Estimating uptake in liver and abdominal tumours for radionuclide therapy dosimetry

Historically, patients for radionuclide therapy have received fixed activities, sometimes normalized to body weight or body surface area. As radionuclide therapy develops, however, practitioners are becoming interested in individualized doses. This requires individual estimates of uptake to be routinely available. Such estimates need not necessarily be highly accurate but the magnitude of associated errors must be known. This paper looks specifically at simple methods for estimating relative (tumour to organ) uptake and also absolute (organ) uptake, for abdominal tumours and the liver. For relative uptake, the ratio of geometric mean counts gives a rough estimate, to within a factor of 2. Absolute liver uptake can be estimated from the geometric mean of liver counts but a correction must be applied for patient attenuation. Such a correction can be calculated solely from the patient's height and weight. The patient weight alone gives a correlation coefficient of 0.86, when plotted against the measured attenuation. However, this correction does not work well for patients heavier than 100 kg. The attenuation index, which is calculated from a patient's weight and height, gives a correlation coefficient of 0.90, for patients ranging in weight from 41 kg to 120 kg.