Control-point-specific plan robustness in volumetric modulated arc therapy-based cranial radiotherapy

Progress in mitigating plan degradation due to intrafraction patient motion may involve the identification and management of specific control points that are sensitive to motion. Robust planning in this manner could improve deliverable dosimetry and support advancements toward reducing planning target volume (PTV) margins.

To improve radiotherapy plan quality robustness in the presence of intrafraction motion by identifying the control-point-specific dosimetric sensitivities. This work explores control-point-specific plan characteristics that impact dosimetry by retrospectively assessing the consequence of simulated patient scenarios for cranial radiotherapy.

Single target cranial volumetric modulated arc therapy (VMAT) treatment plans (n = 30) were converted into static field plans and reconstructed by applying 3D control-point-specific motion traces (n = 100) using our in-house MATLAB application. PTV coverage (volume covered by 100% of the prescription isodose, VRx) and the differences in minimum dose delivered to 99% (D_99%) of the gross tumor volume (GTV) were examined across the patient cohort as these are pertinent metrics for each structure. To identify the individual control points where motion led to target coverage loss, three patient plans (5 and 14 were randomly chosen, and 19 with the greatest range in prescription dose coverage) were selected for an area under the curve (AUC) analysis of control point dose volume histograms (DVHs). The mean dose difference in the area under the curve of control point DVHs (mAUC), and the standard deviation of differences (sAUC) were the metrics used in the investigation. Multileaf collimator (MLC) aperture areas were also explored as a function of these metrics.

Under conditions of simulated intrafraction motion, PTV coverage spanned from -2.8% to +0.73% of target volume with 78.6% of the three thousand motion traces resulting in coverage loss. There were no changes in GTV D_99% that exceeded ± 1.5%. For the in-depth control point analysis, MLC aperture areas formed weak to moderately weak correlations with sAUC (r = -0.19, r = -0.42, and r = -0.32, p < 0.01 for patient plans 5, 14, and 19 respectively). In addition, two statistically distinct sub-populations of MLC aperture areas were confirmed by Welch corrected t-tests (p < 0.0001, p = 0.02, p = 0.005 for cases 5, 14 and 19) across a threshold of ± 0.05 mGy in mAUC.

This work has demonstrated that the dosimetric impact of intrafractional motion reflects the inherent motion sensitivity of specific control points. Our findings suggest that motion sensitive control points could be selectively targeted for gating to enhance robustness against intrafraction motion and improve dosimetry in support of a PTV margin reduction strategy. Single target cranial plans serve as ideal cases to characterize the consequences of motion at the control point level with the aim of expanding the analysis to other anatomical regions.