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Evaluation of Patient-Scatter Factors for Radiation Therapy Shielding Using Physical Measurement in a "Good" Geometry

Bogue, Jonathan Nelson
http://rave.ohiolink.edu/etdc/view?acc_num=mco1525447575821026

Abstract Details

2018, Master of Science in Biomedical Sciences (MSBS), University of Toledo, Biomedical Sciences (Medical Physics: Radiation Oncology).
The application of high-energy radiation for therapeutic uses mandates signi cant shielding to protect both radiation workers and the general public. Modern radiation therapy procedures make use of a linear accelerator to accelerate electrons at potentials up to 20 MeV and deliver either photons or electrons at dose rates of several Gy per minute at isocenter. The scattering and penetrating abilities of high energy photons requires the construction of vaults, using attenuating materials to shield the outside world from the radiation produced by the machine and scatter interactions. Vault design requires careful consideration to minimize footprint and cost while maximizing the attenuation of the barriers. The National Council on Radiation Protection and Measurements (NCRP) provides standardized vault design, guidelines, and properties of various shielding materials in Report Number 151 to help guide physicists. However, many values provided in Report No. 151 are dependent on specific vault geometry, generalized radiation beams, and are not evaluated for flattening filter free beams. The University of Toledo Dana Cancer Center has constructed a unique vault that allows for direct measurement of various shielding parameters. This includes tenth and half-value layers, patient-scatter factors, and differential dose albedo in a geometry that is not often available. These parameters can be evaluated by use of a purpose-built test port within the maze barrier that allows for primary and scatter radiation to be collimated and measured. The test port consists of a 20 cm x 20 cm opening to the main vault and then a primary opening of 30 cm x 30 cm that can be filled with different density attenuators or left open for different measurements. To evaluate patient-scatter factors, physical measurements were performed to insure the accuracy of recommended values under measurement conditions that were not previously possible. Measurements were performed by placing a large cylindrical PMMA phantom centered at isocenter to simulate the patient with a farmer-type ionization chamber to measure the dose at isocenter. All high-density blocks were removed from the test-port and a large volume chamber placed directly at the opening along the maze wall to measure the scatter contribution. This project evaluates measurements within a "good" geometry where primary radiation was not directly incident upon the scatter detector for all reported measurements and leakage from the linear accelerator treatment head was subtracted. Measurements were performed for patient-scatter factors for the Varian Edge linear accelerator at all available accelerating potentials for various gantry angles and eld sizes, to compare against NCRP Report No. 151. This allows for validation of the assumptions within NCRP Report No. 151 and comparison to previously reported results.
E. Parsai, Dr (Committee Chair)
Diana Shvydka, Dr (Committee Member)
Nicholas Sperling, Dr (Committee Member)
54 p.
Physics; Radiation
radiation protection, patient-scatter, radiation shielding

Recommended Citations

Citations

  • Bogue, J. N. (2018). Evaluation of Patient-Scatter Factors for Radiation Therapy Shielding Using Physical Measurement in a "Good" Geometry [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=mco1525447575821026

    APA Style (7th edition)

  • Bogue, Jonathan. Evaluation of Patient-Scatter Factors for Radiation Therapy Shielding Using Physical Measurement in a "Good" Geometry. 2018. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=mco1525447575821026.

    MLA Style (8th edition)

  • Bogue, Jonathan. "Evaluation of Patient-Scatter Factors for Radiation Therapy Shielding Using Physical Measurement in a "Good" Geometry." Master's thesis, University of Toledo, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=mco1525447575821026

    Chicago Manual of Style (17th edition)

mco1525447575821026
639
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This open access ETD is published by University of Toledo Health Science Campus and OhioLINK.