ESTRO 2020 Abstract Book

S289 ESTRO 2020

moving structures as used during delivery. MR-based motion correction can be applied to reconstruct the PET image at the highest spatial resolution, providing accurate visualization of small tumour deposits in thorax and abdomen. This allows for treatment of small tumours and multiple oligometastases in thorax and abdomen. The system will also be used for tumour characterization and response assessment. Compared to other methods based on direct use of PET imaging for treatment guidance (RefleXion), the Unity/MRI/PET combination allows stereotactic targeting and provides additional morphological information due to superior soft-tissue contrast. Material and Methods The authors have together developed a new implementation of the integrated MR-PET with characteristics specifically tailored for the application in radiotherapy. This system is based on the Philips 1.5T Ingenia platform. The PET detector array, with axial length of 25 cm, has been integrated in the body RF coil using an innovative RF shielding design which maintains the original MR performance while maximizing the bore size (only 3 cm reduction compared to Ingenia bore). A physical prototype of the MR-PET body coil was built and MR performance was compared to the Ingenia body coil. The body coil was then equipped with two prototype PET detectors. The PET performance was evaluated with and without RF pulses and switching gradients being emitted by the MRI system simultaneously to PET acquisition. Results The prototype MRI-PET body coil achieved nearly the same B1+ field homogeneity as the clinical body coil, with only slight deviations in the periphery of the field of view. The B1+ efficiency was 38% lower compared to the clinical body coil driven at the same RF power, but this can be compensated by adding a second RF amplifier. The PET system has been tested in a regular 1.5T Ingenia radiotherapy simulator. The 2 PET modules demonstrated a stable PET data flow under MR operational conditions with a coincidence timing resolution 282 ps and an energy resolution of 11%. The tests showed that using the proposed shielding design, wide bore MR-PET is feasible. All regular radiotherapy MRI scans were feasible. Conclusion A dedicated MR-PET has been designed for multi-modality radiotherapy simulation. The combination of the MR-linac with the MR-PET allows the use of PET information in the stereotactic targeting of the Unity system. The system has been especially designed for the search for small tumours and multiple oligometastases in thorax and abdomen.

Conclusion After adjusting for prognostic factors from the GPA, SES has no independent prognostic value for survival in the full cohort of patients with brain metastases undergoing radiation therapy. No robust prognostic value of SES was seen in any of the tumour-based subgroups either, although an effect in patients with a RCC cannot be excluded. This leads us to conclude that SES does not impact survival in this cohort.

Poster Highlights: Poster highlights 18 PH: MRI guidance

PH-0527 The design of an MR-PET for radiotherapy treatment simulation. The search for small tumour volumes J. Lagendijk 1 , C. Beijst 2 , B. Woutjan 2 , H. Erik 2 , S. Bart 2 , A. Cezar 2 , W. Bjoern 3 , S. David 3 , G. Pierre 3 , G. Nicolas 3 , D. Thomas 3 , B. Martino 4 , M. Jurgen 5 , S. Volkmar 3 , S. Andre 5 , L. Oliver 5 , V.D.M. Peter 5 , V. Marc 5 , D.J. Hugo 2 , K. Dennis 2 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands ; 2 UMC Utrecht, Department of Radiology, Utrecht, The Netherlands ; 3 Rheinisch- Westfaelische Technische Hochschule, Radiology, Aachen, Germany ; 4 Futura Composites, Development, Heerhoguwaard, The Netherlands ; 5 Philips Medical Systems, Development, Best, The Netherlands Purpose or Objective The Unity 1.5T MR-linac is able to provide stereotactic precision for every (moving) location in the body. This potential to steer the dose at sub-mm scale stresses the need for accurate multi-functional imaging for target definition. An integrated 1.5T MR-PET simultaneously provides excellent anatomical localization of the PET signal, due to the intrinsic registration between both modalities. The 1.5T MR could also provide the same real-time imaging of