PET/MRI guided GTV delineation for radiotherapy planning in patients with locally advanced squamous cell carcinoma of the oropharynx and the oral cavity

Background: Evaluation of possibilities offered by PET/MRI in GTV definition of primary tumors during radiotherapy planning in patients (pts) with locally advanced carcinoma of the oral cavity and the oropharynx. Methods: 15 pts with SCC of the oral cavity and the oropharynx underwent CECT (Aquilion ONE, Toshiba) and PET/MRI (3T Siemens Biograph mMRI) examination. Delineation of GTV was done using two methods: visual interpretation of CT (GTV-CT), PET (GTV-PETvis) and MRI (GTV-MRI) images, as well as the quantitative automatic method (Syngovia, Siemens) based on a selected threshold value (20%, 30%, 40%, 50%) of SUVmax from a PET examination (GTV-PET20%, GTV-PET30%, etc). Obtained volumes were compared and differences were statistically analysed. Results: Statistical analysis demonstrated that GTV-MRI (p=0.0010), GTV-PET20% (p=0.0409), GTV-PET40% (p=0.0309) and GTVPET50% (p=0.0018) diverged significantly from GTV-CT. The remaining measurements were not significantly different from GTVCT. 73% of GTV-PETvis were inside the GTV-MRI contours. 27% of increased FDG uptake was present outside the GTV-MRI boundaries. Conclusions: Hybrid PET/MRI may provide improved accuracy in radiotherapy planning since it can identify regions of the tumor which are not clearly visible in other examinations. GTV-MRI, GTV-PET20%, GTV-PET40% and GTVPET50% diverge significantly from GTV-CT.