ESTRO 2021 Abstract Book

S1053

ESTRO 2021

Conclusion Isotoxic re-irradiation SABR is feasible in LRRC with significant dose escalation achievable in a meaningful proportion of cases. Trials are needed to determine the safety of OAR constraints using 15% annual tissue recovery and investigate impact of isotoxic SABR re-irradiation on outcomes. Whilst dose was decreased relative to current SoC in a proportion of patients, OAR dose maybe further reduced when planning prospectively with revised OAR limits. PO-1276 Differential Response of FDG Uptake in Pelvic Bone Marrow to Concurrent Chemoradiotherapy M. Robinson 1 , R. Muirhead 2 , K. Chu 1 , C. Jacobs 2 , S. Ng 3 , M. Hawkins 4 1 University of Oxford, MRC Oxford Institute for Radiation Oncology, Oxford, United Kingdom; 2 Oxford University Hospitals NHS Trust, Oncology, Oxford, United Kingdom; 3 University of Oxford, Oncology Clinical Trials Office, Oxford, United Kingdom; 4 University College London, Medical Physics and Biomedical Engineering, London, United Kingdom Purpose or Objective 18 F-Fluorothymidine (FLT) PETCT best identifies active bone marrow (BM) regions but is not a routinely performed scan. 18 F-Fludeoxyglucose (FDG) PETCT is routinely used and widely available. Active pelvic BM volumes identified on FDG PETCT have been shown to be more variable than FLT PETCT defined volumes but to also correlate strongly. The link between FDG uptake and hematological activity is unclear. We aim to identify whether active BM as defined by FDG uptake is representative of underlying cell physiology by determining if a differential response in FDG uptake to concurrent chemo radiation can be seen in regions of high uptake versus low uptake, defined on baseline PET, in pelvic BM in patients receiving concurrent chemo radiation (CRT) for anal cancer. Materials and Methods Paired scans from 12 patients enrolled in the ART trial (Cancer.gov NCT02145416), a single center observational study with FDG PETCT prior to (baseline scan) and at fraction 8-10 (mid-treatment scan) of CRT were analysed. Mid-treatment scans were registered to baseline using deformable registration of CT scan. Voxel activity seen in PET scans (g/ml) was then extracted and voxel by voxel response was calculated. Baseline pelvic BM was then sub divided into 2 populations: active and inactive bone marrow, defined as FDG uptake greater or less than the mean of the whole structure respectively. Both active and inactive pelvic BM voxels were then cropped to having corresponding CT HU <250 to exclude definitive bone. Mann Whitney U testing was performed on active versus inactive PBM response for each patient. Results All patients showed a statistically significant difference in response of active versus inactive bone marrow, p<0.05. Summary results are shown in table below.

Median absolute response of active bone marrow for all patients was -0.25 g/ml and inactive bone marrow mean response was -0.02 g/ml. The figure below shows absolute response histograms of active and inactive bone marrow for all patients. Skewness of active PBM was -0.37 and inactive was 0.12 when excluding outliers, defined as a factor of 6 greater than the interquartile range. Outliers represented significantly less than 0.1%