A method to evaluate intra-operative gamma probes for sentinel lymph node localisation

A method is described to allow objective evaluation of intra-operative gamm a probe performance for the task of sentinel lymph node localisation. The m ethod uses simple simulation based upon standard sensitivity and spatial re solution measurements at depth in water, with technetium-99m sources. The a im is to predict the minimum separation between the injection site and lymp h node required to allow the sentinel lymph node to be identified in the pr esence of high injection site activity. The simulation methodology allows r apid investigation of probe performance for a range of node and injection s ite activities, and a range of node and injection site depths, without the need to perform a large number of physical measurements. Examples of practi cal performance simulations are given from five probes, showing that nodes at less than 115 mm from the injection site may be poorly localised, with e ven the best performing probe requiring at least 51 mm separation to allow detection in the high background from the injection site. This method provi des data to allow the ranking of probe system performance in terms of the p ractical task of sentinel lymph node localisation, rather than arbitrary ra nking based upon basic physical performance measures such as spatial resolu tion and sensitivity. The best probes allow sentinel lymph node localisatio n at between 20 and 30 mm closer to the injection site than the poorest per forming probes, for situations which represent intra-operative localisation in melanoma and breast surgery. The method is also shown to assist in opti mising system settings such as energy detection thresholds, and may allow u sers to understand the limitations and capabilities of intra-operative gamm a probes.