ESTRO 2025 - Abstract Book

S3918

Radiobiology - Normal tissue radiobiology

ESTRO 2025

References: 1.https://journals.lww.com/amjclinicaloncology/abstract/2009/02000/a_radiation_oncologist_s_guide_to_contouring _the.4.aspx 2. https://pmc.ncbi.nlm.nih.gov/articles/PMC5469746/ 3.https://pmc.ncbi.nlm.nih.gov/articles/PMC4075526/ 4.https://pmc.ncbi.nlm.nih.gov/articles/PMC10900111/ 5.cajpcglclefindmkaj/https://pdfs.semanticscholar.org/fd2d/ef44a7ae4b99769552c05fb4981ebca9bf88.pdf

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Digital Poster Dosimetric parameters of radiation-induced optic neuropathy based on visual field perimetry in patients undergoing proton therapy Thao-Nguyen Pham 1 , Nathan Azemar 2 , Mathieu Seraphim 1 , Jean-Claude Quintyn 3 , Juliette Thariat 1,2 1 Radiation Oncology, François Baclesse Center, Caen, France. 2 Laboratoire de physique corpusculaire, UMR6534 IN2P3/ENSICAEN, Université de Caen- Normandie, Caen, France. 3 Ophthalmology, University Hospital, Caen, France Purpose/Objective: Current guidelines for preventing radiation-induced optic neuropathy (RION) refers to severe RION based on clinical symptoms 1 . This study aims to explore the relationship between RION based on visual field perimetry with various dosimetric parameters and baseline visual status in patients undergoing proton therapy. Material/Methods: We conducted a prospective study involving 238 patients with para-optic head and neck cancer (HNC) treated with proton therapy at Baclesse Center, France. Visual field perimetry (VF) measured sensitivity to light at various points across the visual field in decibels (dB). VF was applied to evaluate visual deficits before and post-radiotherapy (6, 12, 18 months and yearly). Organs-at-risk (OAR) delineation was performed for the optic nerves (ON) and chiasma. Grade-1 optical toxicity was defined as mean deficit from the VF (MD) lower than -3 dB till -6 dB (Grade-2:(-6;-9], Grade-3:(-9;-12], Grade-4:<-12). Changes in VF deficits over time were analyzed in correlation with the dosimetry. Regression models were utilized to predict MD. Results: At baseline, the median MD of VF was -1.3 dB (range:-21.3–1.5), with 66.6% (263/395 eyes) below the threshold for vision deficiency. The median Dmax to the chiasma and ON was 50.8 and 51.5 Gy, respectively. Among the eyes without vision deficiency at baseline, the median follow-up time was 1.93 years (range:0.28–4.98), the median time to develop vision deficiency post-radiotherapy was 1.07 years (range:0.21–4.98). The proportion of patients without toxicity declined from 66.6% at baseline to below 49.1% four years post-radiotherapy (26/53 eyes). MD decreased by 0.53/year on average, with a faster decline of 0.78 dB/year in those without baseline toxicity (Grade 0–1). A negative correlation was observed between MD at last follow-up and both Dmean/ON and Dmax/ON (Dmean, Dmax; R=-0.15 and -0.13, respectively, p≤0.05). A random forest model identified both Dmean and Dmax to the chiasma and ON as key predictors of changes in VF per year (parameter importance higher than 50%). The doses required to achieve a 50% probability of toxicity (TD50) was 51.3 Gy for the Dmax/ON and 27.2 Gy for the Dmean/ON. Patients with Dmax ≤51.3 Gy had a 9.7% toxicity rate, compared to 12.5% for Dmax >51.3 Gy. Similarly, for Dmean ≤27.2 Gy, toxicity was 9.9%, increasing to 12.6% for higher doses.

Conclusion: Our results highlight the impact of the Dmean to the ON on RION, alongside the Dmax to the ON and chiasma in

HNC patients undergoing proton therapy. Keywords: Proton therapy, Optical toxicity