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

ABSTRACT 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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    • "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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    ABSTRACT: This topical review provides an up-to-date overview of the theoretical and practical aspects of therapeutic kilovoltage x-ray beam dosimetry. Kilovoltage x-ray beams have the property that the maximum dose occurs very close to the surface and thus, they are predominantly used in the treatment of skin cancers but also have applications for the treatment of other cancers. In addition, kilovoltage x-ray beams are used in intra operative units, within animal irradiators and in on-board imagers on linear accelerators and kilovoltage dosimetry is important in these applications as well. This review covers both reference and relative dosimetry of kilovoltage x-ray beams and provides recommendations for clinical measurements based on the literature to date. In particular, practical aspects for the selection of dosimeter and phantom material are reviewed to provide suitable advice for medical physicists. An overview is also presented of dosimeters other than ionization chambers which can be used for both relative and in vivo dosimetry. Finally, issues related to the treatment planning and the use of Monte Carlo codes for solving radiation transport problems in kilovoltage x-ray beams are presented.
    Physics in Medicine and Biology 02/2014; 59(6):R183-R231. DOI:10.1088/0031-9155/59/6/R183 · 2.76 Impact Factor
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    • "Midgley [8] measured the linear attenuation coefficient of low atomic number materials, including several tissue-equivalent materials (polyethene, nylon, PMMA, breast equivalent and bone-equivalent), using fluorescence x-ray of energies 32 to 66 keV and gamma rays of 140 keV. Hill et al. [3] determined the linear attenuation coefficients for water and four solid phantoms (RMI457, Plastic Water, RW3, and Perspex ) by using a technetium-99 m radionuclide source and a nuclear gamma camera as detector in order to evaluate the water equivalence of solid phantoms. Ferreira et al. [9] used a polyenergetic x-ray beam at the energy range 12 to 54 keV to determine experimentally the linear attenuation coefficient of several tissue-equivalent materials used in CT (water, PMMA). "
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    ABSTRACT: In this work seven tissue-equivalent materials (Nylon, Polyacetate, Polymethylmethacrylate (PMMA), water, muscle-equivalent, bone-equivalent and adipose-equivalent) were characterized, through their attenuation (linear attenuation coefficient) and scattering (scattering profile) properties. An energy dispersive X-ray system (EDXS) was used to analyze these properties simultaneously. The EDXS consisted of a tungsten anode X-ray tube operating at 60 kVp, a goniometer, and two detectors: a Cadmium Telluride (CdTe) detector, positioned at 7 degrees with relation to the incident beam, used for detecting the energy distribution of the scattered photons, and a Silicon Drift Detector (SDD), positioned at zero degree with relation to the incident beam, used for detecting the energy distribution of the transmitted beam (with the sample) or the incident beam (without the sample). The preliminary results obtained in this work show the potential of combining the linear attenuation coefficient and the scattering profile for characterizing and choosing the most suitable tissue-equivalent materials to simulate human tissue. Our results show that adipose-equivalent, water and bone-equivalent would be adequate to simulate adipose, muscle and bone tissue respectively.
    IEEE Transactions on Nuclear Science 04/2013; 60(2):566-571. DOI:10.1109/TNS.2013.2248382 · 1.28 Impact Factor
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    • "For 9 and 16 MeV electron beams, the relative dose reduction with depth of genipin gel is 3% higher than water and reduces to 2% for a 20 MeV electron beam. Hence, for 9 and 16 MeV electron beams, corrections factor may be needed to convert the measured dose in genipin gel to the absorbed dose in water (Hill et al 2008, Brown et al 2008b). Table 2 shows the percentage relative differences of density, electron density, number of electrons per gram, effective atomic number as well as kilovoltage and megavoltage x-ray beams depth doses to water for genipin gel, the most water equivalent PRESAGE R formulation, PAGAT and MAGIC 3D dosimeters (Venning et al 2005, Brown et al 2008b, Gorjiara et al 2010c, Gorjiara et al 2011). "
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    ABSTRACT: The genipin radiochromic gel offers enormous potential as a three-dimensional dosimeter in advanced radiotherapy techniques. We have used several methods (including Monte Carlo simulation), to investigate the water equivalency of genipin gel by characterizing its radiological properties, including mass and electron densities, photon interaction cross sections, mass energy absorption coefficient, effective atomic number, collisional, radiative and total mass stopping powers and electron mass scattering power. Depth doses were also calculated for clinical kilovoltage and megavoltage x-ray beams as well as megavoltage electron beams. The mass density, electron density and effective atomic number of genipin were found to differ from water by less than 2%. For energies below 150 keV, photoelectric absorption cross sections are more than 3% higher than water due to the strong dependence on atomic number. Compton scattering and pair production interaction cross sections for genipin gel differ from water by less than 1%. The mass energy absorption coefficient is approximately 3% higher than water for energies <60 keV due to the dominance of photoelectric absorption in this energy range. The electron mass stopping power and mass scattering power differ from water by approximately 0.3%. X-ray depth dose curves for genipin gel agree to within 1% with those for water. Our results demonstrate that genipin gel can be considered water equivalent for kilovoltage and megavoltage x-ray beam dosimetry. For megavoltage electron beam dosimetry, however, our results suggest that a correction factor may be needed to convert measured dose in genipin gel to that of water, since differences in some radiological properties of up to 3% compared to water are observed. Our results indicate that genipin gel exhibits greater water equivalency than polymer gels and PRESAGE formulations.
    Physics in Medicine and Biology 08/2011; 56(15):4685-99. DOI:10.1088/0031-9155/56/15/004 · 2.76 Impact Factor
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