ESTRO 2022 - Abstract Book

S67

Abstract book

ESTRO 2022

From 2001 to 2021, we studied the MRI brainstem’s modification and clinical brainstem toxicity for patients ≤ 22 years old from 5 reference French centers treated with surgery and radiation therapy (RT) for a localized intracranial ependymoma. Patients with relapse at the first follow-up MRI were excluded. Pre operative, post operative and follow-up MRI during 18 months after RT (or until relapse if it appeared sooner) were analyzed. Post-gadolinium T1, T2, FLAIR and diffusion weighted imaging were used. Survival rates estimated by the Kaplan-Meier method are presented with their 95% confidence intervals (CIs). Results Eighty-three patients were included in the study, 42 patients were treated with PBT, 37 with XRT and 4 with both. Median follow-up was 5.6 years [95%CI: 4.8-6.2], The 5-years overall survival and progression free survival rates were respectively 86.1% [95%CI: 75.7-92.3] and 68.7% [95%CI: 56.4-78.3]. First relapse occurred for 60% in the field of treatment. Neither clinical or MRI brainstem radionecrosis nor new or progressive brainstem symptoms were found. Four patients, all with posterior fossa tumour, presented a punctiform brainstem enhancement (3/4 also had a FLAIR hypersignal), with a median time of onset of 3.5 months [3.0-9.4] after RT and a median time of disappearance of 7.6 months [3.7-7.9] for 3 on 4 patients. Two of them had received PBT, one was treated with XRT and one with mixed XRT-PBT. Median prescribed dose and median brainstem mean dose were respectively 57.6 Gy [54-59.4] and 47.2 Gy [45.3-53.2]. None of the 4 patients suffered from new or progressive brainstem symptoms after completion of RT. For patients with posterior fossa tumour without MRI abnormality, median of brainstem mean dose was 48.5 Gy [17.6-55.3]. Conclusion Brainstem imaging change occurred in 4,8% of children in a large series including PBT. No radionecrosis or symptomatic MRI change was found within the brainstem. The pursue of the study with the others French centers will strengthen our results. C. Bang Overgaard 1 , B. Singers Sørensen 2 , N. Bassler 3 , P. Poulsen 4 , C. Grau 3 , J. Overgaard 1 , M. Krzysztof Sitarz 3 , H. Spejlborg 5 1 Aarhus University Hospital, Experimental Clinical Oncology, Aarhus, Denmark; 2 Aarhus University, Danish Centre for Particle Therapy, Aarhus, Denmark; 3 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 4 Aarhus University Hospital , Danish Centre for Particle Therapy, Aarhus, Denmark; 5 Aarhus University Hospital, Oncology, Aarhus, Denmark Purpose or Objective Protons deliver the highest physical dose in a specific-targeted tissue area. This feature makes protons compared to x-rays, in some cases, a more favorable radiation modality to use in treatment of different cancers. Proton radiation has an increased relative radiobiological effectiveness (RBE) compared to X-rays. Despite that RBE is not constant and depends on factors such as tissue type, dose per fractions, position in the Spread-Out Bragg Peak (SOBP) and endpoint, a constant RBE value of 1.1 is used clinically. Several in vitro studies have been conducted within the area of cell response, but in vivo RBE data are still scarce. In the current study both acute and late normal tissue response are analyzed in the same animals to assess the RBE in vivo . Materials and Methods Included in the study were 243 12–14-week-old C 3 H/HeNRj mice that were fixated in jigs and placed on top of a heated (25°C) water phantom. Their right hind leg was submerged into the water phantom and placed within a well-defined radiation field. Single doses of 22-46 Gy were delivered in the center of a SOBP by an 83-107MeV pencil beam scanning proton beam. As reference, a 6 MV photon beam from a clinical linear accelerator was used. Early and late normal tissue damages were investigated by two endpoints: (1) acute moist desquamation of the skin within 30 days post irradiation, and (2) radiation induced fibrosis developed within 9 to 53 weeks post irradiation. Results The late effect ED50 (the dose producing radiation induced fibrosis in 50% of the mice) were 31.63 Gy (29.72 Gy-33.34 Gy) for protons and 38.26 Gy (34.44 Gy-50.21 Gy) for photons indicating a significant higher ED50 for protons (p=0.035). No significant difference was found between protons and photons for the acute skin damage. The corresponding RBE values were 1.05 (1.00-1.10; moist desquamation) and 1.21 (1.05-1.36; fibrosis) (see figure 1). OC-0092 Enhanced RBE for late compared to early normal tissue damage in vivo