ESTRO 2025 - Abstract Book

S894

Clinical - Haematology

ESTRO 2025

This analysis emphasizes the significant differences in the preferences regarding quality or length of life of patients with primary cutaneous lymphoma, highlighting the need for a personalized approach and shared decision-making for each group of patients.

Keywords: Mycosis fungoides, TSEBT, dose deescalation 1764

Mini-Oral Decrease of metabolic active tumour volume of target- and unintentionally irradiated lesions after bridging radiotherapy prior to CAR T-cell therapy Mette W Steen 1 , Kylie Keijzer 1,2 , Allard E van Ark 1,3 , Lisanne V van Dijk 1 , Janneke W de Boer 2 , Jaap A van Doesum 2 , Anne PG Crijns 1 , Hans HG Verbeek 1 , Tom van Meerten 2 , Anne GH Niezink 1 1 Department of Radiation Oncology, University Medical Centre Groningen, Groningen, Netherlands. 2 Department of Haematology, University Medical Centre Groningen, Groningen, Netherlands. 3 Department of Radiation Oncology (MAASTRO), Maastricht University Medical Centre, Maastricht, Netherlands Purpose/Objective: Bridging radiotherapy (BRT) prior to chimeric antigen receptor T-cell therapy (CART) has shown to improve outcomes in patients with large B-cell lymphoma (LBCL), by reducing metabolic active tumour volume (MATV). However, the optimal radiation schedule and dose to achieve volume reduction are unknown. Therefore, we investigated the MATV dynamics of irradiated, unintentionally irradiated, and non-irradiated lesions in relation to radiation dose in LBCL patients receiving BRT prior to CART. Material/Methods: Patients with relapsed/refractory LBCL after ≥2 lines of immunochemotherapy treated with CART between January 2018 and March 2024 and BRT alone were included. Lesions’ MATV was determined before and after radiotherapy using 18 F-FDG PET/CT scans and median radiation dose per lesion was calculated. Lesions were split into categories based on dose and differences were assessed with the Mann-Whitney U test. Univariable logistic regression was performed to evaluate RT dose and other factors influencing MATV. Results: In total, 25 patients with 146 lesions were included. The prescribed target dose was 5x4Gy (n=19), 15x2Gy (n=3), 8x2.75Gy (n=1), 10x2Gy (n=1) and 2x4Gy (n=1). Median time interval between last radiotherapy treatment and the post-BRT 18 F-FDG PET/CT was 12 days (IQR 9-16). Lesions receiving a dose of ≥4Gy had a significantly higher median MATV before BRT compared to those receiving lower or no radiation doses (p<0.01). Median ΔMATV for the dose groups was: non-irradiated (+84%), 0.1-4Gy (+8%), ≥4-8Gy (-100%), ≥8-20Gy (-100%) and ≥20Gy (-97%) (Figure 1). Irradiated lesions reduced significantly compared to non-irradiated lesions (p<0.01). Lesions receiving doses of ≥4-8Gy, ≥8-20Gy and ≥20Gy showed a significant MATV decrease of ≥80% (all p<0.05). No associations were found between MATV decrease and the time interval after BRT, extranodal lesions, lesion volume or histology (Table 1). The best overall response (complete or partial response) after CART was 88% and the median progression-free survival was 8.9 months. Interestingly, only 1 in 10 patients with progression after CART experienced an in-field relapse, but this lesion also failed to respond to radiotherapy.