ESTRO 2022 - Abstract Book

S62

Abstract book

ESTRO 2022

Conclusions: In our experience, PSMA-PET guided SBRT resulted in excellent results in terms of clinical outcomes, representing a helpful tool with the aim to delay the start of ADT.

PD-0087 Developing rapid response MRI-guided palliative radiotherapy for metastatic spinal cord compression

R. Benson 1 , A. Sideris 2 , L. McDaid 1 , R. Chuter 3 , R. Portner 4 , L. Freear 3 , A. Clough 1 , C. Nelder 1 , E. Pitt 1 , M. Daly 1 , M. Vassiliou 1 , A. Rembielak 4 , P. Hoskin 4 , A. Choudhury 4 , C. Eccles 1 1 The Christie NHS Foundation Trust, Radiotherapy, Manchester, United Kingdom; 2 The University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom; 3 The Christie NHS Foundation Trust, Medical Physics and Engineering, Manchester, United Kingdom; 4 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom Purpose or Objective Implementation of a rapid response metastatic spinal cord compression (MSCC) pathway using diagnostic (dCT) imaging and adaptive MR Linac treatment has the potential to reduce waiting times, align healthcare processes, and improve the patient experience during a global pandemic. This work presents the preliminary feasibility testing of a rapid response, single appointment MSCC pathway on the Elekta Unity MR Linac (MRL) using dCT imaging, as the diagnostic MR field of view is too small. Materials and Methods Retrospective radiotherapy plan data were collected from ten patients who had received urgent/emergency palliative spinal radiotherapy on conventional linear accelerators at our institution. The most recent dCT images prior to the treatment planning scans were imported from the picture archiving communication system (PACS) to the MRL treatment planning system (Monaco V5.40, Elekta). New treatment plans were then created on these dCT images to prepare for delivery on the MRL (figure 1). In order to facilitate this target contours were required and generated by the treatment planner with support from a clinical oncologist. Departmental policy for single dose radiotherapy was used in the creation of these plans (8Gy treated in a single post field). To test implementation on the MRL, MR scans were acquired and treatment delivered to a 3D abdominal phantom (CIRS) using MR-CT registration and the adapt to shape (ATS) workflow. ATS ensures that if anatomical changes have occurred since dCT the contours can be adapted on the day to reflect these changes, allowing for online plan adaptation Results Ten plans were created on imported dCT. Treatment site ranged from upper thoracic spine to sacrum. Target volumes ranged from 156- 508 cm 3 , D95% ranged from 767-874cGy (figure 2). Acceptable coverage was achieved on all plans but proved more challenging on those with larger treatment volumes. Approved plans were exported to Mosaiq (V2.83, Elekta) to test delivery on a phantom on the MRL. Conclusion For MSCC we were able to import and transfer data, produce acceptable treatment plans on dCT images and preliminary testing on the MRL. Attempted delivery of these plans highlighted technical issues that need to be overcome prior to clinical implementation. These included the lack of origin and landmarking information on dCT can make patient positioning challenging due to limitations of online shifts. Additionally, bulk density overrides are required for Hounsfield unit to electron density conversion as dCT images may come from different diagnostic CT scanners for which commissioning data is not available. Further work is on-going to confirm dosimetric accuracy and overcome positioning limitations related to plan delivery prior to clinical implementation.

Proffered Papers: Proton beam therapy

OC-0088 Reduced tube feeding dependence by continuous improvements in head and neck radiotherapy

L. Van den Bosch 1 , H.P. van der Laan 1 , A. van der Schaaf 1 , J.G. van den Hoek 1 , E. Oldehinkel 1 , T.W. Meijer 1 , H.H. Verbeek 1 , R.J. Steenbakkers 1 , J.A. Langendijk 1

1 University Medical Centre Groningen, Radiation Oncology, Groningen, The Netherlands

Purpose or Objective The aim of this study was 1) to assess the prevalence of tube feeding dependence (TFD) during the course of radiotherapy in head and neck cancer (HNC) patients treated over a time span of 13 years of technological improvements and improved organ sparing capabilities and 2) to build and validate NTCP models to estimate the benefit of proton therapy (IMPT) over VMAT with regard to reducing TFD. Materials and Methods The study population was composed of 1181 HNC patients who underwent definitive (chemo)radiation. Patients were treated with parotid sparing (ParRT: 2007-2010), swallowing sparing (SwRT: 2011-2017, including additional sparing of pharyngeal constrictor muscle (PCM), supraglottic larynx and oesophageal inlet muscle) or multi-organ sparing (MoRT: 2018- 2020, including additional sparing of oral cavity and submandibular glands) radiotherapy (Figure 1). All patients were subjected to a prospective data registration protocol in which TFD (CTCAEv4.0 grade 4 dysphagia) was scored at baseline (BSL), weekly during treatment and at week 12. Dose-volume data for a wide range of OAR were collected for the clinical plan and (in the case of IMPT) the backup VMAT plans. First, the prevalence of TFD was studied for the aforementioned three subsequent time periods. Subsequently, multivariable NTCP modelling was performed in development (2007-2016)