Lucia Riccardi, Presenter: Nothing to Disclose

Michele Gabusi, Abstract Co-Author: Nothing to Disclose

Marta Paiusco, Abstract Co-Author: Nothing to Disclose

A comprehensive analysis of the experimental results proved that 10 MBq of administered activity can be adequate to image SNs up to a depth of 4 cm, for 1 minute acquisition time. 

Planar intraoperative lymphoscintigraphy are nowadays possible using portable gamma cameras. The real benefit of intraoperative imaging of SN is under debate. For melanoma SN biopsy, a minimum administered 10 MBq activity was indicated. In skin malignancies, uptake by a SN is minimal, making detection levels below 100 kBq quite common in surgical practice. In addition an empirical rule requires that any lymph node at least as active as 10% of the hottest SN should be excised to reduce the risk of lymph node metastases. In this low counting rate condition, the process of image formation can be strongly affected by statistical uncertainty.

The mini gamma camera under investigation was Sentinella 102 (Oncovision/GEM Imaging, Valencia, Spain), The efficiency of the portable gamma camera was evaluated from the images of an 8.0 mm sphere filled by known amounts of 99mTc. The counts were extracted from a circular region of interest (ROI) on the source image. The activity of a sample of 95 SNs was measured after excision and a median uptake of about 0.5% was found. To carry out a consistent identification criterion of a typical SN, a test involving a group of five nuclear medicine physicians was performed. A set of decreasing-quality  images was acquired and submitted to the observers. The inter-operators differences were computed from the ROIs drawn by the doctors. A boundary condition of 50 ROI counts was proposed to ensure a consistent source identification. Then a set of borderline operating conditions for minimum detectability was established. 

Our study confirmed that in a clinical scenario low amounts of radioactivity are present, typically from tenths to hundreds of kilobecquerel. On the basis of an inter-operator investigation, a boundary condition corresponding to 50 ROI counts was proposed for a consistent identification of SNs. 

Riccardi, L, Gabusi, M, Paiusco, M, Imaging of Melanoma Sentinel Nodes by a Portable Gamma Camera: A Physical Investigation.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14019467.html