ESTRO 2023 - Abstract Book

S545

Sunday 14 May 2023

ESTRO 2023

GTV is defined by T1 contrast-enhancing lesion, thickened meninges, and directly invaded bone.

For IMRT, the CTV include: - Grade I: No margin around the GTV. - Grade II: Margin of 5 mm to expand GTV in normal brain tissue, hyperostosis, along the unthickened meninges and venous sinuses if the GTV is coming into contact. - Grade III: Margin of 10 mm to expand GTV in normal brain tissue, hyperostosis, along the unthickened meninges, and optic or cranial nerves in contact with GTV. In case of bone invasion, a margin of 5 or 10 mm in the healthy bone around the GTV is recommended, for grade II or III respectively. Otherwise, it is considered as an anatomical barrier and does not need to be included in the target volume. In case of post-operative radiotherapy, no additional margin is required for CTV for grade I around tumor bed. A 5 and 10 mm margin is required for grade II and III. The cranial flap should only be included in the CTV only over 5 or 10 mm for grades II or III, in case of initially invaded bone. The drill holes and osteotomy areas should be included if they come into contact with target volume. SRT is not recommended for grades II and III, excluding relapse situation. CTV corresponds to GTV without additional margin. Conclusion The current consensus provides a detailed delineation guideline for meningioma, suggesting smaller margins than the major studies published to date. PD-0654 Prospective trial evaluating a novel MR-based 3D-printed head immobilization device (NCT04114786) P. Jablonska 1 , N. LaMacchia 2 , A. Parent 2 , H. Chan 3 , M. Filleti 3 , M. Ramotar 2 , Y. Cho 4 , A. Santiago 5 , M. Braganza 2 , A. Bandzynski 6 , N. Laperriere 7 , D. Shultz 7 , T. Conrad 7 , D. Tsang 7 , B. Millar 7 , T. Tadic 7 , A. Berlin 7 1 Clinica Universidad de Navarra, Radiation Oncology, Pamplona, Spain; 2 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada; 3 University Health Network, Radiation Physics, Toronto, Canada; 4 Cleveland Clinic, Radiation Oncology, Ohio, USA; 5 Princess Margaret Cancer Centre, Biostatistics, Toronto, Canada; 6 Princess Margaret Cancer Centre, Cancer Digital Intelligence Program, Toronto, Canada; 7 Princess Margaret Cancer Centre, Radiation Oncology, Toronto, Canada Purpose or Objective Immobilization and reproducible positioning are crucial for accurate brain radiotherapy (RT). Current RT planning processes require redundant dedicated imaging studies and a bespoke moulding session to create an immobilization device (i.e., thermoplastic mask [T-mask]). Innovative approaches may improve the patient’s journey and value of care. The aim of this study was to prospectively deploy and assess the performance of a patient-specific 3D-printed mask (3Dp-mask) that is generated solely from MR imaging, allowing for the recreation of a reproducible tolerable positioning and immobilization for patients undergoing brain RT. Materials and Methods Patients undergoing LINAC-based CNS RT (primary brain tumor or resected brain metastases) were enrolled prospectively (IRB #18-5753; NCT04114786). In the investigational arm, an in-house designed 3Dp-mask was generated from MR images to recreate the natural random head positioning during MR acquisition and allow coupling with the LINAC table during RT delivery. Differences in inter-fraction motion were compared between patients treated in the control (T-mask) versus the investigational (3Dp-mask) paradigm. Adverse events and tolerability were assessed using a patient-reported questionnaire after conventional moulding session, and by the end of the first and last weeks of treatment for both arms. Results Between January 2020 and July 2022, a total of 40 patients were enrolled (20 on each arm). All participants completed the prescribed brain RT and all study evaluations. Median time from simulation to ready-to-treatment was 4 (range 1 -12) and 8 days (range 6 -14) for the control and investigational arms, respectively. The average time to complete the 3Dp-mask design and printing was 36h 50min (range 12h 56min - 42h 01min). There were no significant differences in the ratings between groups through each questionnaire time point, except for a greater reduction in neck discomfort in the investigational arm compared to the control arm from end of first RT week to end of last RT week (B= -0.28, CI= [-0.49, - 0.06], p=.011). Both arms showed similar shift values for the left, right, posterior, superior, and inferior. There was a larger absolute anterior-posterior displacement in the control arm than in the investigational arm (mean [sd] 0.210cm [0.126] verus 0.096cm [0.076], p=.001). Absolute superior-inferior displacement tended to be larger in the investigational arm (mean [sd] 0.197cm [0.137] versus 0.103c, [0.075] in the control arm, p=.01). Conclusion The proposed total inverse planning paradigm using a 3D-printed immobilization device is feasible, rendering comparable inter-fraction performance while offering a better patient experience compared to the conventional thermoplastic mask. This approach could allow improvements to the brain RT workflows and potentially significant cost savings.