ESTRO 2023 - Abstract Book

S1706

Digital Posters

ESTRO 2023

PO-1948 Robust LET optimization of proton arcs can substantially reduce high LET in critical structures

L. Glimelius 1 , O. Marthin 1 , V. Wase 2 , J. Sundström 2 , E. Engwall 1 , A. Fredriksson 3 , H. Melbéus 1 , F. Tamm 1 , B. Andersson 2 , J. Öden 2 , R. Bokrantz 2 1 RaySearch Laboratories, Physics department, Stockholm, Sweden; 2 RaySearch Laboratories, Research department, Stockholm, Sweden; 3 RaySearch Laboratories, Research Department, Stockholm, Sweden Purpose or Objective High LET at the distal edge of proton beams may cause increased biological dose to OARs close to the target, and it is common practice to avoid spots where the Bragg peak is located right before them. A potential benefit of proton arc therapy is the many degrees of freedom compared to IMPT using only a few beams, thus increasing the possibility to remove high LET in OARs while still maintaining robust target coverage. This study investigates how LET-based robust optimization can improve IMPT and arc plans for a brain case. Materials and Methods A research version of RayStation 12A contains the functionality to include robust optimization functions on the min/max dose-averaged LET (LETd), evaluated above a user defined dose threshold. It is also capable of planning for dynamic proton arcs, where the dose is delivered while the gantry is moving. RayStation has been used to create a two-field IMPT plan and a dynamic arc plan for a brain case. Both were planned with identical robust optimization functions, assuming 3 mm setup and 3% range uncertainty: uniform CTV coverage of 54 Gy(RBE), max dose of chiasm and brainstem of 54 Gy(RBE) and a general reduction of dose in surrounding tissue. The plans were subsequently reoptimized with additional robust max LETd objectives of 3 keV/ µ m (above 5 Gy(RBE)) on the chiasm and brainstem. A robust evaluation (3 mm setup and 3% range uncertainty) was performed for all plans. Results Figure 1 shows the dose, LETd and DVHs in the nominal scenario. All plans have similar target coverage, and the near max dose to the OARs are below clinical thresholds (chiasm D2: 54 Gy(RBE), brainstem D2: 60 Gy(RBE)). High LET is clearly removed from the chiasm and brainstem in the LET optimized plans. This has been achieved by redistributing weight from spots with protons stopping before the OARs to spots from other angles, which can be seen in the LET-optimized IMPT plan where the posterior beam has a higher entrance dose. For the LET-optimized arc plan, spots passing through the brainstem get an increased weight, resulting in a higher low dose.