Early Prediction of Response to Radioimmunotherapy with I-131 Rituximab in Lymphoma Using Sequential F-18 FDG PET/CT

Introduction: We prospectively evaluated the usefulness of sequential 18F-FDG PET/CT scans before and after the first radioimmunotherapy with 131I-rituximab (RIT) to predict response to subsequent repeated RIT in patients with relapsed or refractory non-Hodgkin’s lymphoma.

Methods: A total of 26 clinical patients received RIT and patients who had no progression after the first RIT underwent subsequent repeated RIT until disease progression or up to a maximum of six cycles. Patients underwent 18F-FDG PET/CT and maximum standardized uptake values (SUVs) and the sums of the products of the greatest perpendicular diameters (SPDs) were measured before, 5 d, and 4 weeks after the first RIT. Then, the % changes of SUVs and SPDs were calculated and designated as %SUV5d, %SUV4w, %SPD5d, and %SPD4w, respectively. The response was evaluated 1 month after each cycle of RIT. We assessed the predictive values of imaging parameters for response to the first or repeated RIT. Then, a predictive model for predicting responders to repeated RIT based on imaging parameters were devised.

Results: Objective responses to RIT were observed in 12 of 26 patients (46%) after the first RIT and 19 of 26 patients (73%) after repeated RIT. ROC curve analysis revealed that %SUV4w and %SPD4w predicted response to the first RIT, while %SUV5d, %SUV4w, and %SPD4w predicted response to repeated RIT. In the first step of our prediction model, patients with %SPD4w > 40% and %SPD4w ≤ 0% were predicted to be responders and non-responders, respectively. In the second step, patients with 0% < %SPD4w ≤ 40% were categorized into 2 groups: %SUV5d > 2% (responders) and %SUV5d ≤ 2% (non-responders). By using this prediction model, sensitivity, specificity, PPV, NPV, and accuracy for predicting responders to repeated RIT were 95%, 86%, 95%, 86% and 92%, respectively.

Conclusions: Serial 18F-FDG PET/CT scans before and after the first RIT, including day 5 PET, will be of benefit in the planning of subsequent repeated RIT considering its efficacy and toxicity.

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