Lack of tumor uptake of 131-I labeled rituximab in a patient with a CD20 positive lymphoma lesion.

Radioimmunotherapy has emerged as a treatment modality for patients with CD20 positive B-cell non-Hodgkin's lymphoma (NHL). Prior to administration of a therapeutic dose, confirmation of uptake of the radiolabeled compound in tumor locations and calculation of an appropriate dose can be performed using a diagnostic dose and subsequent imaging. We report the case of a 69-year-old male with a relapsed mantle cell lymphoma scheduled for radioimmunotherapy, where diagnostic imaging with 131-I labeled rituximab revealed unexpected new insights with implications for treatment. Persistence of the mantle cell lymphoma in a lymph node in the left arm was demonstrated by an 18-F fluorodeoxyglucose scan. However, a scan after a diagnostic dose of 131-I labeled rituximab did not show any uptake of the tracer, even though subsequent cytological analysis unequivocally confirmed a CD20 positive B-cell population in the lesion. The administration of a therapeutic dose of 131-I labeled rituximab was therefore cancelled. We here discuss the mechanisms that may explain lack of targeting in a proven CD20-positive lymphoma and provide recommendations for further studies.