Michael K. Fix

Non‐coplanar radiotherapy treatment techniques on C‐arm linear accelerators have the potential to reduce dose to organs‐at‐risk in comparison with coplanar treatment techniques. Accurately predicting possible collisions between gantry, table and patient during treatment planning is needed to ensure patient safety. We offer a freely available collision prediction tool using Blender, a free and open‐source 3D computer graphics software toolset. A geometric model of a C‐arm linear accelerator including a library of patient models is created inside Blender. Based on the model, collision predictions can be used both to calculate collision‐free zones and to check treatment plans for collisions. The tool is validated for two setups, once with and once without a full body phantom with the same table position. For this, each gantry‐table angle combination with a 2° resolution is manually checked for collision interlocks at a TrueBeam system and compared to simulated collision predictions. For the collision check of a treatment plan, the tool outputs the minimal distance between the gantry, table and patient model and a video of the movement of the gantry and table, which is demonstrated for one use case. A graphical user interface allows user‐friendly input of the table and patient specification for the collision prediction tool. The validation resulted in a true positive rate of 100%, which is the rate between the number of correctly predicted collision gantry‐table combinations and the number of all measured collision gantry‐table combinations, and a true negative rate of 89%, which is the ratio between the number of correctly predicted collision‐free combinations and the number of all measured collision‐free combinations. A collision prediction tool is successfully created and able to produce maps of collision‐free zones and to test treatment plans for collisions including visualisation of the gantry and table movement.

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 gantry‐collimator 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 () between two adjacent control points () and maximum absolute change of G (). Four DTRT plans are created with different maximum G&∆G: & = 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 in the lateral, longitudinal, and vertical directions, and machine uncertainties between 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 film measurements for one case. All dose calculations are Monte Carlo based. Results The extension of the DTRT planning process with user‐defined to investigate the impact of the GT rotation gradient on plan quality is successfully demonstrated. With increasing , slight (case C, : up to‍−1‍Gy) and substantial (case A, : up to −9.3 Gy, case‍B,: up to −4.7‍Gy) 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 , 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.