ESTRO 2024 - Abstract Book

S1900

Clinical - Mixed sites, palliation

ESTRO 2024

Humboldt Universität zu Berlin, Department of Radiation Oncology, Berlin, Germany. 11 University Medical Hospital, Hamburg-Eppendorf, Department of Radiation Oncology, Hamburg, Germany. 12 Outpatient Center of the University Medical Hospital Hamburg-Eppendorf, Department of Radiotherapy and Radiation Oncology, Hamburg, Germany. 13 University Hospital, LMU Munich, Department of Radiation Oncology, Munich, Germany. 14 University Hospital Tübingen, Department of Radiation Oncology, Tübingen, Germany. 15 Klinikum rechts der Isar, Technical University of Munich, Department of Radiation Oncology, Munich, Germany. 16 German Cancer Consortium (DKTK), Partner site Munich, Munich, Germany. 17 German Cancer Research Center, DKFZ, Heidelberg, Germany. 18 University Hospital Bonn, Department of Radiation Oncology, Bonn, Germany. 19 University Hospital Bonn, Institute of Experimental Oncology, Bonn, Germany. 20 University Hospital Dusseldorf, Medical Faculty, Heinrich-Heine-University Dusseldorf, Department of Radiation Oncology, Dusseldorf, Germany. 21 Medical Faculty, University of Cologne, Institute of Medical Statistics and Computational Biology, Cologne, Germany. 22 Olivia Newton-John Cancer Wellness & Research Centre, Austin Health, Department of Radiation Oncology, Melbourne, Australia. 23 University of Cologne, Center for Molecular Medicine Cologne (CMMC), Cologne, Germany

Purpose/Objective:

Abscopal effects (AbE) are characterized by regression of non-irradiated lesions (NIL) following radiotherapy (RT). Their induction is of particular interest for patients with progressive disease (PD) during immune checkpoint inhibition (ICI), as AbE are likely immune-mediated. The actual incidence of AbE in the RT-ICI setting is unclear.

Material/Methods:

We retrospectively screened our clinical records for patients receiving RT for PD of metastatic cancer during ICI between 2015- 2021. Eligible patients had ≥1 NIL outside the 10% RT isodose. Patients switching systemic treatment within radiological response assessment (RA) were excluded. Up to 5 NIL were measured. According to iRECIST, size reduction ≥30% was rated abscopal response (AR), increase by ≥20% abscopal progression (AP), every change of size in between was classified as “control” (abscopal control, AC). For NIL measurement, cross -sectional imaging (MRI, CT, PET/CT) before and after RT was required. Sample size calculation was based on an expected AbE rate of 20% and required inclusion of ≥ 62 cases with 10% precision (CI10 -30%).

Results:

We analyzed data from 8 participating centers. 3381 cases were screened to identify 98 eligible patients (63 men, 35 women) with a total number of 195 non-irradiated lesions (NIL). Lung cancer (n=32, 33%) and melanoma (n=30, 31%) were most frequent cancer types, nivolumab (n=48, 49%) most frequent ICI, and hypofractionated (n=55, 56%) the most frequent RT scheme. At final RA after 180 days, in the lesion-based analysis, 180 NILs from 98 patients could be included for AbE assessment. Of these, 51 NILs (28%) showed AR, 101 (56%) were stable and 28 (16%) showed progression. On patient level, 37 (38%) had ≥1NIL with AR with 21 patients (21%) showing AR in all their NIL (see Figure 1 ).