ESTRO 37 Abstract book

S157

ESTRO 37

OC-0303 Treatment plan comparison between a 1.5 T MRI-Linac and a standard Linac for esophageal cancer M. Nachbar 1 , D. Mönnich 1 , P. Kalwa 1 , D. Zips 2,3 , D. Thorwarth 1,2 , C. Gani 3 1 University Hospital Tübingen, Section for Biomedical Physics- Department of Radiation Oncology, Tuebingen, Germany 2 German Cancer Research Center DKFZ, German Cancer Consortium DKTK partner site Tübingen, Heidelberg, Germany 3 University Hospital Tübingen, Department of Radiation Oncology, Tuebingen, Germany Purpose or Objective This study compares radiotherapy treatment plans for a 1.5 T MRI-Linac and a state of the art clinical treatment system for thoracic esophageal squamous cell carcinoma patients. Material and Methods Twelve patients were included in this planning study. Patients were replanned with an initial plan to a total of 50 Gy covering the primary tumor, affected lymph nodes and elective nodal areas. A subsequent boost plan covered the primary tumor to a total of 60 Gy (2 Gy / fraction). The first plan was created for the Versa HD treatment system (Elekta AB, Stockholm, Sweden) with volumetric modulated arc therapy (VMAT) without the influence of a magnetic field. The second plan was created for the hybrid 1.5 T MRI-Linac using a nine beam step-and-shoot technique with optimized beam angles. Realistic MR-linac plans were generated including the following key parameters. Source-to-isocentre distance: 143.5 cm, leaf width at isocentre: 7.15 mm, fixed isocentre 13 cm above the table. Furthermore, the MR- linac table as well as the RF-coil needed for MR imaging were considered during plan optimization. All plans were generated with Monaco Research 5.19.03 (Elekta AB, Stockholm, Sweden). Treatment plans were optimized to achieve similar target volume coverage. Differences between nine relevant DVH parameters were analysed with a Wilcoxon signed rank test in R 3.3.2. Results Treatment planning was feasible for ten patients. For two patients the cranio-caudal extension of the target volume exceeded the maximum field size of 22 cm of the MRI- Linac. For all patients, both generated treatment plans met dose volume histogram (DVH) criteria with respect to organ at risk sparing and target coverage. Three out of nine DVH parameters showed significant differences (p<0.05). Significantly higher values for the MRI-Linac plans were observed for lung V 5 , the mean lung dose and the mean heart dose. The absolute differences of all parameters between the conventional and the MR-linac plan are shown in Fig. 1.

Conclusion In this in-silico study, treatment plans for the MRI-Linac and for a standard linac were compared for esophageal cancer. Most relevant parameters were statistically equivalent, V 5 and D mean in the lung and D mean in the heart showed a significantly increased dose in MRI-Linac treatment plans. However, all MRI-Linac plans were clinically acceptable. Differences may be compensated in the future by margin reduction due to MR-based daily adapted radiotherapy and implementation of VMAT. OC-0304 Using a single knowledge-based proton planning model to create automated plans for different centers A. Delaney 1 , L. Dong 2 , A. Mascia 3 , W. Zou 2 , Y. Zhang 3 , L. Yin 2 , J. Hrbacek 4 , A. Lomax 4 , B. Slotman 1 , M. Dahele 1 , W. Verbakel 1 1 VUMC, Cancer Center Amsterdam- Department of Radiation Oncology, Amsterdam, The Netherlands 2 University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA 3 University of Cincinnati Medical Center, Department of Radiation Oncology, Cincinnati, USA 4 Paul Scherrer Institute, Center for Proton Radiotherapy, Villigen, Switzerland Purpose or Objective While the number of proton centers globally is increasing, proton treatment planning quality is subject to variation, experienced proton planners are limited in number, and planning for complex cases is time consuming. Such circumstances create a potential role for automated planning/optimization solutions. We evaluated whether a pre-clinical automated optimization solution (RapidPlan for protons, Varian Medical Systems), comprising a model based on proton treatment plans (TPs) from one center, could generate knowledge-based proton plans (KBPs) of an acceptable quality for other proton centers. Material and Methods Fifty intensity-modulated proton therapy (IMPT) TPs for locally-advanced head and neck cancer were designed using a 3-field beam arrangement and used to populate the KBP model. Two proton institutions (A and B) provided 5 contoured CT scans, for which KBPs were created (data from 3 rd center will be available at meeting). KBP optimization objectives were generated automatically, excluding organs-at-risk (OARs) which were not included in the model. Occasionally a subsequent “continue optimization” was applied to refine PTV coverage. KBPs were normalized to cover 95% of the boost planning target volume (PTV B ) with 100%/99% of the prescribed dose for centers A/B, respectively. KBPs were compared to the clinical TPs using PTV B homogeneity index (HI B ; 100*((D2%-D98%)/D50%), mean dose to individual salivary and swallowing OARs. The time required to create KBPs was also investigated. Results KBP creation required <10 minutes. Based on PTV coverage and OAR dose, KBPs were generally of comparable quality to clinical TPs. On average, HI B differed by <2.5%. Some KBP mean OAR doses were lower, e.g. for center A patients larynx/esophagus (on average 6.7/6.0Gy lower) and for center B contralateral submandibular gland/pharynx (on average 27.3/10.6Gy lower) (Table). However, on average KBP parotid mean dose was higher for center A patients (0.1/2.1Gy higher in contralateral/ipsilateral parotid gland). Certain improvements to KBPs are required, including a higher minimum dose to PTV B , lower maximum dose, and control of hot-spots outside PTV/under skin. In addition the model did not include all OARs used in certain clinical TPs (e.g. mandible/optic system). We expect that performing a subsequent optimization, using a few additional structures, should address these issues.

Fig.1 Differences between the plans generated for the conventional Versa HD Linac with a VMAT technique and the MRI-Linac with a step-and-shoot technique. Positive differences mark an increase in the respective metric for the MRI-linac plans.