Lung counting: Evaluation of uncertainties in lung burden estimation arising from a heterogeneous lung deposition using Monte Carlo code simulations

A fundamental assumption made in calibrating a lung counter is that the deposition of radioactivity in the lung is homogeneous; however, depositions rarely follow this pattern. There will be an uncertainty on the activity estimate if the lung counter is calibrated using a lung set that has the activity homogeneously distributed throughout the tissue substitute material and the deposition in the subject is heterogeneous. Monte Carlo code simulations have shown that the uncertainty on an activity estimate of an internal deposition that emits 17 keV photons can be underestimated by as much as a factor of four, overestimated by as much as a factor of 26, or missed completely. As the photon energy rises to 60 keV the uncertainty on the activity decreases so that the maximum overestimate (or underestimate) will be a factor of three. Detector arrays minimise the uncertainties arising from the geometry of the lung deposition and should be the preferred counting configuration.