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ABSTRACT
Background
The introduction of advanced technologies in radiotherapy requires high precision and accuracy, particularly in image-guided radiotherapy (IGRT). This study aimed to evaluate the dosimetric impact of setup errors—specifically positional shifts—on planning target volume (PTV) coverage and organ-at-risk (OAR) sparing in volumetric modulated arc therapy (VMAT) for nasopharyngeal cancer. The goal was to support the clinical implementation of IGRT imaging protocols and establish tolerance thresholds that maintain treatment efficacy without compromising safety.
Method
An anthropomorphic SHANE (Shoulder, Head and Neck) phantom was used to simulate patient anatomy. A reference VMAT plan was created and then systematically modified to introduce positional shifts in all six degrees of freedom. The dosimetric consequences of these shifts were analyzed using the Elekta Monaco 6.1.2.0 treatment planning system (TPS). Dose-volume histograms (DVHs), PTV coverage, and dose to OARs were evaluated. Patient-specific quality assurance (PSQA) was conducted using the Octavius® 4D phantom and VeriSoft software to assess dose delivery accuracy under shifted conditions. Gamma analysis and DVH statistics were used to determine clinically acceptable shift tolerances, in line with QUANTEC and IAEA recommendations.
Result
Translational shifts beyond ±2 mm significantly reduced PTV coverage, with reductions of up to 50% at ±20 mm shifts. Inferior and posterior displacements led to the most severe reductions—up to 70% and 40% respectively. Hotspot regions increased with lateral, longitudinal, and vertical shifts, raising the risk of unintended dose escalation. OARs, including the spinal cord, brainstem, brain, and parotid glands, were variably overdosed depending on shift direction and magnitude. Gamma passing rates deteriorated with lateral and vertical displacements, indicating compromised treatment accuracy. Tolerable setup error thresholds were established between 4–6 mm, beyond which treatment efficacy may be compromised.
Conclusion
The study demonstrates that even small setup errors can have substantial dosimetric impacts in VMAT for nasopharyngeal cancer. Precise patient positioning, daily or frequent image verification (e.g., cone beam computed tomography (CBCT)), and adherence to positional tolerances of ±2 mm are essential to ensure effective dose delivery and OAR protection. Quality assurance tools, such as the Octavius® 4D system, and adaptive planning protocols are critical to maintaining treatment accuracy in anatomically sensitive regions.