ESTRO 2021 Abstract Book

S66

ESTRO 2021

Daily online adaptive radiotherapy can ensure the target coverage, achieving all the dosimetric goals for the actual rectum on every session. The full online adaptive planning described in this study is feasible within 45 min.

Proffered papers: Proffered Papers 7: Advances in RT imaging and contouring

OC-0091 Head Coil Head Support: in support of the head and the head coil W. Frans 1 , M. Frantzen-Steneker 1 , J. Osinga 1 , U.A. van der Heide 1,2 , R. Navest 1 1 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands; 2 Leiden University Medical Center, Department of Radiation Oncology, Leiden, The Netherlands Purpose or Objective The most common MRI RT brain setup consists of an individualized mask or stereotactic headframe with flexible loop surface coils. Due to image-based position verification on state-of-the-art RT treatment systems, it is no longer needed to perform the planning MRI for the brain in RT position. This enables the use of a diagnostic head coil instead of an individualized mask or stereotactic headframe with flexible surface coils. Compared to the diagnostic head coil, flexible loop surface coils have several disadvantages. These include suboptimal signal to noise ratio due to the limited number of receiver channels and a longer scan time. Moreover, the positioning of surface coils is MRI RTT dependent and may lead to variable image quality. A disadvantage of a head coil without fixation, however, is the increased risk of motion artefacts. Typically, motion artefacts in the brain require to redo the scan in approximately 10.5% [ 1 ] to 12.7% [ 2 ] of patients. In this study it was investigated whether a purpose made head support placed inside the diagnostic head coil can be used to stabilize the patient’s head and reduce repeated scans. Materials and Methods The Head Coil Head Support (HCHS) was formed out of a solid block of PU-foam. The shape of the HCHS was optimized on multiple volunteers to center the head in the middle of the head coil and provide support for the head and neck, thus minimizing movement, see figure 1.

A single slice T1-weighted gradient echo cine MRI (300 frames, 226 ms/frame) was acquired on three volunteers on a 3T Philips Ingenia scanner using the head coil. For both with and without HCHS the volunteer deliberately kept rotating the head around the left-right (LR) and cranio-caudal (CC) body axis during a sagittal and transversal acquisition, respectively. The displacement amplitude was calculated as three times the standard deviation of the absolute displacement similar to Mandija et al. [ 3 ] Furthermore, whether or not the HCHS could be used was recorded by the RTT’s for in a total 59 patients. Results The absolute displacement measured in a volunteer is shown in figure 2. The average displacement amplitude over all volunteers around the LR axis of 4.5 mm without HCHS and 3.5 mm with HCHS. Around the CC axis, the average displacement amplitude over all volunteers was 8.2 mm without HCHS and 4.7 mm with HCHS.

In 18 out of 59 patients, the HCHS was not used, mainly because lack of space preventing the head coil from closing. For 41 patients, the MRI images scanned with HCHS were clinically used for radiotherapy treatment. For two patients, a repeated scan was required because of motion artefacts (~5%). The HCHS was found to be easy to use by the RTT’s and none of the patients reported physical discomfort.