ESTRO 2020 Abstract book

S353 ESTRO 2020

US-based motion model is a promising IGRT approach to guide 3D proton beam tracking in real-time if patient- specific models are first created based on the simultaneous pre-treatment acquisition of 4DMRI and liver US. However, to further mitigate residual patient/motion- specific effects, it will be necessary to combine tracking with other motion mitigation techniques (e.g. rescanning/gating or a combination) in order to fully restore dose homogeneity close to the static case. MK and AG contributed equally. OC-0579 First clinical experience with proton therapy for HN cancer according to model-based selections F. Hoebers 1 , M. Unipan 1 , R. Emmah 1 , F. Wesseling 1 , I. Compter 1 , P. Van den Ende 1 , J. Schiffelers 1 , C. Stultiens 1 , J. Mannens 1 , J. Van der Stoep 1 , K. Van der Klugt 1 , F. Visser 1 , R. Wanders 1 , K. Limpens 1 , M. Pijls 1 , M. Velders 1 , E. Backx 1 , G. Vilches-Freixas 1 , I. Rinaldi 1 , G. Bosmans 1 , L. Boersma 1 1 MAASTRO Clinic, Radiation Oncology Department, Maastricht, The Netherlands Purpose or Objective Protontherapy (PrTh) for head and neck cancer (HNC) may allow for better sparing of organs at risk (OARs), compared to photon therapy (PhTh) leading to reduced side-effects. In the Netherlands, PrTh is reimbursed for HNC patients, if the ΔNTCP for grade (Gr) 2 and 3 xerostomia and/or dysphagia between a PrTh and PhTh-plan is ≥10 and 5%, respectively. The NTCP is calculated according to a national indication protocol and requires patient parameters as well as dose parameters to OARs, including the salivary glands and swallowing structures. In this study we report on the proportion of patients, qualifying for PrTh, applying the Between Dec-2018 and Oct-2019, 76 HNC patients were screened for potential treatment with PrTh. Patients with early larynx cancer treated with local RT or with oral cavity cancer with metal reconstruction plates at the mandible in place after surgery were not considered for screening. First, a state-of-the-art PhTh plan was generated using VMAT in Eclipse. PTV coverage was evaluated according to ICRU guidelines, i.e. V95% ≥ 98%. If the NTCPs with PhTh were below the threshold to obtain the required ΔNTCP, then a plan-comparison (PC) was meaningless. In some cases, in whom on the planning-CT artefacts due to dental fillings close to the CTV were present, treatment with PrTh was also considered not to be feasible. For PrTh, intensity modulated proton plans were robustly optimized against 0.3 cm set-up and 3% range uncertainties using RayStation with a MC dose algorithm. A beam-setup with 4-5 directions was used. Plan robustness and CTV coverage were evaluated on voxelwise minimum and maximum dose distributions 1 Results In 21/76 cases (27,6%) a PC was judged as meaningless due to NTCP of PhTh or presence of artefacts. In 55 patients a PC between PhTh and PrTh was performed. In 41 patients (75%) the threshold for ΔNTCP was not met. In 14 cases (25%) the PC resulted in a ΔNTCP that met the criteria; 11 of these were treated were PrTh. The other 3 cases were treated with PhTh: 1 proved to have distant metastases, 1 was unable to undergo treatment in the mask due to pain and 1 patient considered the benefit from PrTh as too small. model-based approach Material and Methods

Results Mean motion prediction error was below 2.1mm and absolute voxel-to-voxel dose differences between ideal and realistic tracking for all 5 cases were less than 10% or 5% in 93.2% or 80.1% of the VOI, respectively (Fig 2a - solid line: median, shade: range). Fig 2b shows DVHs for the two tracking scenarios compared to the static and the 4D non- mitigated results. Tracking substantially improves CTV coverage and dose homogeneity compared to non- mitigated delivery, and differences are small between ideal and realistic tracking, indicating the efficacy of the US-driven statistical motion model. However, tracking cannot completely restore dose homogeneity to that of the static case.

Conclusion