ESTRO 2023 - Abstract Book

S1564

Digital Posters

ESTRO 2023

Results

The study started enrolment in 2017. Currently, 104 patients from three Danish centres have been treated on trial, of which the majority had performance status 2 and stage IIIA-IV disease. Clinical and dosimetric data are summarised in Table 1. Mean doses to GTV-T and GTV-Nlarge were escalated to median 65.6Gy, i.e. close to the aim of 66Gy, despite GTV-T volumes up to 529.2cm3. Plans all complied with OAR constraints. In Figure 1, a comparison between a normo-fractionated 66Gy/33fraction plan and an experimental HERAN plan is shown for one patient. It illustrates that the GTV-T receives the same dose (mean dose: 66Gy) for the normo-fractionated plan and the HERAN plan, while the dose to CTV and PTV volumes is reduced for the HERAN plan (1.A and 1.B). The minimum acceptable dose at the PTV border is 47.5Gy. The de-escalation of the dose peripherally in the target leads to decreased dose to the OAR (1.C). In all patients, a high degree of dose escalation was achieved. Figure 1.D shows the dose volume histogram (DVH) distribution for GTV-T for all 104 treatment plans. The HERAN plans were delivered in 24 fractions. Thus, the mean GTV-T dose was approximately 2.75 Gy/fraction. Conclusion Heterogeneous, hypo-fractionated treatment planning is feasible for LA-NSCLC patients not candidates for standard chemoradiotherapy. Tumour dose escalation (66Gy in 24 fractions) is possible while keeping the dose to OAR low in a multicentre setting. Clinical outcomes are expected in 2023.

PO-1832 Small fields with Varian Halcyon: Monte Carlo and measurements

L. Laakkonen 1 , J. Lehtomäki 1 , A. Harju 1 , A. Kulmala 2

1 Varian Medical Systems, a Siemens Healthineers Company, TPS Technology, Radiation Physics, Helsinki, Finland; 2 Clinical Research Institute HUCH Ltd., Department of Oncology, Helsinki, Finland Purpose or Objective Monte Carlo (MC) simulations can serve as computational copies of the radiotherapy treatment machines, and as such can explore the machine accuracy limits. Halcyon (Varian Medical Systems, a Siemens Healthineers Company) treatment system, also used in the Ethos online adaptive workflow, is a new-generation machine to deliver fast and high-quality treatments. The purpose of this work is to investigate the Halcyon accuracy delivering small fields by comparing the measurements with accurate MC model results. We use a MC model, built on top of Geant4, that has a detailed representation of the Halcyon components: the treatment head geometry is modeled closely following the engineering drawings and the dual-layer multileaf collimator (MLC), critical to the model accuracy, is imported to MC as computer-aided design (CAD) files (Laakkonen et al, 2022, Physics in Medicine and Biology (under review)).

Materials and Methods In this work, we consider small fields limited by the dual-layer MLC and compare the results with high-precision film measurements. A dosimetry film was placed at 5 cm depth inside a plastic water phantom of 40 x 40 x 10 cm ³ having a