ESTRO 2021 Abstract Book

S1504

ESTRO 2021

CT (CE-CT) and 18 F-FDG PET/CT. Materials and Methods 21 patients with newly diagnosed oesophageal cancer and those who underwent both 18 F-FDG and 68 Ga-FAPI PET/CT scans were selected. GTVs of the primary tumour based on CE-CT (GTV CT ), PET/CT, and CE-CT plus PET/CT were delineated. Gross tumour lengths in GTVs and endoscopes were measured and recorded. Results 68 Ga-FAPIPET showed significantly higher radiotracer uptake than 18 F-FDG PET (median SUVmax 16.71 vs. 11.47; P =0.002) in the primary tumour.SUV thresholds of FAPI ×20%, 30%, 40%, and FDG ×40% showed similar lesion lengths compared with that in endoscopic examination( P >0.05). GTV CT demonstrated the largest volume (median: 48.80 mm 3 , range: 14.83-162.23 mm 3 ) than PET-based GTVs. For PET/CT-guided complementary contouring of GTV CT , four patients (19%) were increased by FAPI ×20% and 30%, two patient s (9.5%) were increased by FAPI ×40%, and only one patient was increased by FDG ×40%. Furthermore, the volume of GTVbased on CE-CT plus FAPI ×20%, 30%, and 40%showed no significant differencewith GTV CT and planning target volume based CE-CT plus FAPI-PET meet the standard of organ at risk.

Examination approach Median (cm) Range (cm) P-value (Endoscope vs. other approach) Endoscope 6 3-10 NA CE-CT 8.3 3.5-12 0.007 FDG ×40% 5.5 2-9 0.119 FAPI ×20% 6.5 3-11 0.113 FAPI ×30% 6.3 3-10 0.793 FAPI ×40% 5.8 1.5-9.5 0.242 FAPI ×50% 4.8 1.5-9 0.009 FAPI ×60% 3.5 1-9 0.001 FAPI ×70% 2.5 0.5-8.5 <0.001

Conclusion 68 Ga-FAPI PET/CT showed favourable tumour-to-background contrast in oesophageal cancer and may provide additional information for target volume delineation to avoid tumour geographic misses.

PO-1778 MR radiomic features as predictive factors of ADT response in high-risk prostate cancer H. Tharmalingam 1 , Y. Tsang 2 , A. McWilliam 3 , R. Alonzi 1 , A. Padhani 4 , A. Choudhury 3 , P. Hoskin 1 1 Mount Vernon Cancer Centre, Clinical Oncology, Northwood, United Kingdom; 2 Mount Vernon Cancer Centre, Radiotherapy, Northwood, United Kingdom; 3 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom; 4 Paul Strickland Scanner Centre, Radiology, Northwood, United Kingdom Purpose or Objective The benefits of neoadjuvant androgen deprivation therapy (nADT) in the management of high-risk prostate cancer patients have been well-established. This study aims to identify radiomic prognostic features derived from routine anatomic magnetic resonance imaging (MRI) sequences that can predict the response of the 19 patients with high-risk prostate cancer (with one of clinical stage ≥ T2c, Gleason Score ≥ 7 or pPSA ≥ 10) who received three months of androgen deprivation therapy prior to radical external beam radiotherapy (EBRT) were enrolled into this study. The relative blood volume (rBV) and the relative blood flow (rBF) were used as dynamic MRI kinetic parameters to quantify vascular changes as responses to nADT. For all pre- and post-nADT data sets, a prostate cancer to nADT. Materials and Methods