, 30 cm × 30 cm slabs of Solid Water® phantom (RMI Gammex), presenting various thickness of 0.1 cm–5 cm, a density of 1.04 g/cm 3 , and made from epoxy resins, were used. The phantom is designed to attenuate and scatter radiation in the same way as water, and fulfills the ICRU requirements for water equivalency in low-and high-energy systems [21] [22]. Furthermore, water equivalent Perspex (PMMA) holders were designed with holes fitted to the cuvette size in each experiment to reduce the air effect. "
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    • "Gamma ray transmission measurements from a technetium-99m source were used to evaluate radiological water equivalence of RMI-457 Solid Water, PW and RW3 white polystyrene (PTW-Freiburg, Freiburg, Germany) (Hill et al 2008). Linear attenuation coefficients were determined from the transmission values and compared with calculated values determined from Monte Carlo calculated transmission curves using the EGSnrc Monte Carlo code and NIST values. "
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