Background: To improve organ at risk (OAR) sparing, dynamic trajectory radiotherapy (DTRT) extends VMAT by dynamic table and collimator rotation during beam-on. However, comprehensive investigations regarding the impact of the gantry-table (GT) rotation gradient on the DTRT plan quality have not been conducted.
Purpose: To investigate the impact of a user-defined GT rotation gradient on plan quality of DTRT plans in terms of dosimetric plan quality, dosimetric robustness, deliverability, and delivery time.
Methods: The dynamic trajectories of DTRT are described by GT and gantrycollimator paths.The GT path is determined by minimizing the overlap of OARs with planning target volume (PTV). This approach is extended to consider a GT rotation gradient by means of a maximum gradient of the path (G_max) between two adjacent control points (G = |Δtable angle∕Δgantry angle|) and maximum absolute change of G (ΔG_max).Four DTRT plans are created with different maximum G&ΔG: G_max&ΔG_max = 0.5&0.125 (DTRT-1),1&0.125 (DTRT-2),3&0.125 (DTRT-3) and 3&1 (DTRT-4), including 3-4 dynamic trajectories, for three clinically  motivated cases in the head and neck and brain region (A, B, and C). A reference VMAT plan for each case is created. For all plans, plan quality is assessed and compared. Dosimetric plan quality is evaluated by target coverage, conformity, and OAR sparing. Dosimetric robustness is evaluated against systematic and random patient-setup uncertainties between ±3 mm in the lateral, longitudinal, and vertical directions, and machine uncertainties between ±4° in the dynamically rotating machine components (gantry, table, collimator rotation). Delivery time is recorded. Deliverability and delivery accuracy on a TrueBeam are assessed by logfile analysis for all plans and additionally verified by filmmeasurements for one case.All dose calculations areMonte Carlo based.
Results: The extension of the DTRT planning process with user-defined G_max&ΔG_max to investigate the impact of the GT rotation gradient on plan quality is successfully demonstrated. With increasing G_max&ΔG_max, slight (case C, D_{mean,parotid l.}: up to -1 Gy) and substantial (case A, D_{0.03cm³,optic nerve r.}: up to -9.3 Gy, case B, D_{mean, brain}: up to -4.7 Gy) improvements in OAR sparing are observed compared to VMAT, while maintaining similar target coverage. All plans are delivered on the TrueBeam. Expected and actual machine position values recorded in the logfiles deviated by <0.2° for gantry, table and collimator rotation. The film measurements agreed by >96% (2% global/2 mm Gamma passing rate) with the dose calculation. With increasing G_max&ΔG_max, delivery time is prolonged by <2 min/trajectory (DTRT-4) compared to VMAT and DTRT-1. The DTRT plans for case A and B and the VMAT plan for case C plan reveal the best dosimetric robustness for the considered uncertainties.
Conclusion: The impact of the GT rotation gradient on DTRT plan quality is comprehensively investigated for three cases in the head and neck and brain region. Increasing freedom in this gradient improves dosimetric plan quality at the cost of increased delivery time for the investigated cases. No clear dependency of GT rotation gradient on dosimetric robustness is observed.