ESTRO 2025 - Abstract Book

S4069

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

1794

Digital Poster MR guided motion analysis of open faced masks versus closed faced masks in head and neck radiotherapy James Tallon 1 , Abigael Clough 1 , Claire Nelder 1 , Benedict Dobby 1 , Rachael Bailey 1 , Alice Greenwood-Wilson 1 , Michael Dubec 2 , Marcel VanHerk 1 , Cynthia Eccles 1 , Ananya Choudhury 1 1 Radiotherapy Research and Innovation, The Christie, Manchester, United Kingdom. 2 Medical Physics and Engineering, The Christie, Manchester, United Kingdom Purpose/Objective: Thermoplastic masks in head and neck (H&N) radiotherapy (RT) can stimulate patient anxiety (Lastrucci A, 1). Open faced thermoplastic masks aim to reduce patient distress without compromising treatment accuracy. This work assessed inter and intra fraction positional reproducibility and motion using no mask versus standard of care (SoC) masks versus open faced masks on an MR Linac. Material/Methods: Ten H&N cancer patients, oral cavity (n=1), larynx (n=1) and oropharynx (n=4) undergoing RT (55-66 Gy in 20-30 #) consented to an ethics approved imaging study. Patients underwent ≥3 MRI sessions on a 1.5 T MR Linac (Elekta, Sweden) that included six dynamic T2 weighted Fast Spin Echo 3D (T2W 3D FSE) images acquired during the first, middle and final week of RT. This work was done in two phases. Phase 1: Six patients were imaged to compare the SoC thermoplastic mask to no mask. Phase 2: Four patients were imaged to compare the open faced mask to SoC mask. Each dynamic image (n6) was registered to the first image, to assess intra-fraction motion using rigid registrations performed by three observers. A clip box was used to establish rotations and translations in three regions of interest (ROIs): whole scan, tumour, and chin. To investigate inter fraction motion, differences in mean position of the dynamic sequences were determined for the first, middle, and final week of RT. Results: Population mean intra-fraction motion was > than institutional tolerance -> 3mm and 3 degrees rotation after four minutes in the roll (Y) and yaw (Z) planes with no immobilisation for all three ROIs. Without immobilisation, inter fraction motion for the tumour ROI increased not significantly (p=0.08) for 20 fraction patients. For 30 fraction patients this was significant (p=0.01) (Table 1).

Population mean intra fraction motion for tumour and chin ROIs were > for open faced masks compared to the SoC mask in all planes, however displacements were within institutional tolerances. Over the treatment course, inter fraction motion for the tumour ROI increased not significantly (p=0.22) when wearing the open faced mask (Table 2).