Patient-Specific Three Dimensional (3D) Dosimetry for 90Y Ibritumomab Tiutexan (Zevalin®) in Patients with Follicular Lymphoma.

Objectives: Dosimetry calculations for radioimmunotherapy are usually performed using a two dimensional (2D) protocol based on planar whole body (WB) scintigraphy and fixed organ masses derived from the MIRD anthropomorphic model (Wiseman GA et al., J Nucl Med2003:44;465–74). We compared the absorbed dose obtained with this method with an approach involving patient-specific 3D dosimetry based on quantitative SPECT and patient organs mass estimation. Methods: Five patients with follicular lymphoma in partial or complete remission underwent a CT scan, and five successive WB and SPECT scans at day 0 (D0) 1 hour, D0 4 hours, D1, D4 and D6 after injection of 111In ibritumomab tiutexan (185 MBq). WB scans were used to estimate the activity in the liver, spleen and kidneys. SPECT images were reconstructed using a quantitative processing including corrections for scatter, attenuation, and partial volume effect. The organs mass and activity values were estimated in 3D volumes of interest manually drawn on the CT scan and registered to the SPECT scan. Absorbed doses after administration of 15 MBq/kg of 90Y ibritumomab tiutexan were derived from both 111In WB and SPECT data accounting for the fractions of injected activity measured in each organ at the five time points. Results: The mean differences between the organs mass estimated from the patient CT and the MIRD anthropomorphic model were 8%, 105% and 12% for liver, spleen and kidneys respectively. Measurements on an abdominal phantom filled with 111In showed that errors in organ activity estimates ((estimated activity - true activity)/true activity) for WB and SPECT protocols, were 37% and −6% respectively in the liver, 20% and −10% in the spleen, −26% and −23% in the left kidney, and 47% and −9% in the right kidney. Patient liver, spleen and kidney activity values determined from the SPECT scans were on average 35%, 60% and 39% less than those found from the WB scans. The absorbed doses calculated with the 3D patient-specific protocol were less than those calculated with the 2D protocol, by 46±14%, 77±14% and 70±22% in the liver, spleen and kidneys respectively, except in one patient kidney where SPECT dose exceeded WB dose by 2%. Conclusions: Accounting for patient-specific organ mass and using SPECT activity quantification have a great impact on estimated absorbed doses of 90Y ibritumomab tiutexan if compared with the 2D protocol. This method could improve the correlation between dosimetry and clinical consequences of this treatment, especially in view of dose escalation with stem cell rescue.