ABSTRACT
Morton et al. in 1992 , who brought this modality to the clinical arena by demonstrating its utility in identifying “at risk” node basins among 223 cutaneous melanoma patients. Since this report, validation of the strategy has occurred in a number of solid tumors including, head and neck, endocrine, gastrointestinal, genitourinary, breast, and reproductive tract cancers. The ideal radiocolloid must gain access to the lumen of the initial lymphatic channel in sufficient quantity for the lymph vessels to be seen on the dynamic scans. It should combine a rapid and predictable transport toward the sentinel node with persistent retention. The particle size of the radiocolloid is a critical factor in the ease with which these tracers enter the lymphatic system ( Table 1 ). Large particles (500-2000 nm) remain trapped at the injection site and small particles (4-5 nm) will penetrate the capillary membranes and will not be available to migrate through the lymphatic channels ( Ege 1976 , Henze et al. 1982 ). In the United States, the most commonly used radiopharmaceutical is filtered technetium-99 m sulfur colloid. This agent has a small particle size (<100 nm), it is uniformly dispersed, highly stable, and has a short half-life (gamma-emitter). Injection flow rate of a radiocolloid is important in the success of sentinel node identification. Once the particles enter the lumen of the lymphatic capillaries, they will move freely and uniformly toward
Table 1 Characteristic of Radionuclides
the draining lymph nodes. The valves in the lymphatic vessels will generally not allow retrograde flow. Lymphatic flow is fastest in the leg and foot and slowest in the head and neck. The study is performed by injection of 2 to 4 cm 3 (1-2.5 mCi) intradermally and perilesionally as with blue dye. A lymphoscintigram is then made to visualize localized uptake ( Fig. 6 ). Dynamic images are usually acquired for a total of 20 minutes. The lymphatic channels are best appreciated by summing the individual dynamic frames to produce a composite dynamic image. Delayed scans are then performed at 2.5 to 3 hours following injection of the radiocolloid tracer. These delayed scans should include all node fields that can possibly receive drainage from the injection site. Each static acquisition should be 5 to 10 minutes in length to ensure that even very faint sentinel nodes are detected ( Uren and Howman-Giles 2002 ). Newer radiologic technologies such as SPECT-CT (Fig. 7) are becoming more routinely used. This exam combines traditional planar lymphoscintigraphy with computed topography (CT) to locate the sentinel node in a three-dimensional image as opposed to the traditional two dimensions seen on lymphoscintograms. The intraoperative detection of the sentinel node relies not only on the visual inspection of the lymphatic basin, but also on the assessment of the radioactive colloid in the sentinel node through the aid of a gamma detection device. This hand-held sensor contains a gammasensitive crystal with a preamplifier, and a reading unit ( Fig. 8 ). There are several gamma probes for intraoperative use including laparoscopic devices that can be used in a large spectrum of clinical scenarios. Specificity and accuracy of these devices can be augmented with the use of collimation which will help to reduce background and “bleed-through” radioactivity, frequently encountered in sites where the primary tumor is near its drainage lymphatic basin ( Fig. 9 ). The parametrial nodes in uterine cervical primary and the medial inguinofemoral nodes in an anterior vulvar cancer are good examples of challenging mapping areas. Use of these radiopharmaceuticals appears to be safe given their low dose energy, particle size, and rapid washout rate. Extensive testing has been conducted to determine the safety to health care workers. The amount of radiation exposure from the technique is very small and the cumulative effect is still well within acceptable levels ( Eshima et al. 2000 ).
