Early evaluation of tumor response to 90 Y-ibritumomab radioimmunotherapy in relapsed/refractory B cell non-Hodgkin lymphoma: what is the optimal timing for FDG-PET/CT?

Purpose: To determine the earliest optimal timing for assessment of early response following radioimmunotherapy in non-Hodgkin lymphoma patients using FDG-PET/CT.
Methods: FDG-PET/CT was performed prior to treatment (PET1), at 2 (PET2) weeks, and at 6 (PET3) weeks after ⁹⁰ Y-ibritumomab radioimmunotherapy in 55 patients. Response was evaluated based on the Deauville 5-point scale and Lugano criteria as well as semiquantitative analysis and compared with progression-free survival (PFS).
Results: PET 2 showed complete metabolic response (CMR), partial metabolic response (PMR), stable metabolic disease (SMD), and progressive metabolic disease (PMD) in 33, 13, 6, and 3 patients, respectively, while PET 3 in 41, 8, 3, and 3 patients, respectively. Mean SUV _max of 168 target lesions decreased over time (PET1, 2, 3; 5.58 ± 2.58, 1.87 ± 1.78, 1.75 ± 2.25, respectively). Progression or recurrence after a median of 12.6 months (range 2.6-72.0 months) was seen in 44 patients. Patients with CMR or metabolic response (CMR + PMR) on PET2 showed significantly longer PFS as compared to those who did not (p = 0.00028 and p = 0.029, respectively). A similar significant difference was observed based on PET3 (p = 0.00013 and p = 0.017, respectively). The same trend was observed when analyzing only the subgroup of patients with follicular lymphoma (N = 43/55) (p < 0.0001).
Conclusion: Use of FDG-PET/CT findings with Lugano criteria for assessing early response to radioimmunotherapy after 6 weeks allowed for accurate evaluation and prognostic stratification, though scanning after 2 weeks was too soon to precisely evaluate response.
Key Points: • The optimal timing of FDG-PET/CT to obtain a suitable tool for assessment of response after ⁹⁰ Y-ibritumomab radioimmunotherapy of lymphoma has not yet been defined. • Assessment after 6 weeks by FDG-PET/CT using the Lugano criteria accurately evaluates treatment response and prognosis. • FDG-PET/CT performed 2 weeks after radioimmunotherapy is too early as it significantly misses objective responses.