ESTRO 38 Abstract book

S106 ESTRO 38

Results Twenty-five patients with grade IV ORN were identified. Median follow-up was 76 months (range 24-183) and median time to development of ORN was 22 months (range 5-132). Median age at diagnosis was 61 years (range 47- 72), and 84% were men. The site of tumor origin was base of tongue, tonsil, and posterior pharyngeal wall in 12, 12, and 1 patient(s), respectively. Median radiation prescription dose was 70 Gy in 33 fractions. Only two patients developed ORN contralateral to the tumor site. The average of minimum dose to ORN-VOIs (i.e. the isodose line that covers 100% of the ORN volume) was 54.3 Gy. The first, second, third, and fourth quartile minimum dose distribution for ORN-VOI ranged from 32.4-46.7, 46.7-54.6, 54.6-64.6, and 64.6-68.5 Gy, respectively (Figure 2). The averages of mean and maximum doses to ORN VOIs were 66.3 Gy (range 54-75) and 72.5 Gy (range 64-78), respectively.

Conclusion The mandibular areas of origin of advanced ORN in OPC patients treated with IMRT received at least 55 Gy in approximately half of the examined cohort. However, lower doses, in the intermediate dose range (32-55 Gy), were also associated with mandibular ORN. Our findings suggest that the intermediate dose beam-path results in long-term bone toxicity for OPC survivors. Dose-volume constraints in both intermediate and high dose range of the mandibular organ at risk should be adopted in future IMRT plans. PV-0203 Adaptive proton therapy for patients with Head and Neck tumors involving skull base S. Karaman 1 , R. Malyapa 2 , U. Langner 2 , T. Houser 2 , Z. Fellows 2 , J. Moreau 2 , Z. Vujaskovic 2