Master of Science (MS)
Physics and Astronomy
Purpose: The purpose of this work was to evaluate the ability of a treatment delivery verification system to detect clinically significant mechanical errors in VMAT delivery utilizing data contained within a commercial record and verify (RV) system, and to test whether the system can be used to reconstruct dose distributions that agree more closely with measured dose distributions than do the original treatment plans. Methods: VMAT treatment plans from five prostate patients, five head and neck patients, and five post-mastectomy chest wall patients treated at our clinic were selected for this study. Known mechanical errors were introduced into each plan, and the dosimetric effect of each error was evaluated by a radiation oncologist to determine mechanical error thresholds for clinical acceptability. Next, shifted plans were delivered on an Elekta linear accelerator, and the resulting dose distribution was measured with a two-dimensional diode array. During delivery, positions of the gantry, multi-leaf collimator (MLC), and jaws were captured in both the RV system and the linac verification interface (LVI). Following plan delivery, recorded delivery data were analyzed to characterize the ability of the recording systems to recognize true mechanical error. Finally, delivery parameters from both recording systems were imported to the treatment planning system and used to recalculate the dose delivered to the measurement phantom to test whether the system can be used to reconstruct dose distributions that agree more closely with measured dose distributions than do the original treatment plans. Results: Minimum mechanical error thresholds over all fifteen cases were found to be 1 mm for systematic MLC error, 2 mm for Gaussian-random MLC error, 3 mm for uniform-random MLC error, 3 mm for central leaf pair error and 2° for gantry error. Analyzing delivery data yielded p-values less than 0.05 for both RV and LVI systems for all central leaf pair and gantry shifts (1, 2, and 3 mm central leaf pair shifts and 1, 2, 3, and 5° rotations), which means that each system could reliably detect errors at threshold values. Reconstruction of delivery data by both RV and LVI systems failed to yield p-values less than 0.05 for prostate, head and neck, and chest wall cases. Conclusion: Both delivery verification systems examined were capable of detecting mechanical errors of 1 mm of the MLC and 2° of the gantry that produce clinically unacceptable changes in the planned dose distribution. However, dose reconstructed using the recorded delivery parameters showed no significant improvement in agreement with measured values. Thus, while the delivery verification system can detect mechanical errors, it cannot more accurately estimate the daily delivered dose than the original plans do.
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Grenier, John-Paul, "Evaluation of an automated delivery verification system for volumetric modulated arc therapy treatments" (2013). LSU Master's Theses. 546.