Nodal status (i.e., whether the regional lymph nodes contain metastatic disease or not) is still regarded as an important prognostic factor in breast cancer and influences treatment decisions. Lymph nodes are not identical, and those, which are directly connected to the primary tumor site by means of their afferent lymphatic vessels, are the ones called sentinel nodes. The remaining lymph nodes of the same regional nodal basin are connected with each-other and/or the sentinel node(s), but do not have direct connection with the primary tumor site, and using a dichotomic categorization, they are labeled as nonsentinel nodes. The above definition implies that tumor cells which spread from the primary tumor to the regional lymph nodes as emboli via lymphatics reach the sentinel node(s) (first echelon nodes) first and nonsentinel nodes (second or further echelon nodes) only after. This explains why the sentinel nodes are the most likely site of nodal metastasis, and also suggests that if they are not involved by metastatic disease, the rest of the lymph nodes can also be considered free of metastases. Therefore the sentinel nodes “guard” the regional nodal basin, what explains their name.
The kinetics of tumor cell migration from the primary site to the sentinel nodes also apply to tracer molecules which travel via lymphatics, and first accumulate in the sentinel lymph nodes. Vital dyes (such as patent blue, isosulfan blue, or isocyanine green), 99-meta-technecium labeled sulfur or albumin colloids, or super paramagnetic iron oxide nanoparticles all use the same path, i.e. the lymphatic vessels to reach and selectively label the sentinel nodes. Depending on the tracers used, the sentinel nodes can be selectively removed after being detected by their color, (probe-detected) radioactivity or magnetism. Identification of the sentinel nodes with such tracers is called lymphatic mapping, and their removal sentinel (lymph) node biopsy or sentinel lymphadenectomy.
Lymphatic mapping studies have revealed that most sentinel nodes are located in the lower axilla, but some can be located at higher levels of the axilla or at extra-axillary sites like the subclavicular or supraclavicular regions, the breast itself (intramammary sentinel nodes), or along the internal mammary artery (internal mammary or parasternal sentinel nodes). The theoretical definition of the sentinel node given at the beginning of the present description can be complemented with a second practical one: the sentinel nodes are the ones that are identified as such during lymphatic mapping. Their number is highly variable, but is generally 1–3. If the number of sentinel nodes is consistently higher than this, one must doubt about the adequacy of lymphatic mapping; the American Joint Committee on Cancer has adopted a rule of allowing the (sn) classifier in the Tumor Node Metastasis (TNM) classification only for breast cancers where the number of sentinel nodes does not exceed 6. The pN0(sn), pN1(sn), or pN2(sn) TNM categories denote pathological nodal stage categories established with sentinel node biopsy.
Nodal staging of clinically node-negative (cN0) early breast cancer patients with sentinel node biopsy followed a stepwise evolution reflected in the surgical consequences of finding metastases in sentinel nodes. The concept of sentinel nodes was validated in different clinical trials, where lymphatic mapping was followed by complete axillary lymph node dissection. These studies proved that the sentinel nodes are really more likely to harbor metastasis than nonsentinel nodes. As sentinel nodes were often more thoroughly analyzed than the remaining of the lymph nodes, a few validation studies have also subjected nonsentinel nodes to the same scrutiny as the sentinel nodes, and these studies also reinforced the theory that sentinel nodes are more often involved by metastases (Turner et al. 1997; Weaver et al. 2000). Following these initial validation studies and early learning periods (all completed with axillary nodal dissection), the scenario of performing axillary lymph node dissection (as a surgical treatment of the axilla) only in patients with positive (i.e., metastatic) sentinel lymph nodes became the rule. Patients with negative sentinel nodes are spared from axillary clearance and its potential morbidity (lymphedema, motor dysfunction, sensory loss), although it must be admitted that sentinel node biopsy can also have complications, but less frequently and to a lesser degree. This new era of axillary nodal staging has also initiated a novel approach to teaching sentinel node biopsy: supervision by surgeons already trained in the procedure has become general and no complete axillary dissection was mandated any more to prove proficiency in removing the proper lymph nodes. As the decision to clear the axilla or not was dependent on the status of the sentinel nodes, intraoperative assessment has gained emphasis similarly to preoperative staging by axillary ultrasound. Patients with negative axillary palpation and axillary ultrasound findings (cN0 patients) are offered lymphatic mapping and sentinel node biopsy, whereas those with suspicious findings have ultrasound-guided sampling (mostly fine needle aspiration, but sometimes core needle biopsy) of the suspicious nodes or the most suspicious one. The microscopic evidence of nodal involvement excludes patients from lymphatic mapping, as the procedure is advocated for cN0 patients. Later, a few studies explored the idea that once nodal positivity was proven by means of sentinel node biopsy, there was no more need for axillary lymph node dissection in the majority of patients. The American College of Surgeons Oncology Group (ACOSOG) trial Z-0011 concluded that patients with limited nodal involvement (up to two metastatic lymph nodes independently of the size of the metastasis) undergoing breast conserving surgery complemented with whole breast irradiation needed no axillary nodal dissection (Giuliano et al. 2017). The 6-year results of the European Organization for Research and Treatment of Cancer (EORTC) After Mapping of the Axilla: Radiotherapy Or Surgery? (AMAROS) trial (Donker et al. 2014) and the 8-year results of the Hungarian National Institute of Oncology Optimal Treatment of the Axilla: Surgery Or Radiotherapy (OTOASOR) trial (Sávolt et al. 2017) both concluded that axillary irradiation was as effective as axillary lymph node dissection in preventing regional recurrences in patients with metastatic sentinel nodes. These results have also led to a change in policy: at present many patients with positive sentinel nodes do not have an axillary lymph node dissection (Lyman et al. 2016). This change had obviously resulted in a decrease in intraoperative examinations of the sentinel nodes, as the result had no impact on subsequent surgery.
Another setting of applying sentinel node biopsy as a staging procedure with lower morbidity is its use after primary systemic (neoadjuvant) treatment of locally advanced breast cancer. Neoadjuvant therapy is used for both clinically node-negative and node-positive patients. In the second set, it is widely accepted that sentinel node biopsy should be offered only to patients who become clinically node-negative after systemic treatment (ycN0). The false-negative rate (the proportion of cases without sentinel node involvement but with metastasis in nonsentinel nodes) of sentinel lymphadenectomy after neoadjuvant treatment of initially cN0 patients has been found to be comparable with that of sentinel lymphadenectomy of cN0 patient not requiring neoadjuvant treatment (Geng et al. 2016). It is therefore common to perform sentinel node biopsy after the completion of primary systemic treatment, but the procedure has also been performed before. For patients with initially node-positive disease, the false-negative rates have been found to be somewhat higher, but with at least two sentinel nodes identified, and adequate patient selection, post-neoadjuvant therapy sentinel node biopsy has also been found reliable enough (El Hage Chehade et al. 2016; van Nijnatten et al. 2015).
Most surgeons remove only axillary sentinel nodes and do not search for extra-axillary ones. The latter, to be known about, require lymphoscintigraphy, which visualizes them. Even if they are evidenced by lymphoscintigraphy, internal mammary sentinel nodes are rarely removed by surgeons, despite the fact that reports indicate that metastatic involvement of a parasternal sentinel node may impact on the planning of radiotherapy and the indication of systemic therapy. As a rule, internal mammary sentinel nodes are smaller than axillary ones, often less than 5 mm in size, and commonly they are identified by gamma-probes only on the basis of their radioactivity, and do not label with dyes.
The advantages of intraoperative cytology specimens include fast preparation, relatively low costs, lack of tissue loss during the procedure, and a microscopic verification of malignant cell morphology. Its disadvantages include the inability to measure the size of the metastasis, a higher false negative rate (metastases detected only after the histological examination of the sentinel node), a few false positive reports, and the need for training in cytopathology. The false negative rate can be diminished by increasing the surface sampled (i.e., several cut surfaces). Fast immunohistochemistry for the detection of epithelial cells can also be combined with intraoperative cytology; this increases the accuracy, the time required for reporting, and the costs of the examination.
Frozen sections have also often been used for the intraoperative examination of sentinel lymph nodes. They allow a better morphological evaluation of metastases including the measurement of size, and many pathologists are more familiar with the method. However, frozen section morphology is still suboptimal when compared to the gold standard of formalin-fixed and paraffin-embedded tissue histology. The tissues undergo artifacts, which may prevent ideal interpretation of the cells and structures seen. Some tissue is lost during assessment. It also takes longer time to prepare the sections. The costs are higher than those of imprints. Frozen sections are also not perfect, false negative cases do occur. The use of fast immunohistochemistry and/or the investigation of step sections improve the accuracy of the evaluation. Some centers have used the painstaking approach to investigate the whole sentinel nodes by frozen step sections to allow the most precise intraoperative microscopic sampling, but most have found the costs and workload of this approach prohibitive. Meta-analyses have assessed the accuracy of imprint cytology and frozen section evaluation, and it is suggested that the latter has a pooled sensitivity about 10% higher than the former for detecting macrometastases (those greater than 2 mm) intraoperatively. The choice of which morphologic intraoperative method to use depends on many factors, including personal preference and experience of the pathologist in charge (Cserni et al. 2003).
Molecular intraoperative examinations substantially differ from imprint cytology and frozen section analysis, as they do not use microscopy for detection. In contrast, they destroy the tissue structures and cells to allow the extraction of messenger ribonucleic acid (mRNA) of proteins thought to represent metastatic tumor cells proportionally to their amount. Different amplification systems have been used, most often one step nucleic acid amplification (OSNA) of cytokeratin 19 mRNA or reverse transcription polymerase chain reaction (RT-PCR) of cytokeratin 19 and mammaglobin mRNAs. These methods are more sensitive than classical microscopic examinations, but obviously are rather epithelium than metastasis specific. Validation studies have been biased by the fact that microscopic evaluation and molecular analyses cannot be performed on the same tissue parts, therefore different parts of the same sentinel lymph nodes were investigated by the different methods. It is accepted that different parts of the sentinel nodes have different risks of being involved by the metastatic process, i.e., the areas at the junction of the tumor draining afferent lymphatic vessels are involved at a much higher frequency than areas away from this point. This is why, the best comparisons were made when alternating slices of the lymph nodes were subjected to molecular and microscopic examinations. The comparison of larger series suggests that molecular methods are substantially more sensitive in detecting micrometastatic involvement of the sentinel nodes than either frozen sections or imprint cytology, but when it comes to macrometastases, frozen sections are nearly as sensitive as molecular methods (Cserni 2012). To allow a nearly complete assessment, many laboratories using a molecular intraoperative assay use most if not all nodal tissue to adequately sample the sentinel nodes, and this leaves no tissue for a morphological evaluation. A common recommendation proposes to use at least one slice of tissue (generally the central one) for histology and use only the remaining parts of the sentinel node for the molecular assay. Recommendations are in agreement that intraoperative assessment of any type should be restricted to cases where it has immediate influence on the intervention to follow (Wells et al. 2012).
At the beginning, the definitions of isolated tumor cells were not straight forward and uniform, and this has limited the proper prognostic evaluation of this subcategory, and its discrimination from micrometastasis. At present, it seems that neither isolated tumor cells or clusters nor micrometastases have major impact on breast cancer patients and therefore should not initiate any therapeutic intervention on their own. This clarification of the prognostic impact of low-volume tumor deposits in sentinel nodes discovered by enhanced pathological evaluation has resulted in recommendations proposing not to search for micrometastases and smaller volume involvement, but to devise assessment protocols that identify virtually all macrometastases (Cserni 2012).
Similarly, epithelial displacement by needling procedures (e.g., core needle biopsy, wire localization of the primary tumor, or intratumoral injection of the tracer) or massage used to help tracer migration during lymphatic mapping can lead to the misdiagnosis of displaced epithelium as a real metastasis. Papillary lesions have been reported to be especially prone for misplacement: most misplaced epithelial tissue reported to date was related to papillary lesions. These may differ from metastatic cells or fragments by location (in the sinuses rather than the parenchyma of the sentinel node), morphology, and/or immunophenotype; and not uncommonly the primary site has also features of misplaced epithelium (e.g., papillary fronds in lymphatics in an area of traumatized breast tissue showing wound healing, granulation, hemosiderin, or frank hematoma). As displaced epithelium never takes the form of a macrometastasis, misdiagnosing them as metastatic in nature should imply no erroneous indication of surgical or systemic treatment or radiotherapy.
Trials have been organized for identifying subsets of breast cancer patients who might not require sentinel node biopsy either.
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