ESTRO 2023 - Abstract Book

S1859

Digital Posters

ESTRO 2023

In our preliminary results, the volumetric and positional deviations in DWI from the reference T2W images on both MRsim and MRL were small, suggesting that the use of geometrical distortion in DWI images might not much compromise the geometric accuracy in prostate MRgRT. Meanwhile, DWI ADC was consistent between MRsim and MRL, so it might serve as a potential quantitative imaging biomarker in MRgRT.

PO-2078 Timing of hypoxia PET/CT after 18F-Fluoromisonidazole injection in non-small cell lung cancer

P. Bourigault 1 , J.(.M. Low 2 , M. Skwarski 1,2,3 , R. Macpherson 4 , G. Higgins 1,2 , D. McGowan 1,5

1 University of Oxford, Department of Oncology, Oxford, United Kingdom; 2 Oxford University Hospitals NHS Foundation Trust, Department of Oncology, Oxford, United Kingdom; 3 Guy’s and St Thomas’ NHS Foundation Trust, Department of Clinical Oncology, London, United Kingdom; 4 Oxford University Hospitals NHS Foundation Trust, Department of Radiology, Oxford, United Kingdom; 5 Oxford University Hospitals NHS Foundation Trust, Department of Medical Physics and Clinical Engineering, Oxford, United Kingdom Purpose or Objective Positron emission tomography (PET)/computed tomography (CT) using the radiotracer 18F-Fluoromisonidazole (FMISO) has been widely employed to image tumour hypoxia and is helpful in guiding radiation treatment (RT) planning. Yet, the optimal timing of hypoxic imaging following 18F-FMISO injection remains unclear in patients with non-small cell lung cancer (NSCLC). Therefore, we investigated the correlation between hypoxia-related quantitative values in FMISO-PET acquired at two- and four-hours post-injection (p.i.) in NSCLC patients. Materials and Methods Patients with resectable NSCLC were recruited into the ATOM clinical trial (NCT02628080) (1). Cohort 1 (n=14) received atovaquone treatment, while cohort 2 (n=15) did not. Two-hour and four-hour FMISO PET/CT images acquired at baseline and pre-surgery visits (n=58) were compared. Two-hour images were registered to their respective four-hour ones. Spearman’s rank correlation coefficients ( ρ ) assessed the relationship between hypoxia-related metrics, including standardised uptake value (SUV), tumour-to-blood ratio (TBR), and tumour hypoxic volume (HV) defined by voxels with TBR ≥ 1.4. In-house MATLAB code was used to separate the tumour voxels into four subregions or distance categories: edge (the outermost shell of voxels), outer (voxel centre up to 5.5 mm of the tumour outline), inner (between 5.5 and 11 mm of the tumour outline), and central (superior to 11 mm inside the tumour outline). Results In tumours overall, strong correlation (P<0.001) was observed in the following parameters: SUVmax ρ = 0.87, SUVmean ρ = 0.91, TBRmax ρ = 0.83 and TBRmean ρ = 0.81. Tumour HV was moderately correlated (P<0.001) with ρ = 0.69. Yet, in tumour subregions, the correlation of HV decreased from the centre ρ = 0.71 to the edge ρ = 0.45 (P<0.001). Additionally, SUV, TBR, and HV values were higher on four-hour scans than on two-hour scans. For instance, TBRmax mean, median, and interquartile range was 1.9, 1.7, and 1.6-2.0 two hours p.i., and 2.6, 2.4, and 2.0-3.0 four hours p.i., respectively. As the primary imaging-related trial endpoint used to evaluate the action of atovaquone on tumour hypoxia in patients with NSCLC, change in tumour HV from baseline was assessed. Changes in HV in atovaquone-treated patients were relatively similar at two- and four-hours p.i.. However, discrepancies were observed in changes in HV of six (40%) untreated patients, with HV increasing from baseline on four-hour scans while decreasing on two-hour scans, or vice versa. Conclusion Given the relatively low tracer-to-background contrast on FMISO-PET images, our results support that scans should be performed at four hours p.i. to evaluate tumour hypoxia in NSCLC, and may better allow hypoxic subvolume definition to potentially guide future RT planning. Reference: 1. Skwarski, M., et al., Mitochondrial Inhibitor Atovaquone Increases Tumor Oxygenation and Inhibits Hypoxic Gene Expression in Patients with Non-Small Cell Lung Cancer. Clin Cancer Res, 2021. 27(9): p. 2459-2469. 1 The Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom; 2 The Royal Marsden Hospital, Clinical Oncology, London, United Kingdom Purpose or Objective Purpose/Objective: Acquiring functional magnetic resonance imaging in the form of diffusion weighted imaging (DWI) on a 1.5T MR integrated linear accelerator (MR-Linac) platform can be beneficial for longitudinal response assessments. In pelvic cancers such as bladder, cervix and rectal cancer this could inform on patients that may require treatment adaptation to improve response. Technical validation of DWI and apparent diffusion coefficient (ADC) measurements acquired on MR- Linac is required before use in future studies. We present results of a repeatability analysis performed in bladder, cervix and rectal cancers on a 1.5T MR-Linac. Materials and Methods Material/Methods: 30 patients with bladder, cervix or rectal cancer treated on MR-Linac had DWI acquired, as per guidelines, at radiotherapy planning scans (Day 0) and prior to treatment at first fraction (#1) to perform test-retest analysis. Gross tumour volume (GTV) was manually contoured on DWI and rigidly propagated onto ADC maps, which were created using an in-house tool. ADC mean was calculated for GTV on both Day 0 and #1 scans. Repeatability coefficient (RC), %RC and Interclass Correlation of Coefficient (ICC) were calculated. PO-2079 Repeatability of diffusion weighted imaging on 1.5T MR-Linac in Bladder, Cervix and Rectal Cancer M. Ingle 1 , M. Blackledge 1 , S. Hafeez 2 , S. Bhide 2 , A. Wetscherek 1 , S. Lalondrelle 2