ESTRO 2021 Abstract Book

S36

ESTRO 2021

Anonymized administrative data from all 0.35T-MRgRT treatment systems in Europe and Asia were extracted for patients who completed treatment from 2015-2020. Detailed treatment information was analyzed for all MR-linac systems and 2 MR-cobalt systems. Results 22 systems at 21 centers treated 5,777 patients, of whom 3,486 and 2,291 were treated on the MR linac and cobalt systems. 5,153 patients received 53,486 fractions, of which 11,637 (21.8%) were adapted. Nineteen of 21 centers (90.5%) had treated for ³ 1 year, of which 14 treated >100 patients/year and 9 treated >150 patients/year. Ultra-hypofractionation (1-5 fractions) was delivered for 63.2% of all patients. The proportion of fractions adapted in patients receiving ultra-hypofractionation was 53.1%, with an average of 2.7 adapted fractions per course. The most commonly treated tumor types were prostate (23.7%), liver (14.2%), lung (12.4%), pancreas (11.2%), and breast (8.1%), respectively, with increasing compound annual growth rates (CAGRs) in numbers of patients over the 5-year time period (pancreas: 166%; lung: 136%; liver: 132%; prostate 121%). The CAGR in the number of patients was 71.7%, growing from 288 in 2016 to 2,504 in 2020. Ultra- hypofractionation increased from 39.3% of treatment database patients in 2015 to 71.4% in 2020 (n=1,723/2,413). The proportion of adapted fractions increased from 0% in the first year to 31.0% (n=7,212/23,255) in all patients (Table 1) and 60.5% (n=5,316/8,793) in patients receiving ultra- hypofractionation schemea, by the end of 2020. From 2015 through 2017, few patients had adaptive planning. However, adaptive replanning has been increasingly adopted over the last three years. In 2020, the proportion of adaptive fractions was highest for pancreas (55.7%), liver (34.4%) lung (31.8% ) and prostate (31.7%) tumors.

Conclusion This is the first comprehensive study reporting patterns of utilization among early adopters of a 0.35T-MRgRT system in Europe and Asia. Intrafraction MR guidance, advanced motion management, and increasing adoption of adaptive RT has accelerated a transition to ultra-hypofractionation regimens. MRgRT has been predominantly used to treat tumors in the upper abdomen, pelvis and lungs, and increasingly with adaptive replanning, which is a radical departure from legacy radiotherapy practices.

Proffered papers: Proffered Papers 2: Tumour microenvironment

OC-0061 Radiotherapy affects the morphology and phenotype of HNSCC derived Cancer associated fibroblasts

M. Ansems 1 , M. Dotinga 1 , V. Mekers 1 , A. Beerkens 1 , K. Strepi 1 1 Radboudumc, Radiation Oncology, Nijmegen, The Netherlands

Purpose or Objective For decades cancer research has primarily focused on intrinsic mechanisms of carcinogenesis. However, the microenvironment (TME) tumour cells reside in has a major impact on tumour cell proliferation, metastasis and the anti-tumour immune response. The predominant cell type in the TME of many solid cancers are Cancer Associated Fibroblasts (CAFs). Current evidence indicates that different CAF subsets exist, their exact function however is still obscure. In head and squamous-cell carcinoma (HNSCC) CAFs can even consist of up to 80% of the cells in a tumour. Currently, the majority of HNSCC patients will be treated with radiotherapy (RT) during the course of disease. CAFs are often recognized as RT resistant. Although radiation does cause persistent DNA damage, CAFs do not die upon clinically applied doses of RT, but rather become senescent. Despite their abundance in HNSCC, the effect of RT on their function in HNSCC is largely unknown. Materials and Methods To investigate the significance of CAFs in human HNSCC, the Head and Neck Cancer (HNSC) cohort in the TCGA (The Cancer Genome Atlas) database was used. Expression of several CAF associated markers in 520 HNSCC samples was compared to the expression in 44 normal tissue samples. To investigate the impact of radiation on CAFs, primary CAFs were isolated from four previously established HNSCC patient-derived xenografts and were subjected to different radiation regimes. Expression of CAF associated and immunomodulatory markers was measured by quantitative PCR and flow cytometry analysis and their morphology was assessed by microscopy. Results