Comparison of in vivo scintillation probes and gamma cameras for detection of small, deep tumours

An in vivo probe was compared with a gamma camera for the task of detecting radiolabelled tumour models in a water phantom. The probe, which contained a 1 cm diameter NaI(Tl) detector, was designed and used by the authors for surgical staging studies of gynaecological patients. Tumours were spherical sources of different sizes and activities per unit volume of cobalt-57. The phantom was a tank of water (45 l) containing dissolved radioactivity to simulate background activity. Detector-to-source separations and tank depths were also varied. Camera images of 10 min duration were compared with probe counts of 15 s. For each configuration a large number of source and background runs were analysed using an ideal-observer ROC technique. Area under the ROC curve was used as the figure of merit. Results show that for approximately uniform background the probe should perform substantially better than the gamma camera in detecting small, deep tumours provided that the probe can be manoeuvred to within a few centimetres of the tumour. Mathematical modelling of the results indicates that this conclusion is not dependent on radiopharmaceutical dose, tumour uptake or camera type.