Author ORCID Identifier
0000-0002-6600-1104
Date of Graduation
8-2024
Document Type
Dissertation (PhD)
Program Affiliation
Medical Physics
Degree Name
Doctor of Philosophy (PhD)
Advisor/Committee Chair
Stephen F. Kry. PhD
Committee Member
Rebecca M. Howell, PhD
Committee Member
Julianne M. Pollard Larkin, PhD
Committee Member
Christopher L. Nelson, PhD
Committee Member
Christine B. Peterson, PhD
Committee Member
Catharine H. Clark, PhD
Abstract
Consistency of Advanced Radiation Therapy QA Methods and Global Auditing Systems
Fre’Etta M.D. Brooks, B.S.
Advisory Professor: Stephen F. Kry, Ph.D.
Abstract
Auditing systems provide a means to verify the implementation and operation of all equipment and radiation parameters used by an institution providing radiation therapy treatments. The systems are important tools that are used to determine if an institution can accurately deliver radiation therapy that requires complex planning in the form of IMRT or VMAT and credential radiotherapy treatment institutions for specific clinical trial protocols; to assure the quality of treatment to all patients. Auditing techniques vary by region thus resulting in inconsistencies in error detection between systems. Currently a global standard for auditing techniques has not been established and the differences in institutional pass/fail results between global auditing bodies is unknown.
The purpose of this work was to establish a practical framework for creating a set of validated reference plans and a workflow for developing plans that could be used to test and compare the differentiability of various audit methodologies in a manner that has not been previously explored. The workflow is relevant for PSQA systems as they are used for both individual QA and as audit systems for various agencies. This work aims to provide both a procedural foundation for audit methodology and PSQA evaluation in addition to insight into the magnitude of perturbation associated with beam modeling, calibration, and delivery errors that are currently common in clinical practice. Further, a workflow that facilitates agreement amongst the pass/fail results for audit quality assurance centers is proposed.
Through a sensitivity study with a cohort of over 2,000 plan perturbations for phantom and clinical cases, the clinical impact of atypical beam modeling and TPS configuration was quantified. The parameters related to the modeling of the MLC offset and MLC transmission exhibited a substantial impact on the dose calculation accuracy for both phantom and clinical cases, across all clinical anatomical sites, up to ±5.7% and +16.7 to the CTV and OAR respectively. It was found that these dose perturbations were related to plan complexity. The mean MLC Gap and Tongue & Groove index complexity metrics were best suited to identifying clinical plans that are more sensitive to beam modeling errors.
The results from the sensitivity study were used to identify clinically relevant parameter perturbations that were used in a collaborative effort with the Global Harmonization Group (GHG) for developing plans to test and compare the differentiability of various audit methodologies. First, a cohort of suitable reference plans (error free and accurately calculated by the TPS), such that the calculated and measured doses agreed, were created. Next, clinically relevant perturbations, identified in the sensitivity study, were introduced to the cohort. The perturbations spanned a range of errors from minor to clinically compromising target coverage or organs at risk (OAR) sparing. Then, each plan perturbation was evaluated relative to the original, unperturbed plan, by different audit systems, to identify the system's ability to detect poor beam models or delivery issues. In general, there was high agreement regarding each system’s sensitivity in detecting plan errors. However, there was greater disagreement regarding system specificity. Different criteria and assessment methodologies were proposed to facilitate better agreement amongst the systems.
Finally, the results from the sensitivity study and the workflow developed in collaboration with the GHG were used to validate a cohort of clinical plans and introduce a set of known errors to each case. The plans were then utilized to quantify how accurately IMRT PSQA devices can discriminate between acceptable and unacceptable plans when using different assessment criteria and to identify optimal operating thresholds for each device. The devices demonstrated differences in differentiability depending on which passing criteria (based on changes in the mean dose to CTVs) was used. Further, an in-depth investigation into ArcCheck differentiability revealed that changes to various settings and passing criteria can impact system performance. Optimal operating thresholds and settings were identified for each of the devices.
Given that the results demonstrate that system performance is optimal when using current 3%/3mm criteria for point dose measurement and the ArcCheck baseline criteria (used in the current study) when detecting 5% errors, careful consideration needs to be given when commissioning and implementing these PSQA devices. Further, it is equally important to ensure that consistency is maintained when using pass/fail criteria, as adjustment to device settings can impact system performance.
In conclusion, acceptable and unacceptable plans should be classified as such no matter which agencies or institutions are conducting an audit or plan assessment, to ensure consistency in the quality of radiotherapy treatments worldwide.
Keywords
Dosimetry Audit, Phantom, PSQA, Radiotherpy, IMRT, VMAT, TPS Commissioning, Quality Assurance