In the 1990s, Morton et al., introduced the concept of sentinel lymph node (SLN) mapping and biopsy, which revolutionized the assessment of nodal status in melanoma and breast cancer [1
]. The underlying hypothesis of SLN mapping is that the first lymph node to receive lymphatic drainage from a tumor site will contain tumor cells if there has been direct lymphatic spread [2
]. Patients in whom SLN sampling does not reveal the presence of tumor are spared the morbidity of radical lymph node dissection.
Current techniques for SLN mapping involve preoperative injection of a radioactive colloid tracer (e.g., technetium-99m sulfur colloid) followed by intraoperative injection of a visible blue dye (e.g., isosulfan blue). The dye permits limited visualization of afferent lymphatic vessels and the SLN, while the radioactive colloid tracer improves detection rate and confirms complete harvest of the SLN with the use of an intraoperative handheld gamma probe [3
]. The learning curve associated with conventional SLN mapping is steep [4
], the technique itself requires ionizing radiation, and the blue dye is extremely difficult to find in the presence of blood and anthracosis.
There are three important parameters in designing a lymphatic tracer for SLN mapping: hydrodynamic diameter (HD), surface charge, and contrast generation. Molecules with a HD less than approximately 10 nm have the potential to travel beyond the SLN. For very small agents, such as isosulfan blue, this can result in missing the SLN, but more likely, will result in more than one nodal group in the same chain being labeled. Very large molecules, in the range of 50–100 nm have difficulty even entering lymphatic channels, and travel so slowly that up to 24 hours may be required to label the SLN (reviewed in [5
]). With respect to surface charge, anionic molecules have rapid uptake into lymphatics and excellent retention in lymph nodes [6
]. With respect to contrast generation, agents presently being used clinically are either radioactive gamma emitters or colored dyes.
In 2004, our group introduced near-infrared (NIR) fluorescent quantum dots (QDs) for SLN mapping and resection. NIR light, otherwise invisible to the human eye, provides extremely high signal to background ratios (SBRs) without changing the look of the surgical field. A thorough discussion of the use of NIR light in biomedical imaging has been published previously [7
]. When combined with a suitable intraoperative imaging system [8
], the advantage of NIR quantum dots for SLN mapping include high sensitivity, real-time and simultaneous visualization of both surgical anatomy and lymphatic flow, and non-radioactive detection. In this manuscript, we describe in detail the production and use of type II NIR fluorescent quantum dots for SLN mapping and resection. These NIR QDs have been specifically engineered with a HD (15–20 nm) that permits rapid uptake into lymphatic channels but ensures retention in the SLN, a highly anionic surface charge, maximal absorption cross-section, and a suitable quantum yield.