Answer: Nevocellular aggregate

Histology: Within the fibrous capsule of the lymph node are bland cells with a nevoid appearance. Some of the cells contain nuclear clearing (“pseudoinclusions”). In addition, the same cells are seen deeper within the lymph node infiltrating between adipose tissue.

Discussion: Metastatic lobular carcinoma from the breast may have a very bland appearance in the lymph node. However, metastatic lobular carcinoma cells tend to be loosely cohesive in contrast to the nested appearance of the cells seen in the current case. Also, the cells within lobular carcinoma tend to be round, rather than have the ovoid appearance of nevoid cells. Furthermore, metastatic breast cancer cells are present within the sinuses of the lymph node rather than in the fibrous septae. Metastatic melanoma similarly would be within the sinuses of the lymph node and would have a greater degree of cytologic atypia than the cells seen in the current case. Blue nevi may be present ectopically within the lymph node. However, the cells of a blue nevus are heavily pigmented and dendritic in contrast to the cells of a nevocellular aggregate. Nevocellular aggregates within axillary lymph nodes are not uncommon. This case is somewhat unusual for a couple of reasons. First, the extent of the process within the lymph node is greater than typically seen. Usually, only a small collection of nevus cells are seen within the fibrous capsule of the lymph node. Also, this case is unusual in that the nevus cells are present deeper within the lymph node admixed with adipose tissue. Nevocellular aggregates may extend typically deeper into the lymph node but are usually more obviously associated with penetrating fibrous septae within the lymph node. The finding of nevocellular aggregates is of purely academic interest, as it has no clinical ramifications. One must be particularly attuned to this lesion when looking at sentinel lymph nodes removed in patients with malignant melanoma.