ESTRO 2020 Abstract book

S311 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.

PH-0528 Feasibility of MR-guided stereotactic body radiotherapy in 5, 2 or 1 fractions for prostate cancer J. Mohajer 1 , A. Dunlop 1 , A. Mitchell 1 , S. Nill 1 , U. Oelfke 1 , A. Tree 2,3 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom ; 2 The Royal Marsden NHS Foundation Trust, Department of Urology, London, United Kingdom ; 3 The Institute of Cancer Research, Department of Urology, London, United Kingdom Purpose or Objective The drive towards hypofractionated prostate radiotherapy is motivated by a low α/β ratio for prostate cancer (1 to 2 Gy) compared to surrounding organs at risk, implying an improved therapeutic ratio with increasing dose per fraction. Early evidence from studies of ultrahypofractionated (UHF) prostate HDR brachytherapy has shown good tolerability in terms of normal tissue toxicities and clinical outcomes similar to conventional fractionation schedules. MR-guided stereotactic body radiotherapy (SBRT) online plan adaptation and real-time tumour imaging may enable UHF doses to be delivered to the prostate safely, without the invasiveness of brachytherapy. The feasibility of UHF prostate treatment planning for the Unity MR-Linac (MRL, Elekta AB, Stockholm) was investigated for target prescriptions and planning constraints derived from the HDR brachytherapy and SBRT literature. Material and Methods Ten CTs and structure sets (dominant intraprostatic lesion GTV, whole prostate CTV and organ at risk delineations) of prostate cancer patients previously treated were randomly selected. The PTV was defined as a uniform expansion of the CTV by 2 mm on the basis of reduced geometric uncertainty obtained by the combination of inter- and intra-fraction MRL adaptive strategies. Monaco 5.40 (Elekta) was used to generate MRL step-and- shoot IMRT plans for three dose fractionation protocols (Table 1), testing 5, 2 and 1 fraction plans for the 10 patients. Patient-specific bulk electron density (ED) values were assigned to the bones, CTV and external ROIs in order to simulate MR-based treatment planning. Monaco IMRT optimisation and dose calculation settings were selected to facilitate online plan optimisation in less than six minutes (see Table 1). Optimisation prioritised OAR objectives over target objectives. Conformity was assessed by the Monaco reported PTV conformity index (CI). Results Of the ten plans per UHF scheme, all clinical goals were met in all cases for 5 fractions, and in six cases for both 2 and 1 fraction schemes (Table 1). PTV D95% was compromised by up to 6.4% and 3.9% of the associated target dose for 2 and 1 fraction plans respectively. There were two cases of PTV D95% compromise greater than a 5% dose decrease for the 2 fraction plans. PTV CI medians and ranges were: 5 fractions 0.84 (0.81 – 0.86); 2 fractions 0.83 (0.77 – 0.89); 1 fraction 0.83 (0.78 – 0.86). Mean and standard deviation treatment delivery times were: 5 fractions (7.9 ± 0.5 min); 2 fractions (11.5 ± 0.9 min); 1 fraction (16.0 ± 1.6 min).