Appropriate margin thresholds for isocontour metabolic volumetry of fluorine-18 fluorodeoxyglucose PET in sarcoma: A hybrid PET/MRI study

Objectives Metabolic volumetry and heterogeneity indexes are increasingly being used in fluorine-18 fluorodeoxyglucose (¹⁸F-FDG) PET, mostly using isocontour methods. However, there is no standard method to determine optimal margin thresholds for isocontour volume of interest (VOIiso). Here, VOIiso with various margin thresholds were compared with exact anatomical tumor volumes in sarcoma using a hybrid PET/MRI scanner. Patients and methods ¹⁸F-FDG PET/MRI images were retrospectively analyzed for 36 lesions of soft tissue or bone sarcoma. VOIiso of tumors was drawn with varying margin thresholds; standardized uptake value (SUV) 1.0-4.0 with an increment of 0.5 and 30-70% of maximal SUV (SUV_max) with an increment of 10%. Exact anatomical tumor volumes (VOIMRI) were drawn manually on the basis of MRI. In each VOI, metabolic tumor volume, SUVmean, total lesion glycolysis, and SD were measured and compared between VOIiso and VOIMRI. Results Significant correlations were observed between the indexes from VOIiso and VOIMRI. For metabolic tumor volume, no significant difference existed between VOIiso and VOIMRI with margin thresholds of 30 and 40% of SUV_max, and SUV 1.5. For SUVmean, no significant difference existed with 30% of SUV_max, and SUV 1.5. For total lesion glycolysis, no significant difference existed with 40% of SUV_max, SUV 1.5, and 2.0. For SD, no significant difference existed with 30% of SUV_max, SUV 1.5, and 2.0. Conclusion Margin thresholds of 30 or 40% of SUV_max and SUV 1.5 or 2.0 may be used for the VOIiso method in ¹⁸F-FDG PET images of sarcoma. VOIiso with these margin thresholds are expected to produce reliable metabolic, volumetric, and heterogeneity indexes on ¹⁸F-FDG PET images.