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Quantitative Analysis of the Head Scatter and Jaw Transmission Correction Factor for Commissioning of Enhanced Dynamic Wedge Fields Using a MapCHECK 2 Diode Array

Dickerson, Edward
http://rave.ohiolink.edu/etdc/view?acc_num=mco1340906780

Abstract Details

2012, Master of Science in Biomedical Sciences (MSBS), University of Toledo, College of Medicine.
Quality assurance in radiation oncology treatment planning requires independent verification of dose to be delivered to a patient through ¿¿¿¿¿¿¿second check¿¿¿¿¿¿¿ calculations for simple plans as well as planar dose fluence measurements for more complex treatments, such as intensity modulated radiation treatments (IMRT). Discrepancies between treatment planning system (TPS) and second check calculations created a need for treatment plan verification using a two dimensional diode array for Enhanced Dynamic Wedge (EDW) fields. While these measurements met clinical standards for treatment, they revealed room for improvement in the EDW model. The purpose of this study is to analyze the head scatter and jaw transmission effects of the moving jaw in EDW fields by measuring dose profiles with a two dimensional diode array in order to minimize differences between the manufacturer provided fluence table (Golden Segmented Treatment Table) and actual machine output. The jaw transmission effect reduces the dose gradient in the wedge direction due to transmission photons adding dose to the heel region of the field. The head scatter effect also reduces the gradient in the dose profile due to decreased accelerator output at increasingly smaller field sizes caused by the moving jaw. The field size continuously decreases with jaw motion, and thus the toe region of the wedge receives less dose than anticipated due to less head scatter contribution for small field sizes. The Golden Segmented Treatment Table (GSTT) does not take these factors into account since they are specific to each individual machine. Thus, these factors need to be accounted for in the TPS to accurately model the gradient of the wedge. The TPS used in this clinic uses one correction factor (transmission factor) to account for both effects since both factors reduce the dose gradient of the wedge. Dose profile measurements were made for 5x5 cm2, 10x10 cm2, and 20x20 cm2 field sizes with open fields and 10¿¿¿¿, 15¿¿¿¿, 20¿¿¿¿, 25¿¿¿¿, 30¿¿¿¿, 45¿¿¿¿, and 60¿¿¿¿ wedges for 6 MV and 18 MV beams and compared with TPS generated profiles. The transmission factor was adjusted for the 18 MV beam to obtain a better correlation between planned and measured dose gradient by reducing the gradient of the wedge in the TPS. This correction resulted in an average and maximum pass rate improvement for patient plans at a distance to agreement of 3% 3mm of 1.07% and 3.9% respectively. The off axis ratio data in the second check calculation software was also adjusted to bring the dose agreement between the initial TPS calculation and second check calculation within clinical standards. This study demonstrated the ability to adjust the EDW gradient in a treatment planning system to improve the differences in machine output specific to each machine and the manufacturer provided GSTT.
Diana Shvydka, PhD (Committee Chair)
E. Ishmael Parsai, PhD (Committee Member)
David Pearson, PhD (Committee Member)
76 p.
head scatter; Enhanced Dynamic Wedge; EDW;

Recommended Citations

Citations

  • Dickerson, E. (2012). Quantitative Analysis of the Head Scatter and Jaw Transmission Correction Factor for Commissioning of Enhanced Dynamic Wedge Fields Using a MapCHECK 2 Diode Array [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=mco1340906780

    APA Style (7th edition)

  • Dickerson, Edward. Quantitative Analysis of the Head Scatter and Jaw Transmission Correction Factor for Commissioning of Enhanced Dynamic Wedge Fields Using a MapCHECK 2 Diode Array. 2012. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=mco1340906780.

    MLA Style (8th edition)

  • Dickerson, Edward. "Quantitative Analysis of the Head Scatter and Jaw Transmission Correction Factor for Commissioning of Enhanced Dynamic Wedge Fields Using a MapCHECK 2 Diode Array." Master's thesis, University of Toledo, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=mco1340906780

    Chicago Manual of Style (17th edition)

mco1340906780
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© 2012, all rights reserved.
This open access ETD is published by University of Toledo Health Science Campus and OhioLINK.