CT-SPECT fusion plus conjugate views for determining dosimetry in iodine- 131-monoclonal antibody therapy of lymphoma patients

A method for performing ¹³¹I quantitative SPECT imaging is described which uses the superimposition of markers placed on the skin to accomplish fusion of computed tomography (CT) and SPECT image sets. Methods: To calculate mean absorbed dose after administration of one of two ¹³¹I- labeled monoclonal antibodies (Mabs), the shape of the time-activity curve is measured by daily diagnostic conjugate views, the y-axis of that curve is normalized by a quantitative SPECT measurement (usually intra-therapy), and the tumor mass is deduced from a concurrent CT volume measurement. The method is applied to six B-cell non-Hodgkin's lymphoma patients. Results: For four tumors in three patients treated with the MB1 Mab, a correlation appears to be present between resulting mean absorbed dose and disease response. Including all dosimetric estimates for both antibodies, the range for the specific absorbed dose is within that found by others in treating B-cell lymphoma patients. Excluding a retreated anti-B1 patient, the tumor-specific absorbed dose during anti-B1 therapy is from 1.4 to 1.7 mGy/MBq. For the one anti-B1 patient, where quantitative SPECT and conjugate-view imaging was carried out back to back, the quantitative SPECT-measured activity was somewhat less for the spleen and much less for the tumor than that from conjugate views. Conclusion: The quantitative SPECT plus conjugate views method may be of general utility for macro-dosimetry of ¹³¹I therapies.