FDG-PET Predicts Response to Fractionated Radioimmunotherapy with ⁹⁰Y-Epratuzumab Anti-CD22 MAB in Patients with NHL.

Objective: To evaluate FDG-PET imaging for early prediction of response in patients with NHL treated with fractionated radioimmunotherapy (RIT).

Methods: Ten patients from a larger ongoing, multicenter, Phase I/II trial of fractionated RIT (2–3 injections 1-week apart of humanized anti-CD22 antibody, epratuzumab, labeled with ⁹⁰Y) underwent FDG-PET imaging together with CT scans of the chest, abdomen and pelvis at baseline and 6 weeks post-RIT, and then every 3 months until progression. Tumor responses evaluated from CT images were classified using Cheson lymphoma criteria as complete response (CR), unconfirmed CR (CRu), partial response (PR), stable disease (SD) or progression of disease (PD). PET images were evaluated for abnormal focal uptake visually, using standard uptake value (SUV) quantitation, and were classified as CR when all tumor foci disappeared, incomplete response (IR) when FDG uptake decreased with persistent foci, or PD when FDG uptake increased or new foci developed.

Results: A total of 36 paired imaging studies were obtained post RIT (including 3 patients after retreatment) and evaluated as CR (n=7), CRu (n=14), SD (n=5) or PD (n=10) by CT and CR (n= 13), IR (n= 8) or PD (n=15) by PET. Of the 14 studies evaluated as CRu by CT, 7 were definitively evaluated by PET as CR, 3 as IR, and 4 as PD. Of 22 studies not evaluated as CRu by CT, PET identified PD in one case evaluated as CR by CT and was otherwise concordant with CT (10 PD/PD, 6 CR/CR, 5 SD/IR). Among PET images acquired at 6 weeks post-RIT, the mean time-to-progression (TTP) was 9.6 months for negative PET images (CR) compared to 4.1 months for positive PET findings (IR, PD) (P=0.16).

Conclusion: In our study, FDG-PET appeared superior to conventional CT in evaluating response to fractionated RIT. For CT scans frequently evaluated as CRu, PET resolved uncertainty regarding residual disease, and PET images acquired 6 weeks after RIT predicted later relapse.