ESTRO 2025 - Abstract Book

S1015

Clinical – Head & neck

ESTRO 2025

2133

Digital Poster Value of diffusion-weighted MRI for persistent skull base invasion of nasopharyngeal cancer after radiotherapy Sergi Benavente 1 , Juan Carlos Tortajada 2 , Sahyly Siurana 2 , Silvana Sarria-Estrada 2 1 Radiation Oncology, Vall d'Hebron University Hospital, Vall d'Hebron Institute of Oncology, Barcelona, Spain. 2 Neuroradiology Section, Radiology Department, Vall d'Hebron University Hospital, Barcelona, Spain Purpose/Objective: Skull base invasion (SBI) in nasopharyngeal cancer (NPC) can be extensive, entail significant morbidity, and confound response evaluation. Diffusion-weighted (DW)-MRI has been used for tumor response evaluation in different cancer types, but little is known about when tumors infiltrate the skull base. We report the outcome of NPC with SBI in patients treated with concurrent chemoradiation (CRT) +/- induction/adjuvant chemotherapy and evaluate the potential role of DW-MRI in SBI response evaluation. Material/Methods: A retrospective analysis was performed on patients with NPC and SBI treated at Vall d’Hebron University Hospital between 2007-2022 (restaged as per AJCC 2018). Overall survival (OS) and local failure-free survival (LFFS) were evaluated. Patients with incomplete data were excluded. In those cases with longitudinal DW-MRI response assessment, we investigated the differences according to the apparent diffusion coefficient (ADC) values, a promising parameter derived from DW-MRI. For conventional MRI evaluation, a complete response (fibrotic) was defined as residual unmodified changes at 12 months post-CRT. MRI with echo-planar DW sequences at 3 b-values (50-800-1000 s/mm 2 ) were acquired, and three experienced neuroradiologists evaluated ADC maps of the ROI and reference regions. Longitudinal minimum ADC values higher than 1.3x10 -3 mm 2 /s in combination with conventional MRI response parameters were considered a complete response (ADC-response). Results: Twenty-one cases were included. Clinical characteristics are shown in Table 1 . With a median follow-up of 4.8 years, the 5-year actuarial OS was 77.9% (95%CI 58.5-97.4), and LFFS 90.5% (95%CI, 77.9-100). Twelve cases had baseline and follow-up DW-MRI. Nine of those thirteen patients achieved ADC-response values 6 months post-RT (6 in the first 3 months) and remained failure-free. The cases with limited SBI showed earlier ADC-response. Only one of the remaining three patients, with extensive bone destruction and posterior osteomyelitis, achieved ADC-response at 20 months post-RT and remains failure-free. The other two patients developed metastatic disease and did not achieve ADC-response.