Sacral scintigraphy for bone marrow dosimetry in radioimmunotherapy

Abstract

Myelosuppression has been identified as the dose-limiting toxicity in radioimmunotherapy studies. Accurate bone marrow dosimetry is, therefore, necessary to evaluate bone marrow toxicity which may result from systemic cancer treatment with radiolabeled monoclonal antibodies. Dose to the red marrow was determined in 20 patient studies with 131I labeled anti-carcinoembryonic antigen, anti-alpha-feto-protein, or anti-human chorionic gonadotropin monoclonal antibody for diagnosis or treatment of diverse metastatic carcinomas, using a new technique involving sacral scintigraphy and a previously reported blood-based methodology. For the sacral technique, anterior and posterior γ camera images of the pelvis were obtained at multiple times. Regions of interest were drawn around the sacrum in order to quantitate activity uptake as a function of time using the conjugate view counting method. Cumulated activity in red marrow was determined by curve integration and division by 0.099, since it has been estimated that 9.9% of the total red marrow is contained in the sacrum of the adult. Red marrow doses were then obtained by multiplying the cumulated activities by the appropriate S factor. These doses were compared to red marrow doses obtained from serial whole blood samples taken from these patients. Cumulated activity in the red marrow was determined from the blood with the assumption that the activity concentration in the blood and red marrow were equal. The mean red marrow dose per injected activity was 2.0 ± 0.9 rad/mCi using the sacral data and 2.7 ± 1.3 rad/mCi using the blood data ( P<0.01). The mean red marrow dose obtained by sacral counting was 21.0% less than that obtained by blood counting. In two patients, there were large differences between the sacral scintigraphic red marrow dose calculations and those measured by blood counting. The degree of myelotoxicity in these cases correlated better with the measurements determined by external sacral scintigraphy. Thus, red marrow dosimetry may be accurately determined using sacral regions of interest from γ camera images.