Radiation-induced optic neuropathy after pencil beam scanning proton therapy for skull-base and head and neck tumours

Objective: To assess the radiation-induced optic neuropathy (RION) prevalence, following high dose pencil beam scanning proton therapy (PBSPT) to skull base and head and neck (H&N) tumours Methods: Between 1999 and 2014, 216 adult patients, median age 47 years (range, 18–77), were treated with PBS PT for skull base or H&N malignancies, delivering ≥45 GyRBE to the optic nerve(s) (ON) and/or optic chiasma (OC). The median administered dose to the planning target volume was 74.0 GyRBE (range, 54.0–77.4). The median follow-up was 5.3 years (range, 0.8–15.9). Results: RION was observed in 14 (6.5%) patients at a median time of 13.2 months (range, 4.8–42.6) following PBSPT. Most (92.9%) of RION were symptomatic. Most affected patients (11/14; 79%) developed unilateral toxicity. Grade 4, 3, 2 and 1 toxicity was observed in 10, 2, 1 and 1 patients, respectively. On univariate analyses, age (<70 vs ≥70 years; p < 0.0001), hypertension (p = 0.0007) and tumour abutting the optic apparatus (p = 0.012) were associated with RION. OC’s V60 GyRBE was of border line significance (p = 0.06). None of the other evaluated OC–ON dose/volume metrics (Dmax, Dmean, V40-60) were significantly associated with this complication. Conclusion: These data suggest that high-dose PBS PT for skull base and H&N tumours is associated with a low prevalence of RION. Caution should be however exercised when treating elderly/hypertensive patients with tumours abutting the optic apparatus. Advances in knowledge: This is the first study reporting the risk of developing RION following proton therapy with PBS technique, demonstrating the safety of this treatment.