ESTRO 2025 - Abstract Book

S1717

Clinical - Sarcoma & skin cancer & malignant melanoma

ESTRO 2025

Material/Methods: In ARTIC (ARO 2022-10, DRKS00032390), we retrospectively screened clinical records of stage IV MM patients with PD under ICI treatment. Eligible patients received RT for metastases (= target lesion) and had ≥1 NIL outside the RT- field (= control lesion). Patients switching treatment within the time of response assessment were excluded. NILs were evaluated according to iRECIST response criteria (size reduction ≥30%: Abscopal Response (AR), increase ≥20%: Progression, all other: Abscopal Control (AC)). Radiological imaging before/after RT was required for lesion count. We categorized patients into groups: patients with AR and/or AC (all lesions) = Abscopal Benefit (AB), patients with AP and/or mixed response = Abscopal Progress (AP). We collected information on RT details and clinical information to identify factors for AR. Results: We analyzed data from 12 contributing centers and screened 3773 cases to identify 47 MM patients (21 women, 26 men) with 115 lesions. Most patients received irradiation of brain (n=18, 38.3%) and lung (n=9, 19.1%) metastasis with a stereotactic (n=14, 29.8%) RT-scheme. Mean time between end of ICI and RT was 3.53 ± 5.67 months, ICI agents were Nivolumab (n=25, 53.2%) and Pembrolizumab (n=21, 44.7%). RT led to AR/size reduction (>30%) in 19.1% of patients (9/47) and in 29.1% of lesions (34/115). Logistic regression revealed hypofractionated RT schemes as significant variable for AB (OR=0.092, p=0.04, 95%-CI:[0.007-0.758]). Longer time interval between end of ICI and RT-start was associated with lower risk of death (HR = 0.703). Patients with AB had longer OS and PFS compared to patients with progression (17 months vs. 9 months and 4 months vs. 2 months ((p= 0.12 and 0.99, respectively)).

Conclusion: In MM patients progressive under ICI, RT can induce AR, therefore delaying a switch in systemic treatment. AB occurs more frequently with hypofractionated RT regimens and longer time intervals between ICI and RT. Our results are in line with case series reporting an AR rate between 18-52% and serve as reference for designing prospective trials.