ESTRO 2021 Abstract Book

S332

ESTRO 2021

the histological type, which allowed to deduce adapted margins for the clinical target volume (CTV). However, to date no study has been able to define the most relevant margins for patients with stage 1 tumors. Materials and Methods We realized a retrospective analysis including patients with adenocarcinoma (ADC) or squamous cell carcinoma (SCC) of localized stage T1N0 or T2aN0 operated at the Strasbourg University Hospitals. We analyzed preoperative chest scans, delineated target volumes and analyzed the boundary between tumor tissue and healthy tissue on the surgical specimens. ME was measured from this boundary. The profile of the type of tumor spread was also evaluated.

Results One hundred and twelve patients with ADC and 42 patients with SCC were analyzed. The margin required to cover the ME of a localized NSCLC with a 95% probability is 4.4 mm and 2.9 mm for SCC and ADC respectively. There is a significant difference in maximum distance of ME between the tumor infiltrating lymphocyte (TILs) 0-10% and 50-90% (p<0.05) for SCC. To cover 95% of the samples in the TILs 0-10%, 20-40% and 50-90% groups a margin of 4.97 mm, 2.13 mm and 0 mm respectively would be required. There is a significant difference in maximum ME distance depending on whether the patient has chronic obstructive pulmonary disease (COPD) or not (p = 0.011) for ADC. To cover 95% of the ADC sample with COPD, a margin of 3.74 mm would be required. To cover 95% of the ADC sample without COPD, a margin of 2.27 mm would be required. Conclusion CTV margins of 8 mm and 6 mm for ADC and SCC respectively seem too large, however the absence of CTV margins in view of our results does not seem wise. We propose a decision tree to assist in the application of margins according to histologic type, COPD status and TILs rate. This approach should be considered in conjunction with the data set and other margins to be applied when stereotactic body radiation therapy is used.

PH-0432 Treatment uncertainty for ultra- vs. standard-hypofractionated breast RT based on in-vivo dosimetry Y.A.C. Fiagan 1 , E. Bossuyt 2 , M. Machiels 3 , D. Nevens 3 , C. Billiet 3 , P. Poortmans 3 , T. Gevaert 4 , D. Verellen 3 1 Iridium Netwerk, Faculty of Medicine and Pharmacy, Vrije Universiteit Brussel, Brussels, Radiation Oncology, Antwerp, Belgium; 2 Iridium Netwerk, Radiation Oncology, Antwerp, Belgium; 3 Iridium Netwerk, Faculty of Medicine and Health Sciences, Universiteit Antwerpen, Radiation Oncology, Antwerp, Belgium; 4 Universitair Ziekenhuis Brussel, Vrije Universiteit Brussel, Radiation Oncology, Brussels, Belgium Purpose or Objective We swiftly implemented postoperative ultrahypofractionated radiation therapy (RT) of 26 Gy in 5 consecutive fractions after breast conserving surgery (BCS) during the coronavirus disease 19 pandemic following publication of the FAST Forward trial (ref: Murray Brunt 2020 FAST Forward), replacing our