Melanocytic Nevi in Lymph Nodes

The presence of benign melanocytes within lymph nodes was first reported by Stewart and Copeland. The frequency of their detection in lymph nodes ranges from less than 1% to up to 25% in some studies. It is likely that this variation reflects differences in study populations, the extent of lymph node tissue sampling, and the utilization of immunohistochemical stains. There is some evidence that the frequency of nevus cells within lymph nodes is correlated with phenotypic characteristics of the patient population. For example, some studies have suggested that nevus cells within lymph nodes are more frequently observed in patients with higher numbers of cutaneous nevi.

There are two main hypotheses to explain the presence of benign melanocytes in lymph nodes. In one theory, nodal nevi are the consequence of aberrant migration of melanocytes during embryogenesis from the neural crest. Another explanation suggests that nodal nevi are derived from cutaneous nevomelanocytes that have traveled to the lymph node through intralymphatic spread. In our view, the latter theory is more plausible, at least for most nodal melanocytic nevi, for two reasons. Firstly, protrusion of melanocytes into lymphatic channels is a not uncommon finding in cutaneous biopsies of nevi, suggesting that on occasion melanocyte aggregates may become dislodged and be transported to the lymph node similar to other benign dermal tissue. Secondly, nodal nevus cells are usually present within intracapsular region near the region of entry of afferent lymphatics into lymph nodes which is also more consistent with this hypothesis. It is possible that both mechanisms and possibly even additional ones (e.g., extravascular migration) may apply depending on the clinical setting (acquired versus large congenital nevus).

Clinical Findings

Nodal melanocytic nevi are usually found incidentally, nowadays most often as part of a sentinel lymph node mapping and biopsy procedure to stage patients with a primary cutaneous melanoma or other tumors (e.g., breast cancer). They are more common in young than old adults and more common in patients with numerous melanocytic nevi.

Histopathologic Findings

Location of Nodal Nevomelanocytes

Melanocytic nevi in lymph nodes are most often located in the fibrous tissue of the capsule ( Fig. 26.1 ) and/or trabeculae ( Fig. 26.2 ) but not exclusively. Isolated solitary units of melanocytes or nest can also be found in the subcapsular sinus or lymph node parenchyma ( Figs. 26.3 and 26.4 ). Intranodal nevomelanocytes are more readily recognized and accepted as such when they are associated with intracapsular or intratrabecular nevus cells, but finding an intracapsular nevus is not a prerequisite for diagnosing an intraparenchymal nevus cell. Depending how a section was taken, nodal nevus cells may be or may seem to be located in the subcapsular sinus or nodal parenchyma only. Sometimes, nodal nevi may also grow into perinodal adipose tissue ( Fig. 26.5 ). In a two-dimensional cross-section of a lymph node, nodal nevi may be present as a single focus, but some lymph nodes contain multiple foci of nodal nevi ( Fig. 26.6 ).