Objectives: In the era of intensity modulated radiotherapy, there has been various approaches to deliver an adequate dose to planning target volume (PTV) and spare the organ-at-risks (OARs) structures. The challenging part is when the OARs are in close proximity to PTV. In such scenarios, there is a trade-off whether to cover the PTV or spare the OARs. This limitation has been addressed in this study utilizing dose optimization during couch motion which provides a new method to deliver the dose in a coronal plane. We are introducing gantry static continuous couch motion optimization (GsCMO) technique where dose is optimized at 1-20 couch increments.

Methods: The GsCMO technique consists of simulated arcs utilizing multiple static fields with continuous couch motion. The static IMRT fields from Eclipse treatment plan (Varian medical systems, Palo Alto, CA) were merged using an in house MATLAB® script and produced a single XML file from those multiple static fields. The first part of this study compared the GsCMO technique with standard coplanar (CP) and non-coplanar (NCP) VMAT arcs in brain study where PTV was in close proximity to brainstem and optic chiasm. Comparison of the dose to OARs and PTV coverage (D95=100%) using dose volume histogram (DVH) comparison, conformity index (CI), and gradient measure (GM) were used as meaningful endpoints. And the second part of this study validated the GsCMO technique on Varian Truebeam® developer mode application. The delivery of all the plans from Eclipse planning system and MATLAB® generated automated deliverable XML files were compared on Varian Truebeam® developer mode application. Each plan was delivered onto an IMRT QA phantom. The planned dose was compared to measured dose using 2-dimensional (2D) gamma analysis with passing criteria of 2% dose difference (DD) and 2 mm distance to agreement (DTA). As a secondary QA check, the LINAC log files of beam delivery were used to verify the leaf position accuracy of MLC per control point for multiple fields.

Results: The percentage reduction of dose to brainstem (D50 of -11%±5% and D30 of -20%±6%) and optic chiasm (D50 of -15%±8% and D30 of -29%±5%) were noticed with max point dose reduction comparing GsCMO with standard coplanar and non-coplanar arcs for the brain study. All XML-based plans were delivered with passing 2D gamma criteria (2mm/2%) of 98±1.2%. The root mean square of log files was calculated as 0.0256±0.0041mm for all GsCMO cases. The average delivery time was ~3 minutes for automated GsCMO delivery as compared to ~30 minutes if all fields delivered individually.

Conclusions: Our novel GsCMO technique results in a significant dose reduction to the organ-at-risks (OARs). We have developed a software that can convert the multiple fields of a treatment plan into a single XML file. The preliminary/feasibility test of file-based delivery using this software demonstrated significant improvements in treatment delivery efficiency without compromising accuracy.