Aggregates of nevus cells in the lymph node hilus, capsule, or trabeculae.

Aggregates of nevus cells within peripheral lymph nodes, first described by Stewart and Copeland (1), have been reported more frequently in recent years since the introduction of sentinel lymph node examination in melanoma and breast carcinoma (2,3,4,5,6,7,8,9,10). Nevus cells in lymph nodes associated with congenital cutaneous nevi and with blue nevi have also been reported (11,12,13,14,15). A greater incidence of nodal nevi seems to occur in the regional lymph nodes excised from melanomas as compared to carcinomas (7,16). Two explanations for the origin of lymph node nevi have been considered; however, because of lack of satisfactory documentation, neither has been generally accepted. According to one opinion, nevus cells in the lymph nodes may represent arrested aberrant embryonal migration from the neural crest. Johnson and Helwig (2), discussing this possibility, indicated that reports of blue nevi in unusual locations, such as the prostate, uterine cervix, and vagina, provide similar examples of abnormal embryonal migration (11,12). According to another opinion, the nevus cells migrate to the lymph nodes in adult life and therefore represent “benign metastases” from a cutaneous site (3,6,17). The mechanism of this cell transfer, possibly through lymph or blood vessels, is not clear. Bell et al. (6), studying serial sections of 124 nevi, found frequent subendothelial hillocks of nevus cells protruding into the lumina of lymphatic vessels and occasional free clusters of cells in the vascular spaces. The breaching of the lymphatic endothelial lining shown in this study provides support for the idea that benign nevus cells can embolize, lodge, and grow in the regional lymph nodes without evidence of further malignant invasion.

Another peculiar aspect of aberrant nevus cells is their exclusive location in superficial skin-draining lymph nodes (3,17). All 46 cases of lymph node nevus inclusions reported as of 1977 were located in peripheral lymph nodes; no inclusions in visceral lymph nodes were reported (5,6). Among all the cases, the axillary lymph nodes were by far the most frequently involved group, followed by cervical and inguinal lymph nodes (7,8). Whether this is because of special local conditions or because the axillary lymph nodes are more often examined is not known. In a study of lymph nodes removed for various visceral and cutaneous lesions, nevus cells were found in 6.2% of all axillary lymph nodes examined (3). However, in the extensive study by Ridolfi and associates (5), nevus cell aggregates were present in only three of 17,504 lymph nodes (0.017%) obtained from 909 mastectomies (0.33% of cases), and in three of 2,607 lymph nodes (0.12%) obtained from 100 dissections for malignant melanoma (3% of cases). Similarly, in other studies, the frequency of nevus cell aggregates in lymph nodes was far greater in those removed for melanoma (22%) than in those removed for nonmelanoma cases (0.33% to 7.3%) (3,7,16,18).