ESTRO 2021 Abstract Book

S1036

ESTRO 2021

Conclusion Local excision followed by radiotherapy or chemoradiotherapy should be reserved for selected patients who are not candidates for radical surgery as it may be associated with worse outcomes specially in T2 stage.

PO-1257 A radiomic signature model to predict chemoradiation-induced change in CD8+ cells in rectal cancer Y.J. Lim 1 , S.H. Jeon 2 , E.K. Chie 2 1 Kyung Hee University School of Medicine, Radiation Oncology, Seoul, Korea Republic of; 2 Seoul National University College of Medicine, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective Identification of altered tumor immune status following cytotoxic treatment is important to understand the differential role of cancer immunotherapy. This study aims to develop a radiomic signature to predict CD8 + tumor-infiltrating lymphocyte (TIL) density before and after chemoradiotherapy (CRT) in rectal cancer. Materials and Methods We used the magnetic resonance imaging (MRI) and immunohistochemistry data both before and after neoadjuvant CRT of 131 patients. The study population included pre-CRT dataset A (n = 113; n = 75 and 38 for training and validation) and post-CRT datasets B (n = 32; predominance of tumor) and C (n = 20; pure fibrosis). Thirty-eight radiomic features from T2-weighted MRI scans were incorporated into the least absolute shrinkage and selection operator method. Results In pre-CRT dataset A, the area under the receiver operating characteristic curve (AUC) values of radiomic score for predicting CD8 + TILs were 0.760 and 0.729 for training and validation subsets, respectively. A significant correlation was observed between the signature and CD8 + TIL density in the post-CRT dataset B (Pearson’s R = ‒ 0.372, P = 0.036), whereas no association was found within the post-CRT dataset C (Pearson’s R = ‒ 0.069, P = 0.77). In the dataset B, the radiomic score difference predicted the change in CD8 + TIL density (AUC = 0.824). Conclusion We established the MRI-derived radiomic signature for predicting CRT-induced alterations in CD8 + TILs of rectal cancer. This study provides insights into the clinical utility of radiomics-immunophenotype modeling to track tumor immune status in the contemporary era. PO-1258 Pragmatic and effective use of short course radiotherapy for stage IV rectal cancer M. Higgins 1 1 St Luke's Radiation Oncology Network, Radiation Oncology, Dublin 6, Ireland Purpose or Objective We assessed management of patients with de novo metastatic rectal cancer, referred for radiotherapy to the rectum. For patients who were candidates for short course radiotherapy (SCRT) and chemotherapy, followed by surgery we assessed surgical outcomes, overall survival (OS) and progression free survival (PFS). Materials and Methods Retrospective review of patients meeting criteria: (1) treatment with SCRT to rectum; (2) locally advanced primary rectal cancer; (3) distant metastases at diagnosis. Data were collected from charts, correspondence and electronic patient records. OS and PFS were calculated using the Kaplan Meier method. Results Between 2016 and 2020, 48 patients with stage IV rectal cancer at diagnosis were treated with SCRT. 15 patients (31%) had resectable metastatic disease, and were intended for SCRT (25Gy/5#), then chemotherapy, followed by resection of all sites of disease. 12 of the 15 surgical candidates (80%) had rectal surgery as planned and 11 of the 15 (73%) had resection of the rectal primary and all metastatic disease. One patient (8%) had a pathological complete response (pCR) and 50% of surgical patients had a Mandard TRG of 1 or 2. Median PFS and OS for the 15 surgical candidates were 12.6 and 25.2 months, respectively, with a median FU of 21.2 months.