ESTRO 2021 Abstract Book

S276

ESTRO 2021

PH-0377 Organ-at-risk sparing in head and neck radiotherapy with dynamic trajectory radiotherapy J. Bertholet 1 , P. Mackeprang 1 , S. Mueller 1 , W. Volken 1 , D. Frei 1 , O. Elicin 1 , D.M. Aebersold 1 , M.K. Fix 1 , P. Manser 1 1 Inselspital, Bern University Hospital, University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland Purpose or Objective Non-coplanar beam arrangements have shown potential benefit in organ-at-risk (OAR) sparing compared to volumetric modulated arc therapy (VMAT) for various treatment sites. One approach is dynamic trajectory radiotherapy (DTRT), which extends VMAT with dynamic table and collimator rotation choosing beam incidences along a continuous path that minimize the fractional overlap between the target and the OARs. For head and neck (HN) radiotherapy, many OARs compete in the beam incidence selection. This study evaluates the potential of DTRT for OAR sparing through optimized path finding strategies for common HN cases. Materials and Methods One physician delineated all OARs on an anthropomorphic phantom CT and propagated target volumes from six clinical HN cases to generate a catalogue of common HN cases (Table 1). All cases were planned following institutional guidelines with a sequential boost technique. A dedicated 2-step framework was used to generate deliverable DTRT plans in a research version of the Eclipse treatment planning system (TPS) (Varian Medical Systems) via a scripting interface. First, gantry-table- collimator paths are determined based on contoured structures by an A* path searching algorithm. Second, the multi-leaf-collimator sequence is optimized using a research version of the VMAT optimizer in the TPS. For each case, different DTRT gantry-table-collimator paths were obtained by selecting different OARs, based on clinical relevance. Each gantry-table path could be used multiple times, by applying a 90° collimator angle offset to the A* determined gantry-collimator path or by splitting the field using secondary collimator jaws. For comparison, coplanar VMAT plans with constant collimator angle were created using the same number of arcs and field splitting/collimator angle offset strategies. Results All cases had similar target coverage for DTRT and VMAT plans. Example DTRT trajectories are shown in Figure 1. For cases 1-3, mean dose to the salivary glands, pharynx, oral cavity and larynx were reduced by 2.6 Gy on average with DTRT compared to VMAT (Figure 2). Further, for case 1, dose to the contralateral carotid exceeded tolerance with VMAT but met tolerance with DTRT. For case 2, maximum dose to the spinal cord and brain stem PRVs was reduced by 6.6 and 7.9 Gy respectively.

For case 4, maximum dose to the optical and auditory OARs, spinal cord and brain stem PRVs was reduced by up to 11.9 Gy with DTRT (Figure 2). For case 5, maximum dose to the brachial plexus and mean dose to the oesophagus were reduced by 0.6 and 0.5 Gy respectively but mean dose to the pharynx was increased by 0.8 Gy with DTRT. For case 6, DTRT presented a small but consistent advantage over VMAT (Figure 2).