ESTRO 2020 Abstract book

S979 ESTRO 2020

ADC mean

increased in all SGs by approximately 15% although

(GTV-P) and lymph nodes (GTV-LN) were delineated on the non-contrast-enhanced T1 and T2 weighted images in 3D- Slicer (v4.10.2) and resampled to the ADC map. Mean ADC values were analyzed for GTV-P and GTV-LN at the different time points. Bland-Altman analysis and a Wilcoxon signed rank test (Matlab) were performed for the ensemble of VOIs at both time points to evaluate ADC agreement between MRL and diagnostic reference MRI. Results Mean ADC values for GTV-P and GTV-LN assessed at the different time points are shown in figure 1. Mean ADC for GTV-P at time of simulation was 0.96, 0.86 and 1.18 x10 -3 mm²/s at the MRL and 1.04, 0.99 and 1.37 x10 -3 mm²/s at the 3 T scanner for patient 1, 2 and 3, respectively. Similarly, mean ADC for GTV-P after two weeks was 1.06, 1.26 and 1.40 x10 -3 mm²/s at the MRL and 1.15, 1.38 and 1.54 x10 -3 mm²/s at the 3 T scanner. Wilcoxon signed rank test did not show any significant discordance between ADC measurements at the MRL and at the 3 T diagnostic scanner (p=0.21). Absolute differences are shown in the Bland- Altman plot (fig. 2). All measurements were within limits of agreement (±1.96 SD of the mean difference). Mean ADC in GTV-P and GTV-LN increased for all patients during RT except for one GTV-LN in patient #2, where the lymph node was initially necrotic and showed already a large ADC value.

SGs with lower baseline ADC mean

showed the largest % rise

in ADC mean .

Even allowing for incomplete delineation, largest volumes were recorded for the PGs, smallest for SLs on both T1PCFS and ADC map at both time points. Highest median ADC mean at baseline was found in SLs and lowest in PGs. ADC mean in SLs showed greatest variation between patients. The overall impression was that smaller ADC values were associated with larger SG volumes and greater heterogeneity was seen in ADC in SGs with small volumes, see figure 1. The reason for this pattern of ADC values is unclear – if this is a feature of gland size or behaviour specific to that particular gland. Absolute volume and % change in volume varied between anatomical and functional sequences though both demonstrated that PG and SMs decreased in volume from baseline to wk 3 of RT whereas SL glands increased in volume. Conclusion Outlining all major paired SGs and analysing changes in volume and ADC on serial MRIs during RT is feasible although scan field of view may need optimised to ensure inclusion of the entire PG in every patient. Median, rather than mean ADC values within SGs may be more representative of underlying physiological change given that mean ADC is affected by the quality of the ADC map, this analysis is currently limited by available software. This is the first study to report on volumetric change in SMs during RT and the first to analyse changes in volume and ADC in SL glands. PO-1685 Serial DWI in HNC treated on a 1.5 T MR- Linac and benchmark to a reference 3 T diagnostic MR- scanner S. Böke 1,2 , R.M. Winter 3 , M. Nachbar 3 , K. Clasen 1 , C. Gani 1,2 , A. Stolte 1 , J. Boldt 1 , C. Marks 1 , S. Gatidis 4 , K. Nikolaou 2,4 , D. Zips 1,2 , D. Thorwarth 2,3 1 University Hospital and Medical Faculty. Eberhard Karls University Tübingen, Department of Radiation Oncology, Tübingen, Germany ; 2 German Cancer Consortium DKTK- partner site Tübingen, and German Cancer Research Center DKFZ, Heidelberg, Germany ; 3 University Hospital and Medical Faculty. Eberhard Karls University Tübingen, Section for Biomedical Physics. Department of Radiation Oncology, Tübingen, Germany ; 4 University Hospital and Medical Faculty. Eberhard Karls University Tübingen, Department of Radiology- Diagnostic and Interventional Radiology, Tübingen, Germany Purpose or Objective MR-Linacs (MRL) are a new development in radiation oncology, offer the higher soft tissue contrast and possess the possibility for daily treatment adaptation. Furthermore, longitudinal acquisition of functional imaging using diffusion weighted imaging (DWI) including the assessment of apparent diffusion coefficient (ADC) values is possible. DWI has shown to be a prognostic biomarker in head and neck cancer (HNC) treated with radiotherapy (RT). Consequently, this might offer the potential for biological adaptation of RT in the future. As a preparatory step for future real-time interventions at the MRL we report on the first three patients with serial weekly DWI at the 1.5 T MRL (Unity, Elekta AB, Stockholm, Sweden) during RT in comparison to two DWI scans at a benchmark diagnostic 3 T MR (Vida, Siemens Healthineers, Three patients with locally advanced HNC were curatively treated at the 1.5 T MRL and DWI (b 0, 200, 500, 800 s/mm 2 ) was performed once per week during treatment. ADC maps were calculated omitting the b0-images. For comparison, DWI (b 50, 800) at the 3 T diagnostic scanner was acquired in treatment position before the start and after two weeks of treatment. GTV of the primary tumor Erlangen, Germany). Material and Methods