Abstract Book

S336

ESTRO 37

followed by dynamic T 1 -w acquisitions with 100% oxygen challenge ( figure 1A ). Non-linear and rigid registrations corrected for breathing and patient motion in dynamic OE- and DCE-MRI acquisitions and allowed combined OE/ DCE voxel-wise analysis. MRI longitudinal relaxation rate ( R 1 ) change induced by oxygen inhalation (denoted Δ R 1 ) was calculated along with perfused Oxy-R, a previously validated combined OE/ DCE hypoxia biomarker ( figure B-E ).

Conclusion This study demonstrated the first online real-time internal respiratory motion monitoring on a standard- equipped conventional linac. Although less accurate than KIM, COSMIK has several advantages over KIM: substantially reduced imaging dose, no loss in kV image quality due to MV scatter, compatibility with couch rotations, robustness against occasionally failed and discarded segmentations and lower latency. COSMIK is expected to have similar accuracy as the clinically used Vero and CyberKnife systems. OC-0632 Oxygen enhanced-MRI is feasible, repeatable and detects radiotherapy-induced NSCLC hypoxia changes A. Salem 1 , R.A. Little 2 , A.K. Featherstone 2 , S . Cheung 2 , Y. Watson 2 , J. Matthews 2 , M.C. Asselin 2 , A. Jackson 2 , G.J.M. Parker 2 , C. Faivre-Finn 1 , J.P.B. O'Connor 1 1 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom 2 University of Manchester, Division of Informatics- Imaging and Data Sciences, Manchester, United Kingdom Purpose or Objective Oxygen enhanced (OE)-MRI, when combined with dynamic contrast-enhanced (DCE)-MRI, has shown promise as a non-invasive hypoxia identifying and mapping method. We performed a first-in-man clinical evaluation of the repeatability of OE-MRI biomarkers and their sensitivity to detect radiotherapy-induced changes in non-small cell lung cancer (NSCLC) hypoxia. Material and Methods Patients with stage I-IV NSCLC were recruited and underwent multi-parametric MRI (OE and DCE) for protocol development ( n =6), twice prior to radiotherapy to assess repeatability ( n =10) and after 14±4 radiotherapy fractions to evaluate sensitivity to treatment effect ( n =14). OE acquisition consisted of baseline T 1 mapping Award Lecture: Company Award Lectures

Results OE-MRI was feasible and well-tolerated in 22 out of 23 patients. Oxygen challenge resulted in significant increase in measured R 1 in the aorta in all patients ( p <0.0001). Signal changes reverted to baseline once the oxygen challenge was removed ( figure 2A ). This demonstrates that OE-MRI produces a readily measurable input function. There was intra-tumor spatial heterogeneity in measured oxygen-enhanced Δ R 1 in all tumors. These changes were spatially coherent. The perfused Oxy-R (hypoxic) volume demonstrated excellent repeatability with interclass correlation coefficient (ICC) = 0.961 (95% CI 0.858-0.990); figure 2B . Using perfused Oxy-R, there was no discordance in the hypoxic status classification of tumors between repeat scans – all tumors retained their classification as being hypoxic or normoxic (perfused Oxy-R = 0). Visual inspection revealed that MRI hypoxia maps were spatially repeatable in tumor, nodal and distant metastatic lesions across a range of tumor and hypoxic volumes ( figure 2C ). In the absence of volumetric tumor change, the perfused Oxy-R (hypoxic) volume decreased at mid-treatment (3.23 cm 3 (95% CI 0- 9.41 cm 3 )), compared to baseline (4.16 cm 3 (95% CI 0- 10.6 cm 3 )); p =0.0150 ( figure 2D ). This was accompanied by an increase in the perfused Oxy-E (normoxic) volume at mid-treatment ( p =0.0426). None of the exclusively normoxic lesions at baseline demonstrated emergence of hypoxia at mid-treatment.