ESTRO 2025 - Abstract Book

S1064

Clinical – Head & neck

ESTRO 2025

then re-stage patients with a repeat MRI maxillofacial to assess response during week 4 of primary chemoradiation. Subsequently, a stereotactic boost is designed to treat gross disease to a cumulative EQD2 dose of 70-72 Gy. Overall treatment is then delivered in shorter amount of time and less fractions compared to conventional fractionation. The purpose of this study is to establish feasibility of stereotactic boosts to primary tumor and to assess acute toxicity among patients receiving boosts. Material/Methods: A process to deliver sequential SBRT boosts has been established. Phase one of treatment is delivered in 23-27 fractions, followed by a stereotactic boost delivered in 3-5 fractions. Maximum EQD2 objectives are set for optic nerves and chiasm, brain stem surface, brain stem core, spinal cord and normal brain as 55-56, 60, 54, 45 and 66-68 Gy, respectively, with α/β ratio set to 2 Gy. Stereotactic boost dose constraints for 3-5 fraction regimens are calculated based on the dose these critical structures receive in the phase one plan. A conservative approach summing the maximum doses from each plan was utilized. Plans were optimized to maximize tumor dose while respecting calculated dose limits. Results: Prescribed dose for the stereotactic boost ranged from 3.00 to 3.54 Gy per fraction. Dose constraints for all critical structures other than brain were achieved for all patients. Patient averaged maximum EQD2 to brainstem surface was 52.4Gy (range 43.9-59.8Gy), brainstem core 47.5 (35.8-53.4), left optic nerve 53.2 (49.6-55.9), right optic nerve 51.6 (50.0-54.2), optic chiasm 48.8 (48.5-49.3), brain 70.8 Gy (68.1-74.3 Gy). Less than 1cc of the normal brain exceeded dose constraints which was deemed clinically acceptable. A EQD2 dose 68-72 Gy was given to the gross disease respecting OAR constraint to brainstem and optic structures. Conclusion: This study demonstrated feasibility and safety of stereotactic boosts for a select group of patients who can benefit from a shorter course of treatment and a larger dose per fraction to the tumor site. Digital Poster Outcomes of Palliative Hypofractionated Radiotherapy with 50 Gy in 20 fractions with or without a treatment Break in Head and Neck Cancer Patients Aarthi Iyer 1 , Shao Hui Huang 1 , Andrew McPartlin 1 , Jie Su 2 , Scott Bratman 1 , B.C John Cho 1 , Ezra Hahn 1 , Andrew Hope 1 , Ali Hosni 1 , John Kim 1 , Nauman Malik 1 , Brian O'Sullivan 1 , Jolie Ringash 1 , C.Jillian Tsai 1 , John Waldron 1 , Li Tong 1 , Enrique Sanz Garcia 3 , Christopher Yao 4 , Wei Xu 2 1 Radiation Oncology, Princess Margaret Cancer Centre,University of Toronto, Toronto, Canada. 2 Biostatistics, Princess Margaret Cancer Centre,University of Toronto, Toronto, Canada. 3 Medical Oncology, Princess Margaret Cancer Centre,University of Toronto, Toronto, Canada. 4 Otolaryngology-Head and Neck Surgery-Surgical Oncology, Princess Margaret Cancer Centre,University of Toronto, Toronto, Canada Purpose/Objective: A radiotherapy (RT) option for durable palliation in advanced head and neck squamous cell cancer (HNSCC) is 50 Gy in 20 daily fractions. A planned 2-week break at mid-point or unplanned ad-hoc break can be considered to reduce acute toxicity. The effect of a break on outcomes have not been described. We report treatment tolerance and disease outcomes following palliative treatment to 50 Gy with or without planned/unplanned break. Keywords: Stereotactic boost, sino-nasal cancer 3172

Material/Methods: All patients receiving 50 Gy in 20 fractions with palliative intent between 2005-2021 were included. Baseline