Dosimetric Approaches for Radioimmunotherapy of Non-Hodgkin Lymphoma in Myeloablative Setting

Radioimmunotherapy (RIT) is a safe and active treatment available for non-Hodgkin lymphomas (NHLs). In particular, two monoclonal antibodies raised against CD20, that is Zevalin (⁹⁰Y-ibritumomab-tiuxetan) and Bexxar (¹³¹I-tositumomab) received FDA approval for the treatment of relapsing/refractory indolent or transformed NHLs. RIT is likely the most effective and least toxic anticancer agent in NHLs. However, its use in the clinical setting is still debated and, in case of relapse after optimized rituximab-containing regimens, the efficacy of RIT at standard dosage is suboptimal. Thus, clinical trials were based on the hypothesis that the inclusion of RIT in myeloablative conditioning would allow to obtain improved efficacy and toxicity profiles when compared to myeloablative total-body irradiation and/or high-dose chemotherapy regimens. Standard-activity RIT has a safe toxicity profile, and the utility of pretherapeutic dosimetry in this setting can be disputed. In contrast, dose-escalation clinical protocols require the assessment of radiopharmaceutical biodistribution and dosimetry before the therapeutic injection, as dose constrains for critical organs may be exceeded when RIT is administered at high activities. The aim of the present study was to review and discuss the internal dosimetry protocols that were adopted for non-standard RIT administration in the myeloablative setting before hematopoietic stem cell transplantation in patients with NHLs.