An Initial Assessment of the Impact of the Normal Tissue Objective on VMAT Radiosurgery Plan Complexity and Deliverability.
Abstract
Purpose
To evaluate the impact of the Normal Tissue Objective (NTO) in the Eclipse (Varian) treatment planning system on volumetric modulated arc therapy (VMAT) radiosurgery plans, focusing on clinical parameters, plan complexity, and patient-specific quality assurance (PSQA).
Methods
A retrospective longitudinal study was conducted on VMAT radiosurgery plans from 18 patients, using the Eclipse (Varian) V15.6 treatment planning system. Plans were generated using different NTO configurations: automatic, off, and drop-off, with the k values of 0.1, 0.5, 0.8, and 1. Evaluated parameters included target coverage, normal tissue dose tolerance volume, conformity index (defined by the RTOG), and minimum and maximum doses. Plan complexity was quantified using the Modulation Complexity Score (MCS) and its components: leaf sequence variable (LSV) and aperture area variability (AAV). Plan deliverability was assessed for 10 plans using gamma index analysis (2%/2 mm, 1%/2 mm, and 2%/1 mm criteria) and measured from a Truebeam STx with SRS MapCHECK coupled to the StereoPHAN phantom. The plans were selected based on field size. Statistical analysis was performed using the Wilcoxon signed-rank test and Spearman correlation.
Results
Target coverage showed no significant dependence on NTO configuration. However, the volume of normal tissue receiving tolerance doses increased with different NTO drop-off values. This was accompanied by increased conformity index values. Plan complexity increased moderately, as measured by the MCS. Substantial variations were observed in the LSV and AAV. Notably, LSV strongly correlated with changes in gamma passing rates.
Conclusion
The use of the Normal Tissue Objective in VMAT radiosurgery does not affect target coverage. It does have a measurable impact on normal tissue irradiation and plan complexity. Variations in NTO parameters moderately increase complexity metrics and influence MLC leaf motion. These changes affect plan deliverability, as assessed by gamma analysis.