Lesion Detection and Administered Activity

TO THE EDITOR: There is a preoccupation in nuclear medicine imaging with the risks posed by the use of radionuclides and with reduction of administered activities (1). Nearly all nuclear medicine presentations include information on the absorbed or effective doses from the radiopharmaceutical under discussion. The tiny carcinogenic risk, an extra 1 in 1,000 risk from a typical diagnostic administered activity, is minimal (2) when the lifetime risk of cancer is up to 1 in 2 (3). The debatable risk (4) of induced cancer from the absorbed dose must be balanced against the risks of misdiagnosis and the consequent effect on potential lifesaving treatment, especially in patients with cancer. Of course, pediatric and benign disease investigation may require a more conservative approach.

Confirmation of the detrimental effects of reducing the administered activity on lesion detection can be seen in a recent paper in The Journal of Nuclear Medicine by Rauscher et al. (5). Their study, on the effect of reducing the administered activity on the sensitivity of ⁶⁸Ga PSMA-11 PET/CT imaging, shows that, as would be expected, the lower the simulated administered activity, the fewer the number of lesions detected. Three readers identified 21 lesions at a rate of 100%, 100%, and 90% with a baseline administered activity of 120–192 MBq and 85%, 81%, and 90% with two thirds of the baseline tracer activity.

The standard recommended activity of ⁶⁸Ga PSMA-11 of approximately 1.8–2.2 MBq/kg of body weight is still under debate (6). If between 10% and 19% of lesions are missed by a reduction of one third of an administered activity of 120–192 MBq (5), this may imply that potentially up to one fifth of lesions are being missed by the standard administered activity compared with increasing the administered activity by one third.

Recommended standard administered activities should be optimized using clinical and phantom studies defining the required lesion size as seen on the image, the lesion-to-background ratio, and the administered activity required to achieve this in a time during which the patient can be expected to be motionless. There is the complication that the fraction of the injected activity that is captured by the lesion will depend on the biodistribution and metabolism of the disease being imaged. There is also the additional complication of the varying sensitivity and resolution of different types of imaging equipment marketed by different manufacturers.

It is time to move on from the situation in which recommended activities are derived from the average of activities that produce an acceptable image to a more scientific approach to ensure that small but clinically important lesions that may change patients’ management are not missed.

• © 2020 by the Society of Nuclear Medicine and Molecular Imaging.