¹⁸F-FDG PET/CT for monitoring the response of lymphoma to radioimmunotherapy

We retrospectively evaluated ¹⁸F-FDG PET/CT for monitoring the response of non-Hodgkin's lymphoma to radioimmunotherapy. Methods: A total of 33 clinical patients received ¹³¹I-tositumomab (n = 23) or ⁹⁰Y-ibritumomab tiuxetan (n = 10) and underwent ¹⁸F-FDG PET/CT scans before radioimmunotherapy and at 12 wk after radioimmunotherapy. A third scan was performed on 13 patients at 24 wk after radioimmunotherapy, 12 of whom did not receive interval therapy. Tumor metabolic activity was assessed before and after radioimmunotherapy visually and quantitatively by lean maximum standardized uptake value (SUV_lean max). Response was assessed by the International Workshop Criteria (IWC) and Revised IWC, which includes ¹⁸F-FDG PET (IWC-PET). Results: Mean SUV_lean max decreased from baseline in 244 target lesions 12 wk after radioimmunotherapy (from 6.51 ± 4.05 to 3.94 ± 4.41; P < 0.01), regardless of response at 12 wk after radioimmunotherapy (P ≤ 0.02). After radioimmunotherapy, SUV _lean max was lower for responders than for nonresponders (P ≤ 0.01). Median percentage change in SUV_lean max of target lesions per patient was -51% (-95% to 97%). No significant difference in decline in SUV _lean max between patients who received ¹³¹I-tositumomab and those who received ⁹⁰Y-ibritumomab tiuxetan was demonstrated (-31% ± 51% vs. -47% ± 46%; P = 0.38). Patients with greater than a 52% decline in SUV_lean max tended toward longer survival (P = 0.09) than those with lesser declines. The 12-wk overall response rate to radioimmunotherapy based on IWC was 42% (14/33); complete response rate was 15% (5/33). Eleven of 12 patients with progression at 12 wk had new disease sites, and in 4 patients, new disease sites were the only sites of progression. Of 108 lesions evaluated at 12 and 24 wk after radioimmunotherapy, 49 resolved at 12 wk and remained resolved at 24 wk, 17 gradually declined in SUV over 24 wk, and 37 initially decreased at 12 wk but increased at 24 wk. PET showed disease progression at 24 wk in 10 of 13 patients; 7 patients had new lesions and 1 was reclassified from partial response to complete response. Conclusion: In non-Hodgkin's lymphoma, ¹⁸F-FDG uptake in tumors typically drops significantly after radioimmunotherapy. A continued decline in tumor SUV _lean max between 12 and 24 wk without additional therapy can occur, suggesting a need for delayed-response assessment. In patients who progress after radioimmunotherapy, new sites of disease commonly develop, rather than recurrence or progression at previous disease sites. Large declines in ¹⁸F-FDG uptake tend to be seen in those with the longest progression-free survival.