The Impact of Treatment Plan Complexity on Calculation and Measurement-Based Pre-Treatment Verifications in Intensity-Modulated Radiotherapy

<h3>Purpose/Objective(s)</h3> Excessive modulation in radiotherapy (RT) treatment plans often introduces heightened complexities and uncertainties in doses, potentially impacting clinical outcomes. This investigation delves into the intricate relationship among plan complexities, calculated doses, and measured doses, providing valuable insights to enhance the robustness of treatment plans. <h3>Materials/Methods</h3> A meticulous selection of 55 plan complexity metrics (PCMs) was conducted, emphasizing features such as small field characteristics and leaf speed/acceleration. Doses were obtained from two beam-matched treatment devices, utilizing the Anisotropic Analytical Algorithm for intended dose computation and validated through Monte Carlo and Collapsed Cone Convolution algorithms. 3D diode arrays with various geometries ("O", "+", and "×" shapes) were employed to measure the delivered dose. Interrelationships were explored using Spearman correlation and principal component linear regression (PCR). <h3>Results</h3> A total of 4448 GPRs for various QA systems corresponding to two Linacs were recorded. Correlation coefficients between calculation- and measurement-based verification quality assurance (CQA, MQA) were found to be below 0.531. Most PCMs exhibited stronger correlations (<i>r_pcm-QA</i>) with CQA (maximum: 0.838) compared to MQA (maximum: 0.646). The pelvis treatment site showed the highest proportion of <i>r_pcm-QA</i> ≥ 0.5, surpassing head-and-neck and chest-and-abdomen cases. At the 1%/1 mm criterion, PCMs exhibited the strongest correlation with QA systems. Specific PCMs for MLC speed and acceleration displayed significant correlations with both CQA and MQA. PCR determination coefficients (R²) underscored that PCMs exhibited higher accuracy in predicting CQA (maximum: 0.756) in contrast to MQA (maximum: 0.440). <h3>Conclusion</h3> This study represents the first comprehensive investigation into the relationship between PCMs, CQA, and MQA. A weak correlation between CQA and MQA was observed. Importantly, the evidence suggested a stronger correlation and higher predictive accuracy of CQA with PCMs compared to MQA. Specific PCMs related to MLC movement effectively evaluated the modulation degree of IMRT plans. Additionally, heightened complexity in pelvis plans requires extra vigilance due to its potential association with reduced accuracy.