ESTRO 2024 - Abstract Book

S5341

Radiobiology - Tumour biology

ESTRO 2024

751

Proffered Paper

The Role of the Tumour Niche on Radiation Response in a Murine Model of Rectal Cancer

Lydia Melissourgou-Syka 1 , Michael A. Gillespie 1,2,3 , Katrina Stevenson 2 , Tamsin R.M. Lannagan 1 , Sean M. O'Cathail 2,4 , Colin W. Steele 1,2 , Owen J. Sansom 1,2 , Campbell S.D. Roxburgh 2 1 CRUK Scotland Centre, Cancer Sciences, Glasgow, United Kingdom. 2 University of Glasgow, School of Cancer Sciences, Glasgow, United Kingdom. 3 University of Glasgow, Academic Unit of Surgery, Glasgow, United Kingdom. 4 Beatson West of Scotland Cancer Centre, Clinical Oncology, Glasgow, United Kingdom

Purpose/Objective:

Non-metastatic rectal cancer is conventionally treated with neoadjuvant radiotherapy (RT) followed by surgery 1 . RT response in rectal cancer is heterogeneous with a current lack of robust, predictive biomarkers 2 . The biology underpinning response and resistance is not well defined but previous studies have reported improved response with increased tumour infiltrating lymphocytes (TILs) prior to treatment 3 . We have developed an orthotopic mouse model of rectal cancer, which displays an aggressive and radioresistant phenotype with a relative absence of tumour infiltrating immune cells. This model employs murine organoids implanted submucosally in the murine rectum, where tumour-extrinsic factors may impact RT response; these include the gut microbiome and the gut mucosal immune system. Our aim was to evaluate whether rectal tumours display a differential response to RT when implanted orthotopically in the murine rectum versus a heterotopic subcutaneous site. In addition, we assessed local and systemic immune activation related to RT to determine whether aspects of the rectal tumour niche may inherently create a radioresistant environment for tumour growth. Autochthonous murine tumour-derived organoids expressing common driver mutations (Apc fl/fl ; Kras G12D/+ ; p53 fl/fl ; Tgfbr1 fl/fl : herein termed ‘AKPT’) were established and maintained in-vitro. Organoid fragments were subcutaneously injected into the right flank of syngeneic C57Bl/6 immunocompetent mice (n=14). Half of the mice (n=7) were treated with 15Gy in 3# (Mon-Wed-Fri) using a Small Animal Radiation Research Platform (SARRP; XStrahl) and half were anaesthetised (sham RT). The dose and fractionation were chosen to recapitulate the short-course radiotherapy regime used in the clinical setting. Irradiation commenced 1 week following tumour engraftment. All mice were aged until clinical endpoint (tumour size 15 mm in any dimension, deteriorating body condition or ulceration) and pieces of primary tumour were collected for bulk-RNA sequencing. The subcutaneous model’s response to irradiation was compared with orthotopic controls, where organoid fragments were transplanted via a colonoscopy-guided injection into the rectal submucosa. Animals in the orthotopic model were treated with the same RT fractionation regime and aged. Material/Methods:

Results:

Subcutaneous tumours developed at 100% engraftment rate (14/14) compared to an average of 97% in the orthotopic model. Previous work in our lab has shown no tumour response to fractionated radiotherapy in the orthotopic setting