SPECT/CT Lymphoscintigraphy to Locate the Sentinel Lymph Node in Patients with Melanoma

Abstract

The major strength of nuclear medicine imaging has always been its high functional resolution, and a good example of this is lymphoscintigraphy using radiocolloids to map the location of the lymph node receiving direct lymph drainage from a solid tumor site. Using this technique the one or two sentinel nodes of the 1,000 or more nodes in the patient are radiolabelled and clearly visible on lymphoscintigraphy. This same technique, however, also nicely illustrates a limitation of nuclear medicine images: their low anatomical resolution. The surface location of these sentinel nodes (SN) can be marked and we can measure the depth of the SN from the skin, but the planar nuclear medicine images provide no anatomical information. Nuclear medicine images can be improved using single photon emission computed tomography (SPECT), which displays tomographic slices of the distribution of tracer in the patient. SPECT was developed in the late 1960s by Kuhl and Edwards in Philadelphia before computed tomography (CT) scans were developed. SPECT images were better than planar scans as there was improved image contrast of pathological lesions, but anatomical data in these images was still limited; when using SPECT scans in lymphoscintigraphy it is almost nonexistent. The limitations of this low anatomical resolution in nuclear medicine images, whether planar or tomographic, is dramatically illustrated in the case of F-flourodeoxyglucose (FDG) positron emission tomography (PET) scanning. This method was developed in the 1970s and, although of great interest to nuclear medicine physicians as it displayed the glycolytic rate of tumors, it had little impact on clinical practice as clinicians had no confidence in the information. Very little anatomical data was present even in the three-dimensional (3D) tomographic PET displays. It was not until hybrid imaging devices were produced that allowed PET/CT scans to be performed without moving the patient that the situation changed. PET/CT allowed precise image fusion of the PET tomographic slices with the corresponding CT anatomic slices with the PET functional data displayed in color against the grey scale background of the CT image. Immediately clinicians could appreciate the importance of the information in these images and PET/CT has now assumed a major role in the staging of solid tumors and in the detection of recurrence. These scans can also be used to monitor the effectiveness of chemotherapy regimens by directly measuring the metabolic activity of the tumor. It is already clear that the equivalent technology for single photon emitting radionuclides, SPECT/CT, is going to have a similar impact on the clinical usefulness of these tracers. SPECT/CT as part of lymphoscintigraphy is an excellent example of the fusion of the high functional resolution of nuclear medicine images (the radiolabelled SN) with the high anatomical resolution of CT. The technology shows precisely which node is the SN, and its detailed anatomical position can be described. Several studies have shown that, in patients with melanoma, SPECT/CT provides advantages over planar imaging. These include higher sensitivity for SN detection, detection of SNs in new node fields, improved detection of SNs when near the injection site, detection of deep pelvic SNs in patients with leg melanomas, and improved surgical location by providing exact anatomical location. The study in this issue by van der Ploeg and colleagues, in a group of patients with melanoma with a planar study that was difficult to interpret, an unusual drainage pattern or no drainage to a SN, confirmed increased sensitivity for detecting the SN and also improved anatomical location Society of Surgical Oncology 2009