ESTRO 2020 Abstract book

S318 ESTRO 2020

stratification of AJCC stage III rectal cancer patients into different risk groups after surgery.

Conclusion Dose escalation IMRT neoadjuvant chemoradiation treatment offers advantages in terms of ypCR compared to standard arm in these preliminary results. PD-0535 The Effects of Neoadjuvant Treatment on the Tumor Microenvironment in Rectal Cancer J. Yen 1 , Y. Chung 1 , C. Jen 1 1 Koo Foundation Sun Yat Sen Cancer Center, Radiation oncology, Taipei, Taiwan Purpose or Objective Because more than one neoadjuvant treatment is available for advanced rectal cancer, the aim of this study was to compare the differential clinical and pathologic effects of different combinations of chemoradiation regimens, treatment sequencing, and timing to surgery on patient outcomes. Material and Methods Between January 2015 and October 2018, 126 newly diagnosed rectal cancer patients with MRI-based cT3-4 or N+ rectal disease for curative-intent treatment received one of 4 neoadjuvant regimens consisting of short-course (5 x 5 Gy) or long-course radiotherapy (RT) (25 x 2 Gy) with or without concurrent or sequential chemotherapy (capecitabine, 5-fluorouracil, and/or oxaliplatin) followed by immediate surgery or delayed surgery.Whole post- neoadjuvant surgical specimens were assessed to analyze the global tumor pathologic regression grades, residual tumor distribution patterns, the extent of lymphovascular permeation, lymph node positivity, and the overall density of lymphocyte infiltration in the tumor microenvironment. These factors were further examined to identify possible correlations with clinical outcomes. Results Among the 4 neoadjuvant treatment groups, including 2 conventional regimens, we found a significant increase in the post-neoadjuvant stromal lymphodensities in the 3 groups with delayed surgery after different chemoradiation regimens compared to the group with immediate surgery after a short course of RT alone. Independent of neoadjuvant chemoradiation regimens, the postinduction high-intermediate-low stromal- infiltrating CD8+ T-cell densities corresponded to tumor regression grades, distant metastasis rates and disease- free survival and were prognostic factors for the further

Conclusion The effectiveness of induction strategies on tumor remission and disease recurrence in advanced rectal cancer was significantly correlated with an enhanced cytotoxic immune response in the tumor microenvironment. PD-0536 Bone-marrow sparing IMRT for anal cancer patients: a prospective phase II trial F. Arcadipane 1 , P. Silvetti 2 , C. Fiandra 3 , E. Gallio 4 , S. Martini 2 , F. Giglioli 4 , A. Sardo 4 , G.C. Iorio 2 , F. Olivero 2 , A. Gastino 2 , V. De Luca 2 , U. Ricardi 2 , P. Franco 2 1 AOU Citta' della Salute e della Scienza, Department of Oncology- Radiation Oncology, Torino, Italy ; 2 University of Turin, Department of Oncology- Radiation Oncology-, Turin, Italy ; 3 University of Turin, Department of Medical Physics- Radiation Oncology-, Turin, Italy ; 4 AOU Citta' della Salute e della Scienza, Department of Medical Physics- Radiation Oncology, Torino, Italy

Purpose or Objective