PET/MRI-guided GTV delineation during radiotherapy planning in patients with squamous cell carcinoma of the tongue

The aim of the study was to evaluate the usefulness and accuracy of 18-fluorine-labeled fluorodeoxyglucose (PET) and magnetic resonance imaging (MRI) hybrid in gross tumor volume (GTV) delineation during radiotherapy planning in patients with carcinoma of the tongue. Ten patients with squamous cell carcinoma (SCC) of the tongue underwent computed tomography (CT) and PET/MRI examination. The GTV for primary tumor and lymph nodes (nGTV) were defined on CT (GTV-CT) and compared to GTVs obtained from PET (GTV-PET) and MRI (GTV-MRI) images. Two methods of GTV determination were used: visual interpretation of CT, PET (GTV-PET${}_{vis}$) and MRI images and quantitative automatic method (Syngovia, Siemens) based on a chosen threshold value (20%, 30%, 40%, 50%) of standardized uptake values (SUV${}_{max}$) from PET examination (GTV-PET${}_{20\%}$, GTV-PET${}_{30\%}$, etc.). Statistical analysis of differences in GTV values obtained from CT, PET and MRI studies was performed. GTV-CT was used as a reference. In all, 80% of GTV-MRI and 40% of GTV-PET${}_{vis}$ were larger than GTV-CT. Respectively, 20% of GTV-MRI and 60% of GTV-PET${}_{vis}$ were smaller than GTV-CT. Taking into account all threshold measurements, 70% of volumes were smaller than GTV-CT. GTV-PET${}_{30\%}$ were the most closely related volumes to GTV-CT from all threshold methods in 50% of patients. GTV-PET${}_{vis}$ generated the most similar volumes in relation to GTV-CT from all PET measurements. Statistical analysis confirmed those results. Compared to nGTV-CT, 70% of nGTV-MRI and 20% of nGTV-PET${}_{vis}$ were larger. The remaining nGTV-MRI and nGTV-PET${}_{vis}$ measurements were smaller than nGTV-CT. Measurements of all thresholds nGTVs were smaller than nGTV-CTV in 52.5% of cases. nGTV-PET${}_{20\%}$ were the most closely related volumes to nGTV-CT in 40% of the cases. Statistical analysis showed that nGTV-PET${}_{20\%}$ (p = 0.0468), nGTV-PET${}_{vis}$ (p = 0.0166), and nGTV-PET${}_{50\%}$ (p = 0.0166) diverge significantly from nGTV-CT results. nGTV-MRI (p = 0.1141), nGTV-PET${}_{30\%}$ (p = 0.2845), and nGTV-PET${}_{40\%}$ (p = 0.5076) were significantly related with nGTV-CT. Combination of PET/MRI provides more information during target tumor mass delineation in radiotherapy planning of patients with SCC of the tongue than other standard imaging methods. The most frequently matching threshold value was 30% of SUV${}_{max}$ for primary tumor delineation and 30–40% of SUV${}_{max}$ for nGTV determination.