Semester of Graduation

Summer 2025

Degree

Master of Science (MS)

Department

Physics and Astronomy

Document Type

Thesis

Abstract

Purpose: To reduce the high false positive rates (FPRs ≥ 46%) from the evaluation of Gamma Knife stereotactic radiosurgery (SRS) treatment plans for small brain metastases. Specifically, we modified two evaluation indices, Treated Volume Ratio (TVR) and Gradient Index (GI), which exhibited FPRs exceeding 84%. A false positive result incorrectly indicates that an acceptable plan, as determined by a medical physicist and a radiation oncologist, is inacceptable.

Methods: This study utilized training and validation datasets from Gamma Knife treatment plans from The University of Texas MD Anderson Cancer Center (MDACC) and Mary Bird Perkins Cancer Center (MBPCC). Data included doses, volumes, and evaluation indices derived from dose volume histograms (DVH). We evaluated FPRs for three methods, including: (1) the original clinically used index formulae and intervention thresholds; (2) revised formulae and thresholds; and (3) revised formulae, prescribed isodose line (IDL), and original intervention thresholds. Each index was assessed individually. We also assessed the composite FPRs of the logical OR of the dichotomous (pass/fail) results for TVR, GI, and Coverage Index (CI).

Results: The original method produced high FPRs, for TVR, GI, and logical OR yielding 97.8%, 46.2%, and 98.8% in the training dataset, 95.6%, 76.9%, and 94.07% in the first validation dataset and 100%, 85% and 100% in the second validation dataset. The second method reduced the corresponding FPRs to 1.7%, 1.7%, and 5.0% in the training dataset, 6.3%, 8.6%, and 14.9% in the first validation dataset, and all zeros in the second validation dataset. The third method reduced the FPRs to 10.0%, 0.0% and 10.0% in the second validation dataset.

Conclusion: The proposed refinements reduced FPRs from > 46% for GI and TVR to < 20%. The refinements are applied only to small target volumes (Vtgt < 93 mm3). Reduced FPRs can improve clinical workflow efficiency by reducing the frequency of unproductive FPRs investigations.

Date

5-13-2025

Committee Chair

Newhauser Wayne

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