Sentinel Lymph Node Biopsy

Dr Debra Hean - MBChB FC Path (SA) Anat

PATHCHAT NO 17: SENTINEL LYMPH NODE BIOPSY

Introduction

Sentinel lymph node biopsy (SLNB) has evolved over the years to become one of the most useful tools in the management of malignant melanoma. Large, multi-institutional studies have confirmed that in experienced hands it is an accurate and reliable technique to identify tumour in draining lymph nodes. SLNB is also a highly accurate staging method and represents the most important prognostic factor in patients with early stage disease. SLNB is not used for primary diagnosis of melanoma, but is rather a tool for staging and prognostication.

The sentinel node, in the context of SLNB, refers to a node which acts as an “indicator” for the whole lymph node group. It is the node which receives direct drainage from the tumour itself. The affected node is located by injecting blue dye and radioactive material into the primary tumour site. It is assumed that this node would be the first in the group to be affected by metastatic disease, and if it is negative then the other nodes are assumed to be negative, and a complete node dissection is not undertaken. Similar principles apply to sentinel lymph nodes in breast cancer.

The most common site of metastatic disease in melanoma is the draining lymph nodes. Before the modern SLNB era pioneered in the 1980s and early 1990s, patients with clinically localised melanoma considered at intermediate to high risk of metastatic disease were offered prophylactic regional lymph node clearance. However, it was noted that only 20% of these patients actually had nodal disease, exposing the remaining 80% to unnecessary anaesthetic complications and surgical morbidity. By identifying, removing and carefully examining only those lymph nodes receiving direct drainage from the primary site i.e. the sentinel nodes (SNs), it has became possible to accurately determine the status of the entire field, with a minimally invasive technique. Complete lymph node dissection (CLND) is now restricted in most centres to those patients with either clinically detected metastatic disease or demonstrated metastatic disease in sentinel nodes (SNs).

As such, determination by pathologists of the presence or absence of metastatic melanoma in SNs is critical to providing patients with an accurate estimate of prognosis and identifying those patients who may benefit from immediate CLND. There is preliminary clinical trial evidence to suggest that early CLND in patients with positive SNs may improve outcome.

Indications for sentinel lymph node biopsy

In early melanoma, SN status is the most important predictor of survival. In general, SLNB is done for patients with a primary tumour thickness of between 1 and 4mm. The role of SLNB in patients with thin melanomas is not well-defined. The incidence of a positive SN in this patient population is low (approximately 5%).The choice to perform a SLNB in these patients must be weighed with risk factors such as Clark level, ulceration, sex and mitotic rate of the primary tumour. In patients with thick melanomas (>4mm), most studies have supported the prognostic significance of SN status.

Preparing sentinel lymph nodes for pathologic examination: what does the pathologist need from the surgeon?

Sentinel nodes should be removed intact, preferably with a thin rim of adipose tissue, and should be devoid of crush or diathermy artifact. The request form should indicate the number of removed SNs and their anatomical locations. Any second tier or non-sentinel nodes that have also been removed should be indicated as such on the request form. SNs should be sent to the laboratory either fresh or in 10% buffered formalin.

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Sentinel Lymph Node Biopsy