Minimum acceptable sensitivity of intraoperative gamma probes used for sentinel lymph node detection in melanoma patients

The aim of this study was to determine the suspension level for the sensitivity of an intraoperative scintillation gamma probe in the detection of the sentinel lymph node (SLN) in melanoma patients.Thirty-eight consecutive patients with melanoma were enrolled in the study during a 12-month period and underwent lymphatic scintigraphy after the peritumoral intradermal administration of about 14MBq of ^99mTc-nanocolloids. The SLNs were successfully removed during the surgical intervention about 4h later.To identify and localize the SLN, a scintillation NaI(Tl) collimated probe was used. Predictably, the probe sensitivity decreased as the photopeak energy window was progressively narrowed, from 6.9±0.7 counts per second (cps)/kBq (designated as the 'optimum,' or 'OPT,' sensitivity) to 2.5±0.3 cps/kBq (LOW sensitivity) and to 1.4±0.2 cps/kBq (VLOW sensitivity).Maximum lymph node count rates (cps) were determined for the foregoing energy windows prior to skin incision (PRE_OPT, PRE_LOW, PRE_VLOW, respectively) and invivo after incision (INV_OPT, INV_LOW, INV_VLOW).Forty-three SLNs were removed with a mean source-to-detector distance of 46±24mm (min 12mm, max 92mm). Four SLNs could not have been detected using PRE_LOW. This figure would have decreased to 34, with nine undetectable lymph nodes, with PRE_VLOW. One SLN could not have been identified using INV_LOW and four could not have be identified using INV_VLOW.In the clinical scenario of SLN detection in melanoma patients, a system sensitivity of 2.5cps/kBq represents a suspension level, that is, a level under which the equipment must be suspended from clinical use and the poor performance must be investigated.