Validation of prospective whole-body bone marrow dosimetry by SPECT/CT multimodality imaging in 131I-anti-CD20 rituximab radioimmunotherapy of non-Hodgkin's lymphoma

Purpose: Radioimmunotherapy (RIT) for relapsed non-Hodgkin's lymphoma is emerging as a promising treatment strategy. Myelosuppression is the dose-limiting toxicity and may be particularly problematic in patients heavily pretreated with chemotherapy. Reliable dosimetry is likely to minimise toxicity and improve treatment efficacy, and the aim of this study was to elucidate the complex problems of dosimetry of RIT by using an integrated SPECT/CT system. Methods: As a part of a clinical trial of I-131-anti-CD20 rituximab RIT of non-Hodgkin's lymphoma, we employed a patient-specific prospective dosimetry method utilising the whole-body effective half-life of antibody and the patient's ideal weight to calculate the administered activity for RIT corresponding to a prescribed radiation absorbed dose of 0.75 Gy to the whole body. A novel technique of quantitation of bone marrow uptake with hybrid SPECT/CT imaging was developed to validate this methodology by using post-RIT extended imaging and data collection. Results: A strong, statistically significant correlation (p=0.001) between whole-body effective half-life of antibody and effective marrow half-life was demonstrated. Furthermore, it was found that bone marrow activity concentration was proportional to administered activity per unit weight, height or body surface area (p < 0.001). Conclusion: The results of this study show the proposed whole-body dosimetry method to be valid and clinically applicable for safe, effective RIT.