ESTRO 36 Abstract Book

S256 ESTRO 36 2017 _______________________________________________________________________________________________

Germany 3 Ion Beam Applications IBA, Clinical Solutions, Louvain- la-Neuve, Belgium Purpose or Objective With the increase of proton therapy facilities worldwide featuring Pencil Beam Scanning (PBS) as their only treatment modality, PBS is on the way of becoming the standard for proton therapy. However, for some indications in the thoracic region PBS is not widely used due to uncertainties in the planned dose, which can be caused by combined effects of setup errors, range uncertainty, interplay effect, breathing irregularity, anatomical variations, delivery machine uncertainties, etc. By performing pre-treatment plan robustness evaluation that includes these effects, it is evident that actual delivered fractional dose at any instance is highly uncertain to predict. 4D dose accumulation is able to control some of the uncertainties that are affecting pre- treatment evaluation of the plan quality. Therefore, the purpose of this proof-of-concept study is to investigate the feasibility of monitoring and assessing the quality of delivered treatment fractions throughout the treatment course. Material and Methods 4D dose accumulation is performed by utilizing (1) delivery machine log files (IBA, Belgium), (2) breathing pattern records (ANZAI, Japan) and (3) planning 4DCT scans or repeated 4D control CT scans (Siemens, Germany). Dose computation is performed in the RayStation (RaySearch, Sweden) treatment planning system (TPS). For every spot that is delivered during a particular fraction, the spot energy, position, dose (as charge) and absolute time of delivery is retrieved from the machine log file using a dedicated script. Patient’s breathing pattern is analyzed and inhale peaks are determined. Subsequently, all breathing cycles are divided in 10 phases and each phase is associated with absolute time. PBS spots are split in 10 groups according to their corresponding phase and written to 10 treatment sub-plans (DICOM), where every sub-plan corresponds to a particular phase of the 4DCT. Using scripting capabilities of the TPS, sub-plans are imported for dose computation. Eventually dose warping to the reference phase is performed to estimate the delivered fractional dose. Data sets used for the proof-of-concept were not collected during the same treatment fraction. Results By using the described method the timeline of a PBS delivery can be correlated with patient’s breathing pattern as shown in Figure 1. Computation of log based sub-plans on 4DCT results in an accumulated fractional 4D dose distribution as shown in Figure 2. Based on the exemplary case, the method allows to assess the conformity between planned and delivered doses (i.e., CTV V95 has dropped to 96.7% from nominal 100%).

adjacent to or surrounding the bronchi. Fisher’s exact test was used for comparison. p<0.05 was considered significant. Results Fifty three patients (32%) were adapted due to changes in A (8%), T (6%), N (15%) or T+N (3%), and 7% had more than one replan. Atelectasis was seen at planningCT in 50 patients (30%) while in 12 patients (7%), it appeared during RT. Presence of A before or during RT was not significantly correlated with replanning. However, the changes in A during RT significantly increased the probability of replanning. (p=0.03), see Fig.2. Additionally, A within 5mm of T or N was significant (p=0.01). Only 11 patients (6%) had changes in PE, but only in one patient was replanning indicated. Patients with T0 or N0 had a significant low risk of replanning (p=0.01, p=0.03) while patients with two or more N had a high rate of replanning (ns). Nodes in stations 1,2,3 or 4,7 or 10,11,12 had significantly higher rate of replanning as compared to patients with nodes in station 5,6 and 8,9. Node-volume >30cm 3 had a significantly higher rate of replanning (p=0.02). No correlation was found for T location, T size, T or N adjacent to bronchi or for T or N shrinkage. Histology was not significant for replanning. The imaging rate may be decreased for patients with T0 and no A, as none of these were adapted. For patients with N0 and no A, only 11%, were replanned. On the contrary, 60% of the patients with A and N volume>30cm 3 were replanned. Conclusion Prognostic factors for replanning of lung cancer patients are changes in A during RT, A in the vicinity of T or N, large N volume, and the N stations involved. No correlation between risk of replanning and T size or T location was found. The imaging frequency may be adjusted based on these pre-treatment characteristic. Patients without A and T0 or N0 had little risk of replanning. The imaging frequency may be reduced for these patients, while patients with A and large N volumes should be monitored closely.

OC-0488 Thoracic tumor treatment course assessment based on 4D dose accumulation for scanned proton therapy A. Meijers 1 , C. Richter 2 , F. Dessy 3 , J. Widder 1 , E. Korevaar 1 , A. Jakobi 2 , C. Ribeiro 1 , J. Langendijk 1 , A. Knopf 1 1 University of Groningen - University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands 2 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden- Helmholtz-Zentrum Dresden - Rossendorf, Dresden,