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Investigating Prone Breast Treatment Plan Robustness and Attenuation of Prone Breast Board

Wilkinson, Austin Ryan Osborn
http://orcid.org/0000-0001-9363-5694
http://rave.ohiolink.edu/etdc/view?acc_num=mco1588723306761983

Abstract Details

2020, Master of Science in Biomedical Sciences (MSBS), University of Toledo, Biomedical Sciences (Medical Physics: Radiation Oncology).
Breast cancer patients are often treated in the prone position to improve dose homogeneity and cosmetic outcome. This is especially common when the patient’s anatomy is more pendulous. A breast board is used to support the patient on the treatment table. This breast board creates a source of setup error as well as a source of error in the dose calculations of the treatment planning system. This study looks at two different methods for characterizing the board in the treatment planning system, compares the robustness of two different treatment planning methods across seven patients, and investigates the scatter contribution from the breast board. The first characterizing method was the material override method where a bulk density was assigned to the board shell and interior. The second characterizing method was the ROI type external method where the native CT numbers calculated during the simulation were used. A vertical transmission factor profile of the breast board was produced using each method and compared to the profiles obtained from an experimental setup. Seven previously anonymized patients were retrospectively selected, and two plans were made for each patient using the control point method and the augmented wedge method. The control point method is also known as “field-in-field”. This is when hot spots are covered with MLCs and the relative weights of the fields are changed to remove the hot spot. The augmented wedge method was when wedges and control points were used in the same plan. Planning with wedges alone was found to not be feasible. The plans had to have matching prescription coverage of 95%, as well as similar dose statistics. The robustness was compared by calculating the change in dose to a point in the dose shadow of the board when the board was completely removed, as well as the change in hotspot dose when the isocenter was slightly perturbed. The scatter contribution was investigated by measuring the skin dose of a virtual breast phantom with and without the board included in the calculations as a function of angle of obliquity. This study has shown that the breast board is best accounted for by setting the board as an external ROI structure and thus allowing the planning system to use the density assigned by the CT. Comparing measurements to the planning system in this case resulted in a deviation of 1.53% with the ROI type external method. The alternative method of assigning a bulk density was found to be less accurate and modeled to within 9.33% with the material override method. It was found that seven of the seven patient plans saw an improvement in robustness when augmented wedges were used, reducing the percent change in dose to a point in the shadow of the board from 9.97% to 7.57%. The scatter dose contribution was found to be greatest at low angles and decreased with increasing angle of obliquity. After 45° no scatter contribution was found. In conclusion, prone breast treatment should be used sparingly, and should not completely replace supine breast treatment due to the setup error introduced as well as the characterization problems. When prone treatment is deemed necessary, the board should be modeled using the ROI type external method and planned with the augmented wedge method. When planning it should be noted that the skin coverage will be dependent on the angle of obliquity of the beam and board.
David Pearson (Committee Chair)
E Parsai (Committee Member)
Nicholas Sperling (Committee Member)
70 p.
Physics; Radiation
Prone breast; prone radiation; breast cancer; breast board; prone breast board; transmission factor

Recommended Citations

Citations

  • Wilkinson, A. R. O. (2020). Investigating Prone Breast Treatment Plan Robustness and Attenuation of Prone Breast Board [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=mco1588723306761983

    APA Style (7th edition)

  • Wilkinson, Austin. Investigating Prone Breast Treatment Plan Robustness and Attenuation of Prone Breast Board. 2020. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=mco1588723306761983.

    MLA Style (8th edition)

  • Wilkinson, Austin. "Investigating Prone Breast Treatment Plan Robustness and Attenuation of Prone Breast Board." Master's thesis, University of Toledo, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=mco1588723306761983

    Chicago Manual of Style (17th edition)

mco1588723306761983
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© 2020, all rights reserved.
This open access ETD is published by University of Toledo Health Science Campus and OhioLINK.