Intermodality Variability in Gross Tumor Volume Delineation for Radiation Therapy Planning in Oropharyngeal Squamous Cell Carcinoma.

PURPOSE: Multimodality imaging can enhance the precision of tumor delineation for intensity modulated radiation therapy planning. This study aimed to analyze intermodality variation for gross tumor volume (GTV) delineation in locally advanced oropharyngeal carcinomas (LAOCs).

METHODS AND MATERIALS: We examined the pretreatment contrast-enhanced computed tomography (CECT), magnetic resonance imaging (MRI), and fluoro-deoxy-glucose-based positron emission tomography (FDG-PET) image data sets of 33 adult patients with primary LAOC. Automatic segmentation method was used to derive PET-based metabolic tumor volumes (MTVs) at 30%, 40%, 50%, 60%, and 70% of the primary tumor's maximum standardized uptake value (SUVmax). The geometric conformality or spatial overlap was assessed using the Dice similarity coefficient (DSC), which ranges from 0 to 1, indicating no overlap to complete overlap.

RESULTS: The size of the tumor in the anteroposterior dimension of the GTV was found to be more on CT than MRI, with a mean difference of 0.29 cm ( P value .015). Overall, PET-based MTV volumes were smaller than GTVs on CT and MR. Among various intensities on PET, MTV30 was the closest match with GTV-CT/MR. The mean difference for absolute tumor volumes (GTV-CT, GTV-MR, and MTV30) was not statistically significant; however, spatial overlap by DSC score was average, that is, <0.7. DSC was 0.65 ± 0.15 between GTV-CT and GTV-MR, 0.62 ± 0.15 between GTV-CT and MTV30, and 0.576 ± 0.16 between GTV-MR and MTV30 pairs, respectively. On qualitative analysis, overall tumor extension into adjacent muscles, parotid gland, retromolar trigone, and marrow infiltration of mandible was better appreciated on MRI.

CONCLUSIONS: Given the significant spatial variation, multimodality imaging can serve as an excellent complement for target volume delineation on CT scans during intensity modulated radiation therapy planning for LAOC by harnessing the improved soft tissue definition of MRI and the ability of PET to provide metabolic activity information.