Metabolic response of non-Hodgkin's lymphoma to ¹³¹I-anti-B1 radioimmunotherapy: Evaluation with FDG PET

¹³¹I-anti-B1 (CD20) radioimmunotherapy (RIT) is a promising approach for treatment of non-Hodgkin's lymphoma (NHL). We assessed the tumor metabolic response to RIT using FDG PET. Methods: We examined 14 patients with NHL, who were given first a tracer dose of ¹³¹I-anti-B1 and then RIT, each preceded by infusion of unlabeled anti-B1. In 8 of 14 patients, PET was performed at baseline and 33-70 d after RIT. The other 6 patients underwent PET at baseline, 6-7 d after the tracer dose, and 5-7 d after RIT to estimate the early response to tracer dose and RIT. To assess tumor FDG uptake, standardized uptake value normalized for lean body mass (SUV-lean) was measured 1 h after FDG injection. Results: After RIT, complete response was observed in 6 patients, partial response in 6, and no response in 2. At 33-70 d after RIT, mean SUV-lean of 6 responders markedly declined to 41% of the baseline value (P < 0.002). Soon after tracer dose and after RIT, mean SUV- lean of the other 6 responders decreased to 79% and 62% of the baseline values, respectively (P < 0.05). In 2 nonresponders, SUV-lean did not significantly decline from the baseline value at 37 d after RIT. Conclusion: FDG PET metabolic data obtained 1-2 mo after RIT correlate well with the ultimate best response of NHL to RIT, more significantly than the early data after tracer dose or RIT. FDG uptake in NHL may decline gradually after RIT in responding patients.