children (15%–50% versus 80%), but microscopic metastases have been found in up to 80% of the adult patients [8.37]. The central compartment level is involved in approximately 90% of the cases. Involvement of the lateral LNs varies between 51% and 100% in different series, the caudal compartments being involved more frequently than the cranial compartments. Supraclavear LNs are the third site involved in terms of frequency, with a reported rate ranging from 10% to 52%. Contralateral LN involvement is not rare, with an incidence of up to 18% for papillary thyroid cancer [8.38]. It is important to note that distribution of locoregional LN involvement is poorly related to the site of the primary thyroid tumour.
8.3.1.2. Added value of SLNB in differentiated thyroid carcinoma
SLNB was developed as an alternative to elective LN dissection in patients with cN0 disease. Few studies evaluating the utility of SLNB in differentiated thyroid cancer have been published. An initial study by Kelemen et al. [8.39]
using blue dye in 17 thyroid cancer patients demonstrated the feasibility of SLNM in thyroid surgery, although the disruption of lymphatic pathways frequently observed during thyroidectomy can make SLN identification difficult. Moreover, parathyroid glands may stain blue, and this may cause their inadvertent removal.
Similar results were obtained by Pelizzo et al. in a group of 29 patients with differentiated thyroid cancer [8.40]. Gallowitsch et al. [8.41] and Rettenbacher et al. [8.42] described the use of a radiotracer with intraoperative counting using a hand-held gamma probe.
Since these early reports, a number of other studies have demonstrated that SLNB is indeed an accurate technique for obtaining information about cervical LN involvement in patients undergoing thyroidectomy [8.43].
point source can help to delineate the cervical contour and to mark the cutaneous projections of the radiolabelled areas.
8.3.2.1. Added value of SPECT/CT imaging in pre-operative imaging
Interpretation of planar images can be difficult because the anatomy information is limited to outlining the body contour. In particular, in these patients, SLNs in the neck region can be difficult to localize as a result of complex anatomy, interlacing lymphatic vessels, unexpected drainage patterns and because the 3-D surface of the structures of the head is not visualized in planar images. Furthermore, SLNs in proximity to the radiocolloid injection area can easily be missed on planar images because some 98% of the injected activity does not migrate from the interstitial injection point, and therefore masks the average 0.16% that ends up in the SLNs.
SPECT/CT imaging can optimize SLN visualization in the head and neck region, thus leading to improved intraoperative detection [8.44–8.48]. In these cases, such an imaging technique is of high value for identifying the topography of the SLNs in relation to several vital vascular and neural structures, to enable their safe removal (see Fig. 8.1).
FIG. 8.1. SLNM in a 75 year old woman with papillary microcarcinoma of the left thyroid lobe. Left: Anterior planar view obtained 2 h after US guided intratumoural injection of 99mTc nanocolloid; radioactivity accumulation in the liver indicates passage of some radiocolloid into the general circulation at this relatively late time point post-injection. The injection site corresponds to the area with greatest radioactivity accumulation just left of the midline; multiple SLNs are visualized in the left cervical region, but the arrow points to the node with the highest radiocolloid uptake, also visualized as the first draining node on prior sequential imaging (the primary SLN). Right: Fused SPECT/CT sections (transaxial, sagittal and coronal) showing more accurately than planar imaging the exact topographic location of the primary SLN (indicated by yellow arrows, in level IV). Images courtesy of L. Feggi and S. Panareo, Nuclear Medicine Service, University Hospital, Ferrara, Italy.
SPECT/CT imaging can also detect SLNs that are missed on planar lymphoscintigraphy in a substantial number of patients. In head and neck cancer, many SLNs are located at a close proximity to the injection area and are therefore easily overlooked in planar imaging (see Fig. 8.2). Cases of non-LN radiocolloid accumulation (e.g. leakage of radioactivity in the oral cavity after injection or contamination on the skin) can be identified using SPECT/CT imaging, while distinguishing between leakage and a true SLN in planar images.
8.3.2.2. Intraoperative phase
At the time of surgery, intraoperative SLN localization is performed using a hand-held gamma probe. After thyroidectomy, the central compartment is bilaterally scanned, searching for other hot spots. Generally, the most radioactive LN and all nodes with a count rate more than 10% of that of the hottest node are removed. Then, the same exploration is carried out for lateral compartments of the neck. The anatomical location of all resected radioactive LNs is recorded and SLNs are sent for intraoperative frozen section histology. When frozen sections of one or more LNs reveal thyroid cancer metastasis, the surgeon performs an enlarged LDN of the involved compartment.
The surgical bed is then checked again to evaluate the completeness of removal of all hot spots. Definitive histology of all resected specimens is performed with haematoxylin and eosin staining, as well as with immunohistochemistry using an antithyroglobulin antibody.
FIG. 8.2. SLNM in a 71 year old woman with papillary carcinoma of the left thyroid lobe.
Left: Anterior planar view obtained 2 h after US guided intratumoural injection of
99mTc nanocolloid. The injection site corresponds to the area with greatest radioactivity accumulation in the left cervical region; only one LN is visualized, just below the thyroid approximately at the midline (grey arrow). Right: Fused SPECT/CT sections (transaxial, sagittal and coronal) selected so that they show part of the radioactivity accumulation at the injection site (visible in the sagittal section, not indicated by a specific mark) and a draining LN in level III (orange arrow); this second SLN had not been identified in the planar image because of the close proximity to the injection site. Images courtesy of L. Feggi and S. Panareo, Nuclear Medicine Service, University Hospital, Ferrara, Italy.
8.3.2.3. Added value of intraoperative imaging
The surgeon can localize SLNs with the aid of another visual element that facilitates the procedure. SLNB is based on a combination of gamma probe counting and blue dye mapping. Surgeons generally localize the SLNs by combining the auditive signal (gamma probe) with the visual one (blue dye). In head and neck cancer patients, the use of blue dye can be problematic, mostly because the blue dye migrates very quickly. For these reasons, use of the blue dye technique in head and neck patients is limited, considering the high density of LNs in the region and the short distances between the injection site and the SLN(s).
Portable, small FOV gamma cameras have been designed to facilitate radioguided surgery. Intraoperative real time imaging using the portable gamma camera provides an overview of all radioactive hot spots in the whole surgical field. The position of the camera can be adjusted to visualize SLNs near the injection area, which are nodes that are easily overlooked using the gamma counting probe alone. Discrimination between SLNs and second tier LNs is facilitated because the amount of radioactivity within each node can be quantified using the portable gamma camera, and the intraoperative images can be related to the pre-operative scintigraphic images. Furthermore, continuous image monitoring can be used to record the whole procedure, such as stepwise monitoring enabling localization of SLNs and detection of remaining activity after LN harvesting.