ESTRO 2022 - Abstract Book

S573

Abstract book

ESTRO 2022

Figure 2: Population average DVHs of the total plans.

MO-0640 Is robust planning safe? A comparison of margin and robust photon plans on different target shapes

J. Robbins 1 , E. Vasquez Osorio 1 , A. Green 1 , M. van Herk 1

1 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom

Purpose or Objective In radiotherapy, PTV margins are used to account for uncertainties. Margin recipes were derived for targets with low curvature which is unrealistic in head and neck (H&N) patients. Robust planning could potentially better spare organs at risk, but the actual robustness to geometrical uncertainties in photon therapy of H&N cancer is unknown. The aim of this study is to compare, for different target shapes, margin-based plans with robustly optimised plans in terms of dose- probability under realistic set-up error scenarios. Materials and Methods 24 VMAT photon plans were created on a cylindrical phantom in RayStation with a prescribed dose of 66 Gy, using minimum, maximum and uniform target objectives, and dose fall-off objectives. Margin-based and robustly optimised plans were created for a sphere (48 mm diameter), a cube (48 mm length) and 10 actual CTV shapes from H&N patients (shapes shown in Figure 1), all centred in the phantom. A PTV expansion of 4 mm was used for margin plans and a robustness setting of 4 mm in all cardinal directions for robust plans. The mean distances between the CTV and the 95% and 50% isodose surface were computed to evaluate conformality. Then, for each plan, 1000 treatments were simulated under set-up uncertainties. Systematic ( Σ = 1.2 mm) and random ( σ = 1.4 mm) uncertainties were modelled as Gaussian distributions. For each simulation, the minimum cumulative dose to the CTV shape and the D99 was recorded. Results The 90 th percentile of the minimum CTV dose in the simulation results is close to or above the 95% prescribed dose for margin plans of all targets, in accordance with the van Herk margin recipe, but is below this level for all robust plans except for the sphere (Figure 1a). For all plans, the 90 th percentile of the D99 is above the 95% prescribed dose (Figure 1b). This shows that margin plans provide robust coverage for the whole CTV for all shapes yet robust plans fail to cover the entire target in most scenarios. However, regions of high and intermediate dose for the margin plans extend beyond those for the robust plans (Figure 2). The distance between the CTV and the 95% isodose surface is ~5.9 mm in margin plans, and ~4.1 mm in robust plans.