ESTRO 2023 - Abstract Book

S687

Monday 15 May 2023

ESTRO 2023

primary tumors and 127 (38%) were treated to metastatic sites. The median prescribed RT dose was 50 Gy (range, 20-76 Gy) in 5 fractions (range, 1-30) over 7 days (range, 1-50 days), the median prescribed BED10 dose was 100 Gy10 (range, 34- 158 Gy10), and 184 (55%) patients were treated with 100 Gy10 or higher. 1190 of 1893 prescribed fractions (63%) required plan adaptation. The median follow-up was 11 months (range, 1-52 months). Acute grade 2 or 3 toxicity occurred in 52 (16%) and 6 (2%) patients, respectively. Late grade 2 or 3 toxicity occurred in 24 (7%) and 6 (2%) patients, respectively. Grade 2+ toxicity rates were not different for daily compared to every other day treatments (24% vs. 17%, p=0.3). For 159 patients with more than 1 year follow-up (median 22 months), the 1-year rate of Grade 2+ toxicity was 15% (95% CI: 10.6- 19.4%). There were no acute or late grade 4 toxicities, and 1 acute grade 5 toxicity was observed in a pancreatic cancer patient post Whipple procedure, possibly related to RT. Conclusion Dose-escalated RT delivered on a 0.35T MR-linac has been very well tolerated in this large series of consecutively treated patients, with acute or late grade 3+ toxicity being rare even with daily treatment delivery. This is notable given our frequent use of ultra-hypofractionation, frequent use of the adaptive workflow, and ablative dosing in most patients to sites close to gastrointestinal OARs. PD-0815 Inflammatory markers to predict neoadjuvant chemoradiotherapy response in rectal cancer patients H. Alkı ş 1 , G. Özden 1 , Z. Ba ş kan 1 , M. Ba ğ cı Kılıç 1 , H.K. Gündüz 1 , A. Kornienko 1 , B.Z. Devran 1 , M. Adlı 1 1 Marmara University, Radiation Oncology, İ stanbul, Turkey Purpose or Objective Pretreatment inflammatory markers obtained from the complete blood count (CBC) can be predictive for treatment response in rectal cancer patients treated with neoadjuvant chemoradiotherapy (NACRT). The aim of this study was to determine the correlation between inflammatory markers and treatment response in rectal cancer patients treated with NACRT. Materials and Methods A total of 192 rectal cancer patients treated with NACRT were included in the study. Male/female ratio was 1.59. Clinical T stage was T2 in 13 patients, T3 in 162, and T4 in 17. Clinical N stage was N0 in 25 patients, N1 in 160, and N2 in 7. Radiation dose was 50-56 Gy to the primary tumor and 45-50.4 Gy to the regional lymph nodes. All patients received concurrent capecitabine (n=191) or 5-fluorouracil (n=1). Patients with no evidence of residual disease on DRE, MRI, and endoscopic evaluation following NACRT were determined as clinical complete responders. Patients with clinical (n=34) or pathological (n=27) complete response were classified as complete responders (CR) and the other response groups as non- complete responders (nCR) (n=131). Pretreatment absolute values of neutrophils (N), lymphocytes (L), monocytes (M), and platelets (P), plateletcrit (PCT), mean platelet volume (MPV), neutrophil-to-lymphocyte ratio (NLR), lymphocyte-to- monocyte ratio (LMR), and platelet-to-lymphocyte ratio (PLR) were recorded as hematological inflammatory markers. Mann– Whitney U-test was used to compare the variables between the groups. Results Median age was 60 (18-86) years. Mean N, NLR, PLR, L, and LMR are given in the Table. Pretreatment N (p=0.042), NLR (p=0.001), and PLR (p=0.002) were significantly higher, while L (p=0.015) and LMR (p=0.004) were lower in nCR group compared to CR group. Pretreatment M, P, PCT, and MPV did not have any effect on the treatment response. Table. Mean (± SD) N, L, NLR, PLR, and LMR values according to treatment response. Markers CR nCR P value Neutrophil (10 ³ / µ L) 4.65 ± 1.41 5.21 ± 1.70 0.042 Lymphocyte (10 ³ / µ L) 2.86 ± 4.85 1.99 ± 0.71 0.015 NLR 2.27 ± 1.05 3.03 ± 1.62 0.001 PLR 130.36 ± 58.51 164.20 ± 77.92 0.002 LMR 4.71 ± 4.85 3.55 ± 1.62 0.004

Conclusion Rectal cancer patients with lower pretreatment N, NLR, PLR, and higher L and LMR are more likely to have complete response following NACRT. These markers may be used to predict treatment response in rectal cancer patients treated with NACRT.

Poster Discussion: Radiobiology

PD-0816 Spatial distribution of CD8-positive T cells predicts radiation response in oropharyngeal cancer J. Kaufmann 1 , M. Haist 2,3,4 , I. Kur 5 , S. Zimmer 6 , S. Grabbe 2 , H. Schmidberger 1 , A. Weigert 5 , A. Mayer 1 1 University Medical Center of the Johannes-Gutenberg-University, Department of Radiation Oncology and Radiotherapy, Mainz, Germany; 2 University Medical Center of the Johannes-Gutenberg-University, Department of Dermatology, Mainz, Germany; 3 Stanford University School of Medicine, Department of Pathology, Stanford, USA; 4 Stanford University School of Medicine, Department of Microbiology & Immunology, Stanford, USA; 5 Faculty of Medicine Goethe-University Frankfurt, Institute of Biochemistry I, Frankfurt, Germany; 6 University Medical Center of the Johannes-Gutenberg-University, Institute of Pathology, Mainz, Germany Purpose or Objective Effective anti-tumor immune responses are mediated by CD8-positive cytotoxic T cells (CTL) and require organized, spatially coordinated interactions within the tumor microenvironment (TME). Understanding coordinated T-cell-behavior