PURPOSE: To determine if tumor volume, in addition to tumor metabolic activity, can be assessed noninvasively from attenuation-corrected fluorodeoxyglucose (FDG)-PET imaging using a semiautomated method.METHODS: CT and FDG-PET scanning was performed in 14 patients, eight with newly diagnosed untreated malignancies, and six patients with progressive non-Hodgkin's lymphoma (NHL). Tumor volume was determined from CT scans by summation of manually drawn regions of interest over tumor. Tumor volume was determined at FDG-PET with a semiautomated method based on quantitation of (18)F uptake and thresholding.RESULTS: Mean tumor volume was 187 +/- 189 cm(3). Tumor volume determined by means of PET and CT was strongly correlated in the patients with untreated tumors. Correlation was weaker for all patients, mainly due to one previously treated patient with a large disparity between CT and metabolically active tumor volumes at FDG-PET, presumably due to tumor necrosis.CONCLUSIONS: Tumor volume determination by FDG-PET was strongly correlated with tumor volumes determined by anatomic imaging with CT. FDG-PET appears comparable to CT in measuring untreated tumor volumes of this size. FDG-PET may be superior to anatomic techniques in assessing metabolically active tumor volume, and warrants further study in this role.