Malignant melanoma primary in lymph node. The case of the missing link.

When malignant melanoma occurs in a lymph node, it is presumed to be metastatic, and the primary cutaneous/mucosal process is discovered to be either active, spontaneously regressed, previously excised, or occult. Nevus cell aggregates and nodal blue nevi are well-documented morphologic curiosities occurring in lymph nodes. It has been hypothesized that these nevus cell aggregates and blue nevi could be progenitors of lymph nodal malignant melanoma in patients without an obvious extranodal site of origin. We document a prototypical case of primary malignant melanoma evolving from precursor nevus cell aggregates associated with blue nevi in an axillary lymph node. The coexistence of nodal blue nevi and nevus cell aggregates implies a common origin from migratory neural crest cells arrested within mesenchyme.