S773
Clinical - Lung
ESTRO 2026
performed pre-treatment and at 6 and 18 months post-SABR. CT images were reported for suspicion of local recurrence by 3 radiation oncologists using established assessment criteria and compared with parallel serial MRI analysed by an expert thoracic radiologist along with mean apparent diffusion coefficient (ADC) calculation. PET-CT allowed for standardized uptake value measurements and assessment of radiological local or distant recurrence. Results: Seventeen patients with 23 treated volumes underwent CT, MRI, and PET-CT scans as part of this study. Ten patients all assessments and imaging in the 24-months post-treatment study period. No clinically significant adverse events nor any locoregional failures/recurrences occurred within the study population in the follow-up period. Eight patients with suspicious CT findings were confirmed by MRI and PET-CT to have radiologically benign post-treatment changes while PET-CT confirmed radiological distant/nodal recurrence in three patients. CT vs MRI tumour measurements were similar at baseline (25.6mm vs 25.4mm) but CT volumes were larger and more variable at 6 months post-treatment (56.0mm vs 45.9mm). This trend persisted throughout the follow- up period. Qualitative assessment of CT and MRI was highly discordant in terms of shape, internal consistency, and volume estimation. ADC values, where attainable, significantly increased post- treatment and demonstrated a stable trend across all post-treatment timepoints with levels in keeping with benign pathology. Conclusion: CT imaging demonstrates significant and persistent variability compared to MRI, primarily in the first 6- months post-lung SABR. CT-based assessment of post- treatment changes is highly variable and prone to over-estimation of possible recurrence. Patients may benefit from inclusion of thoracic MRI early in the post-treatment setting in addition to routine PET-CT to more accurately characterize post-SABR response. ADC measurements were consistent with those of similar studies in patients without local recurrence and offers potential as a radiomic biomarker of recurrence. Keywords: Stereotactic radiotherapy, Follow-up, MRI Proffered Paper 1691 Acute Toxicities of Moderately Hypofractionated Chemoradiation with Consolidation Immunotherapy in Unresectable Stage III NSCLC: Phase II Study Joanna Ludbrook 1,2 , Mahesh Kumar 1,2 , Girish Mallesara 3 , Eric Hau 4,5 , Senthilkumar Gandhidasan 6 , Shalini K Vinod 7,8
Markers and Efficacy of a Patient-Specific Margin Design Using Deep Inspiration Breath Hold for Stereotactic Body Radiation Therapy for Pancreatic Cancer,” Adv. In Radiat Oncol (2021); 6:100655B Farrugia B et al, “A systematic review of tumour position reproducibility and stability in breath-hold for radiation therapy of the upper abdomen,” Phys. Imaging Radiat. Oncol. (2025); 34:100751.D Nguyen et al, “Reproducibility of surface-based deep inspiration breath-hold technique for lung stereotactic body radiotherapy on a closed-bore gantry linac,” Phys. Imaging Radiat. Oncol. 2023; 26:100448 Keywords: Deep Inspiration Breath Hold, motion, Uncertainty A Prospective Study of Surveillance Imaging with Multiparametric MRI and PET-CT in Lung Patients Post-Stereotactic Ablative Radiotherapy (SIMPLE- AS) Spencer Martin 1 , Andrew Oar 2 , Daniel Moses 3,4 , Shivani Kumar 5,6 , Joseph Descallar 6 , Michael Lin 7 , Wenchang Wong 8 , Mei-Ling Yap 1,6 , Shalini K Vinod 1,6 1 Liverpool & Macarthur Cancer Therapy Centres, Liverpool & Campbelltown Hospitals, Sydney, Australia. 2 Icon Cancer Centre, Gold Coast University Hospital, Gold Coast, Australia. 3 Department of Radiology, Prince of Wales Hospital, Randwick, Australia. 4 Randwick Clinical Campus, Faculty of Medicine, University of New South Wales, Randwick, Australia. 5 Cancer Therapy Centre, Liverpool Hospital, Liverpool, Australia. 6 South West Sydney Clinical Campuses, Faculty of Medicine, University of New South Wales, Sydney, Australia. 7 Department of Nuclear Medicine, Liverpool Hospital, Liverpool, Australia. 8 Department of Radiation Oncology, Prince of Wales Hospital, Randwick, Australia Purpose/Objective: Post-treatment changes are difficult to assess on Computed Tomography (CT) imaging following stereotactic ablative radiotherapy (SABR) for lung tumours. This work assesses and compares post- Digital Poster Highlight 1688 treatment tumour response in patients after undergoing lung SABR across serial CT, Magnetic Resonance Imaging (MRI) and Positron Emission
Tomography (PET) CT. Material/Methods:
Patients undergoing thoracic SABR for primary lung cancer or oligometasases were recruited for thoracic multiparametric MRI (T2 weighted, diffusion weighted, dynamic contrast enhancement) at baseline, 3, 6, 12, 18 and 24 months post-SABR concurrent with standard-of-care CT scans and clinical assessment. Fluorodeoxyglucose (FDG)-PET CT scans were
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