ESTRO 2020 Abstract book

S436 ESTRO 2020

OC-0705 Dose restoration: an online adaptive strategy to fight against range uncertainties in proton therapy E. Borderías 1 , X. Geets 1,2 , E. Sterpin 1,3 1 UCLouvain, MIRO Molecular Imaging Radiotherapy and Oncology, Brussels, Belgium ; 2 Cliniques Universitaires Saint Luc, Radiotherapy, Brussels, Belgium ; 3 KULeuven, Oncology, Leuven, Belgium Purpose or Objective Proton therapy is more sensitive to uncertainties in treatment planning and dose delivery than photon therapy. Density changes in the patient may alter planned proton ranges and compromise plan quality. In this work, we study the clinical benefit of an on-line adaptation strategy, dose restoration (DR), in a challenging location such as lung. DR aims to compensate for density changes by accurately reproducing the planned dose in repeated CT images, without considering re- contouring modifications. We also investigate to what extent stabilizing dose distributions through DR can reduce the need of full adaptation. Material and Methods Our database included a planning CT and two repeated 4D- CTs (1rCT, 2rCT) for 14 lung cancer patients. 4D-Robust optimization on CTV was performed in RayStation 8A, with 5 mm setup error, 3% of range uncertainty, and three phases of the respiratory cycle (end-exhale, end-inhale and MidPosition). DR uses isodose contours generated from the initial dose and patient specific weighted objectives to reoptimize the plan and mimic the initial dose in repeated CTs. Fully automatic robust DR, using the same robustness parameters as in planning, was performed in the two series of repeated 4D-CTs. Robustness tests were also run for initial, distorted (no adapted) and restored (adapted) plans in each CT series. The plot of all evaluation scenarios, each with its corresponding DVH, results in DVH- bands (Figure 1). Local dose differences between restored/distorted and initial dose distributions were also analysed by reporting absolute errors in smalls volumes DE(vol=2%).

Results The evaluation of initial plans on repeated CTs showed large dose distortions, substantially reduced after restoration. In the analysis of local dose differences, median DE(vol=2%) were lowered from 9.2 Gy (1rCT) and 8.2 Gy (2rCT) in distorted plans to 3.1 Gy(1rCT) and 2.8 Gy (2rCT) in restored plans. Deviations respect to the reference dose in dose-volume parameters extracted from DVH-bands (nominal and worst-case values) shown general improvement of DVH metrics in restored plans (Figure 2). After DR, median D95 was increased by 1 Gy, keeping all restored plans within clinical limits, except one patient that would require off-line adaptation including contour deformation. According to CTV coverage criteria (D95%>95%Dprescription), 36% (5/14) and 23% (3/13) of the cases would have needed full adaptation in 1rCT and 2rCT, respectively, without DR being performed.