ESTRO 37 Abstract book

ESTRO 37

S546

Purpose or Objective In rectal cancer, the response to chemoradiotherapy (CRT) varies considerably depending on the aggressiveness of the tumour. The aggressiveness is, amongst other factors, linked to the presence of tumour hypoxia, and the functional MRI parameter R 2 * is suggested to be a non-invasive imaging biomarker of hypoxia. The purpose was to investigate whether static or dynamic R 2 * MRI measurements can predict CRT outcome in rectal cancer. Material and Methods Twenty-six patients with rectal cancer underwent baseline static T 2 *-weighted MRI with multiple echo times (TE) and dynamic susceptibility contrast (DSC)-MRI using a gadolinium-based contrast agent (Dotarem®) before CRT and surgery. To reduce bowel peristalsis, the patients received glucagon (intramuscularly) and Buscopan (intravenously). The static R 2 * (=1/T 2 *) was calculated from the multi-TE acquisition, whereas the peak R 2 * and the area under the R 2 * curve (AUC_R 2 *) were derived from the DSC data. Histopathologic CRT response was evaluated in the resected specimens by ypTN grading and tumour regression grade (TRG) scoring by the AJCC/CAP system, divided into good responders (TRG 0-1; n = 11) and poor responders (TRG 2-3; n = 15). The Mann-Whitney U test was applied to investigate associations between MRI parameters and CRT response, whereas Cox regression evaluated differences in progression-free survival (PFS). The median follow-up in the cohort was 31 * significantly differentiated good and poor responders (p = 0.002) and between ypT0-2 versus ypT3-4 cases (p = 0.028). Poor responders had higher AUC_R 2 * values compared to good responders. The high AUC_R 2 * was also related to poor PFS, although not significant (p = 0.080). Conclusion AUC_R 2 * obtained from DSC-MRI was significantly associated with CRT response in rectal cancer patients, where high AUC_R 2 * was related to poor response. To our knowledge, this is the first evidence of R 2 * MRI associations with CRT response in rectal cancer. The results require validation in larger, independent cohorts. However, a baseline measurement of tumour aggressiveness by AUC_R 2 * can be relatively easily obtained in a clinical setting, and has potential to facilitate non-invasive treatment individualisation. months. Results For static R 2 * and dynamic R 2 * peak, there were no significant differences. However, AUC_R 2 L.B. Nilsen 1 , D. Ingrid 1 , E. Grøvik 1 , C. Saxhaug 2 , O. Geier 1 , E. Reitan 2 , A. Helland 3 , K.D. Jacbosen 3 , B. Breivik 4 , D.O. Sætre 5 , T.H. Paulsen 6 , N.C. Lange 3 , K.E. Emblem 1 1 Oslo University Hospital, Diagnostic Physics, Oslo, Norway 2 Oslo University Hospital, Radiology and Nucelar Medicine, Oslo, Norway 3 Oslo University Hospital, Oncology, Oslo, Norway 4 Hospital of Southern Norway, Radiology, Kristiansdand, Norway 5 Østfold Hospital Trust, Radiology, Kalnes, Norway 6 Oslo University Hospital, Medical Physcis, Oslo, Norway Purpose or Objective Stereotactic radiosurgery (SRS) is commonly given to patients with metastatic intracranial brain tumors. The understanding of radiobiological responses to SRS in PO-0984 Vascular responses to stereotactic radiosurgery of metastases to the brain

tumor and normal brain tissue is largely unknown. Here we aim to gain insight into vascular responses following SRS using Vascular Architectural Imaging (VAI). Material and Methods Eighteen patients with 27 brain metastases from primary non-small-cell lung cancer (N = 10) and malignant melanoma (N = 8) received SRS (15/18/25 Gy single fraction or 21/24 Gy in three fractions) to the tumor bed (visual metastasis + 2 mm margin). The 3D dose distribution was divided into five different dose bins: 0-2 Gy, >2-5 Gy, >5-10 Gy, >10-15 Gy, and >15 Gy (Figure 1). Six patients had received WBRT (30 Gy) (N=4), SRS (N=2), or both (N=2) within one year prior to MR baseline and SRS of current investigated lesions. Maps of Cerebral Blood Volume (CBV), Cerebral Blood Flow (CBF), vessel caliber and relative oxygenation status (∆SO2) were obtained by conventional perfusion MRI and VAI. Regions of enhancing lesion and edema were identified on T1w post contrast and FLAIR images, respectively. Binned dose distributions, lesion and edema ROIs were all co- registered to the VAI-maps.

Results Before SRS, tumor and edema were characterized by low blood volume ( p<0.05 ) and blood flow ( p<0.05 ), and high degree of oxygen-deprivation ( p<0.01 ), as well as higher abnormal tumor vessel calibre compared to normal brain tissue in the lowest dose bin region (Figure 2). No differences in vascular parameters were found in normal brain tissue (planned dose regions <15 Gy) between patients previously treated with WBRT and/or SRS. After SRS, vascular changes were most pronounced in regions of edema. However, large variations in all vascular parameters were observed, particularly in tumor regions. In normal brain tissue, transient changes in blood volume and flow were observed at three and six months (Figure 2A-B), whereas in regions receiving less than 10 Gy the oxygenation status remained stable on all follow- up MR time points (Figure 2C).