Feasibility of bremsstrahlung dosimetry for direct dose estimation in patients undergoing treatment with 90Y-ibritumomab tiuxetan.

purposeRadioimmunotherapy with (90)Y-ibritumomab tiuxetan has been used successfully used in the treatment of CD20-positive non-Hodgkin's lymphoma (NHL). Pretherapy imaging with (111)In-ibritumomab tiuxetan has been used in provisional dosimetry studies. Posttherapy imaging of (90)Y-ibritumomab tiuxetan for clinical use is appealing as it would simplify the data acquisition process and allow measurements of actual doses absorbed during treatment.

methodsThe study included 29 patients with non-Hodgkin's lymphoma, of whom 16 (group I) received a pretherapy (111)In-ibritumomab tiuxetan diagnostic study and (90)Y-ibritumomab tiuxetan treatment 1 week later, and 13 (group II) received only (90)Y-ibritumomab tiuxetan treatment. Planar imaging and blood sampling were performed in all patients. The doses absorbed by organs at risk were calculated using a whole-body average attenuation correction factor (relative dosimetry approach) and, in the case of the (111)In-ibritumomab tiuxetan image sets, also using organ-specific attenuation correction factors (absolute dosimetry method). Red marrow absorbed doses were based on gamma counting of blood samples.

resultsThe estimated red marrow absorbed doses from (111)In and (90)Y data were equivalent. In all cases, the doses absorbed by organs at risk were found to be within prescribed limits. The relative dosimetry approach applied to both the (90)Y and (111)In data significantly underestimated the doses relative to those obtained with the (111)In absolute dosimetry method which is generally accepted as the reference method (MIRD 16). In the case of (111)In, the relative dosimetry approach values were highly correlated (R(2) = 0.61) with the reference method values. Relative dosimetry estimates may be adjusted multiplying by a correction factor of 2.8. The (90)Y-ibritumomab tiuxetan relative dosimetry data correlated poorly with the reference method values (R (2) = 0.02).

conclusionBased on patient-specific dosimetry, the administered activity may be increased by an average factor of 2.4, indicating that most patients could be undertreated. The relative dosimetry approach based on planar imaging largely underestimates doses relative to reference values. Dosimetry based on planar bremsstrahlung imaging is not a dependable alternative to (111)In dosimetry.