Divya Rattanpal: Beam data precision affects multi-target brain radiosurgery dosimetry by CyberKnife

The Dosimetric Impact Of Beam Data Precision In Multi-Target Brain Stereotactic Radiosurgery: Quantitative Assessment Using Independent Dose Calculation For CyberKnife
Final Masters Project Presentation by: Divya Rattanpal
Supervisors: Dr. Martin Ebert and Mr. Godfrey Mukwada
Submitted to the School of Physics, Mathematics & Computing for the degree of MASTERS OF PHYSICS (Medical physics)
ABSTRACT
Aim: This project aims to enhance the precision of dose calculations in stereotactic radiosurgery (SRS) for patients with multiple brain metastases by systematically analyzing how alterations in radiation beam data
affects dose accuracy. Specifically, the study aims to investigate the impact of modifying off-center-ratio (OCR) data within the umbral region on dose calculations across various targets. This includes examining
dose deviations from the standard treatment planning system (TPS) when faced with complex, multi-target SRS scenarios, focusing on the sensitivities of different Gross Tumor Volumes (GTVs) to changes in OCRs,
as well as their geometries, orientations, and special proximity.
Method: Utilizing a novel approach, this project adjusts the original OCR beam data using MATLAB to better represent peripheral doses. A MATLAB-based code, which integrates a Ray-Tracing algorithm, utilizes five sets of scaled OCR tables in conjunction with the original beam data tables is developed in the study (based on the code by Pavel Dvorak of the University Hospital Vinohrady, Prague). This enables precise dose computation by integrating several steps: reconstructing patient model and structures from DICOM CT image series and DICOM RT structure set, reconstructing dose cube using DICOM RT Dose file, exporting essential parameters from Plan XML and BeamDataList XML files, and accurately calculating the dose at the reference and center-of-mass (COM) voxel of selected structures. The code generates a plot that illustrates the percentage dose deviation of the calculated doses from the TPS against the scaling factors applied to modify OCRs. Additionally, it displays the axial and sagittal views of CT images with overlays of selected structures, providing a visual representation of the treatment plan in relation to
treatment anatomy.
Results: In the study, we examined eleven treatment plans for patients with multiple brain metastases, focusing on the impact of scaled OCR on dose calculations. A key finding of this study was the proportional relationship between out-of-field dose and dose deviations, accompanied by notable sensitivity variations among different GTVs. The study uncovered how these deviations vary across different tumor sensitivities and collimator sizes due to scaling adjustments. Additionally, our results showed that the percentage deviations of independently calculated dose (IDC) from TPS for all plans and GTVs consistently fell within the acceptable tolerance of ± 5%.
Conclusion: The methodology employed in the project allows for the detailed analysis of dose accuracy relative to the varying geometries and orientations of different targets, thereby enhancing the accuracy and
reliability of dosimetric assessments in SRS. Our findings significantly contribute to improved preparedness for managing the complexities in radiotherapy, potentially leading to more effective and safer treatment
outcomes for patients with brain metastases. To further enhance the validity and applicability of these results, it is recommended to expand patient plan dataset and to integrate additional modified beam data,
such as Tissue Phantom Ratio and Output Factors. This expansion will provide a more comprehensive foundation for refining treatment approaches and optimizing patient care.
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