ESTRO 2022 - Abstract Book

S224

Abstract book

ESTRO 2022

Results The following consensus guideline was developed, specifically targeting identified at-risk regions.

GTV should encompass all macroscopic visible tumour, including involved areas of any organ. If no distinction can be made between fibrosis and GTV, the fibrotic area should be included in the GTV. Otherwise, the fibrotic area can be included in the CTV. After IC, GTV may be adjusted for regression (towards other structures). CTV includes GTV with a 1 cm margin and all pre-treatment GTV. CTV should not be adjusted towards other organs, except towards the pelvic bones if bony invasion is clearly absent. Multifocal recurrences should be encompassed in one CTV, with the upper and lower limit 1 cm beyond the most cranial and caudal GTV respectively. For RT naïve patients, elective CTV is advised, conform treatment of LARC, i.e. lymph nodes, remaining mesorectal fat and, depending on recurrence location, the anal sphincter. For previously irradiated patients, no elective fields should be irradiated. Average DSC of GTV and CTV increased from 0.60 to 0.66 and 0.71 to 0.81 between case 1.2 and 2.1 respectively.

Institution Case

1

2 3 4 5

Mean

1.1 1.2 1.3 2.1 2.2 2.3

0.84 0.86 0.72 0.28 0.75 0.69 0.82 0.85 0.80 0.27 0.82 0.71

0.83 0.94

0.64 0.86 0.81

0.82 0.87 0.80 0.81 0.73 0.81

0.66 0.75 0.76 0.71 0.85 0.35 0.89 0.89

0.72

0.74 Table 1: DSC of CTV in reference to median delineation.

Conclusion Currently, there is limited evidence for delineation guidelines in LRRC. This study provides a first multidisciplinary, consensus-based guideline. Verification in re-recurrences is needed to understand disease behaviour and recurrence patterns and to optimize delineation guidelines accordingly.

OC-0268 Cell-free DNA as predictor of pathological complete response in locally advanced rectal cancer

C. Truelsen 1 , C. Kronborg 2 , B. Singer Sørensen 2 , L. Callesen 3 , K. Spindler 3,4

1 Aarhus University Hospital, Department of Clinical Experimental Oncology, AArhus, Denmark; 2 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 3 Aarhus University Hospital, Department of Clinical Experimental Oncology, Aarhus, Denmark; 4 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Neoadjuvant radiotherapy followed by total mesorectal excision is the standard treatment strategy for locally advanced rectal cancer (LARC). For patients achieving clinical complete response (cCR) after neoadjuvant radiotherapy the watch and wait strategy (W&W) is proposed as an organ-sparing approach surpassing surgical morbidity. There are, however, concerns related to persisting, viable cancers cells despite cCR- as cCR is only in partial concordance with pathological complete response (pCR), which substantiates the need for complimentary information for patient stratification for a non- operative management. The aim of this study was to investigate circulating cell-free deoxyribonucleic-acid (cfDNA) as a biomarker for prediction of pCR. Materials and Methods A prospective biomarker study was conducted at Aarhus University Hospital, Denmark from 2017 to 2020 including patients with LARC treated with neoadjuvant radiotherapy. Plasma obtained at baseline, mid- and end of therapy was quantified