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Developing a Quantitative Means for Evaluating Single Isocenter Multi-Target SRS Plans

Oakey, Mary E
http://rave.ohiolink.edu/etdc/view?acc_num=mco1556908025631349

Abstract Details

2019, Master of Science in Biomedical Sciences (MSBS), University of Toledo, Biomedical Sciences (Medical Physics: Radiation Oncology).
Stereotactic Radiosurgery (SRS) is a common treatment method for multiple metastasis in the brain. SRS is prescribed to a high dose with low fractionation, Two properties of the dose distribution desired in SRS treatments are conformity and quick dose fall off. A common means of evaluating these properties is the Paddick Conformity Index, the Paddick Dose Gradient Index, and the RTOG Conformity Index. With the advances in multi-leaf collimators (MLCs) design, new means of treatment have been developed; this include Single Isocenter Multi-Target SRS (SIMT SRS). SIN'IT SRS differs from SRS in that it utilizes a single isocenter and treats all of the lesions in one plan. This single isocenter approach allows for a lower treatment time and less monitor units (MUs) delivered to the patient. Evaluating and comparing SINIT SRS plans can become difficult due to the fact that the isodose lines from one lesion, particularly the 50% isodose line, often intersects with another lesion. This complicates the analysis of both the dose fall off and conformity of each lesion. The purpose of this project is to develop a quantitative means to evaluate the conformity and dose fall off of an Single Isocenter Multi-Target SRS plan. For this project, a standard patient was created with seven randomized spherical targets varying in diameters (8mm-2()rnm). Good, intermediate, and bad plans were generated for each size target for this patient using Raystation v8A SPI. Each plan uses 0.1x0.1x0.1 cc dose grid, 9 non-coplanar arcs, the same arc angles, and a single isocenter. Each lesion was prescribed to 20 Gy for a single fraction To evaluate the dose distribution for each plan, the volume of 12 Gy (V12), volume of 10 Gy (VIO), and volume of 20 Gy (V20) in the brain were analyzed. These dose volumes were selected because an accumulation of these doses can lead to radionecrosis (RN) which has been used as a common indicator to evaluate the generated plans. [I] Two methods have been used to analyze the generated plans; the first analysis was done by combining the individual planning target volumes (P TVs) to one large PTV, Then the Paddick Conformity Index (PCI) and Paddick Dose Gradient Index (PGI) were applied to the combined dose statistics. The second method involved determining a defined volume to be applied to each target lesion. This volume was the exact area where the conformity and dose fall off were evaluated. The defined volume was determined by measuring from the centroid of a target to the 50% isodose line in multiple directions. The expansion volume was calculated to be within a 95% confidence interval of the mean for each size. Once the expansion volume was created, the previously mentioned indices were applied to it. It was observed that both the composite method and the expansion method are able to distinguish the difference in quality between plans. There seemed to be a trend occurring between target size and expansion volume; but, because only four expansion volumes were used in this study a conclusion cannot be drawn on this. It was noted that the PCI and PGI for the smaller lesions were significantly impacted by plan quality versus the larger lesions. Contrary to other studies, we found no observable relationship between target coverage and the distance from the treatment isocenter. Overall, the two methods proposed can be applied in a clinical setting various types of cases.
David Pearson, PhD (Advisor)
Diana Shvydka, PhD (Committee Member)
Nicholas Sperling, PhD (Committee Member)
62 p.
Health Sciences; Medicine; Physics; Radiation
medical physics; stereotectic radiosurgery; single isocenter; multiple brain metastases; radiation therapy

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Citations

  • Oakey, M. E. (2019). Developing a Quantitative Means for Evaluating Single Isocenter Multi-Target SRS Plans [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=mco1556908025631349

    APA Style (7th edition)

  • Oakey, Mary. Developing a Quantitative Means for Evaluating Single Isocenter Multi-Target SRS Plans. 2019. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=mco1556908025631349.

    MLA Style (8th edition)

  • Oakey, Mary. "Developing a Quantitative Means for Evaluating Single Isocenter Multi-Target SRS Plans." Master's thesis, University of Toledo, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=mco1556908025631349

    Chicago Manual of Style (17th edition)

mco1556908025631349
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This open access ETD is published by University of Toledo Health Science Campus and OhioLINK.