ESTRO 2021 Abstract Book

S265

ESTRO 2021

Centre+, Maastricht, The Netherlands

Purpose or Objective Intensity modulated proton therapy for lung tumors with a large tumor movement is challenging due to loss of robustness in target coverage. Often an upper cut-off at 5 mm tumor motion is used for proton patient selection. In this study, we propose 1) a robust and easily implementable treatment planning strategy for lung tumors with a motion larger than 5 mm and 2) a 4DCT robust evaluation strategy for evaluating the dose on We created a treatment planning strategy based on the internal target volume (ITV) concept (Fig 1). The ITV was created as a union of the clinical target volumes (CTVs) on the eight 4DCT phases. The ITV expanded by 2mm was the target during robust optimization on the average CT (avgCT). Clinical plan acceptability was judged on a robust evaluation, computing the voxel-wise min and max (VWmin/max) doses over 28 error scenarios (shifts in x,y,z direction and range shifts) on the avgCT. Plans were created for 16 lung cancer patients with tumor movement of 4-26 mm in RayStation 10A (RaySearch Laboratories) using Monte Carlo dose for our Mevion S250i Hyperscan system (Mevion Medical Systems). We developed a new 4D robust evaluation approach (4DRobAvg). The 28 scenario doses of a robust evaluation were computed on each of the 8 individual 4DCT phases. For each scenario the dose distributions on the individual phases were deformed to the reference phase (50% expiration, CT50ex) and combined to a weighted sum using equal weights, resulting in 28 average scenario dose distributions. From these 28 scenario doses, VWmin/max doses were computed. This newly developed evaluation strategy was compared to two other possible 4D evaluations: re-computing the nominal plan on each individual 4DCT phase (4DNom), and computing the robust VWmin/max doses on each individual phase (4DRobInd). the breathing phases. Materials and Methods

Results Despite large ITV volumes, a clinically acceptable plan fulfilling target and organ at risk (OAR) constraints was feasible for all patients. The 4DNom and 4DRobInd evaluation strategies were found to under- or overestimate the dosimetric effect of the tumor movement, respectively. 4DRobInd showed target under-dosage for five patients, not observed in the robust evaluation on avgCT or in 4DRobAvg (Fig 2C). OAR dose levels were also judged to be more clinically relevant in the 4DRobAvg method compared to the other two methods. The accuracy of dose deformation was quantified and found acceptable (Fig 2E).