Purpose: To investigate the influence of freedom in dynamic table rotation on treatment plan quality, deliverability, delivery time and robustness against systematic machine position errors for dynamic trajectory radiotherapy (DTRT). DTRT extends VMAT by dynamic table and collimator rotation during beam-on, described by gantry-table and gantry-collimator paths.
Methods: The gantry-table path is determined by minimizing overlap of OARs with PTV under following restrictions in freedom for table rotation: Maximum gradient of the path (G) between two adjacent control points (Δtable-angle / Δgantry-angle) and maximum change of G (ΔG). A VMAT plan and 4 DTRT plans are created for different table rotation constraints G&ΔG= 0.5&0.125 (DTRT1), 1&0.125 (DTRT2), 3&0.125 (DTRT3), 3&3 (DTRT4), including 2-4 arcs/trajectories, for a nasopharynx, a glioblastoma and a bilateral oropharynx case. For all plans, plan quality is evaluated by target coverage and OAR sparing, delivery time is estimated and dosimetric robustness against systematic errors between +-1° in gantry, table, collimator rotation and combination thereof is evaluated. Delivery accuracy is verified with film measurements on a TrueBeam for the nasopharynx plans. All dose calculations are Monte Carlo based.
Results: With increasing G&ΔG, slight (oropharynx) and substantial (nasopharynx, glioblastoma) improvements in OAR sparing are observed compared to VMAT, while maintaining similar target coverage. However, delivery time was prolonged by up to 5 min. The VMAT (oropharynx), DTRT2 (nasopharynx) and DTRT1 (glioblastoma) plan revealed the best dosimetric robustness. Film measurements agreed by >98% (2%/2 mm Gamma-passing-rate) with the dose calculation for VMAT and DTRT1&2. DTRT3&4 resulted in interlocks due to gantry exceeding its tolerance limits.
Conclusion: With increasing freedom in table rotation, plan quality can be improved for DTRT on cost of increased delivery time for the investigated cases. No clear dependency of dosimetric robustness on table rotation constraints was visible. This work was partially supported by Varian Medical Systems.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by Varian Medical Systems.