Background-based Delineation of Internal Tumor Volume in Static Positron Emission Tomography in a Phantom Study

OBJECTIVES Considering the fact that the standardized uptake value (SUV) of a normal lung tissue is expressed as x±SD, x+3×SD could be considered as the threshold value to outline the internal tumor volume (ITV) of a lung neoplasm. METHODS Three hollow models were filled with 55.0 kBq/mL fluorine-18 fluorodeoxyglucose (18F-FDG) to represent tumors. The models were fixed to a barrel filled with 5.9 kBq/mL 18F-FDG to characterize normal lung tissues as a phantom. The PET/CT images of the phantom were acquired at rest. Then, the barrel was moved periodically to simulate breathing while acquiring PET/CT data. Volume recovery coefficient (VRC) was applied to evaluate the accuracy of ITVs. For statistical analysis, paired t-test and analysis of variance were applied. RESULTS The VRCs ranged from 0.74 to 0.98 and significantly varied among gross tumor volumes for delineating ITV (P<0.01). In two-dimensional PET scans, the motion distance did not affect VRC (P>0.05), whereas VRC decreased with increasing distance in three-dimensional PET scans (P<0.05). CONCLUSION The threshold value (x+3×SD) had the potential to delineate the ITV of cancerous tissues, surrounded by lung tissues, particularly in two-dimensional PET images.