In patients with suspicious recurrence based on the Tg level after a normal postablation WBS, FDG PET/CT is the preferred scintigraphic method to localize disease rather than postempiric (131)I WBS. Empiric (131)I administration may be used only in patients who do not have a significant FDG uptake.

This study investigated whether the reference background above which a residual mass is considered positive in the International Harmonization Project criteria should be modified for early 18 F-FDG PET evaluation. Methods: In 92 patients with newly diagnosed diffuse large B-cell lymphoma, the maximal standardized uptake value (SUVmax) was measured on postcycle 2 PET in the most intense residual mass (or, in the case of negative PET findings, in the area of most intense tumor uptake before therapy), in the mediastinal blood pool (MBP) and the liver, as potential reference background tissues. Results: With MBP as a reference (SUVmax, 2.0 6 0.6), PET was unable to distinguish early responders from nonresponders. In contrast, with liver as a reference (SUVmax, 2.5 6 0.7), 2-y progressionfree survival was significantly different between patients with PET-negative findings (81.8% [95% confidence interval, 71%-93%]) and patients with PET-positive findings (51.8% [95% confidence interval, 35%-69%], P 5 0.003). Conclusion: When assessing early response, particularly in risk-adapted therapeutic trials, it seems preferable to refer to a background tissue (liver) with a higher level of uptake than that of current international criteria (MBP) which were designed for end-of-treatment evaluation.

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