False-positive rate of the immunoperoxidase stains for MART1/MelanA in lymph nodes

MART1 and MelanA are considered sensitive markers of melanocytic differentiation and are used to increase the detection of melanoma micrometastases in sentinel lymph nodes (SLNs). However, the false-positive rates of these two antibodies have not been adequately evaluated. We examined 217 lymph nodes (LNs) from patients with no history of melanoma: 117 SLNs from breast cancer patients, 79 LNs from other nonmelanoma malignancy patients, and 21 reactive LNs. Capsular melanocytic nevi were identified in 5 SLNs from 5 breast cancer patients by both antibodies. Two of these 5 SLNs with capsular nevus also contain MART1- and MelanA-positive cells within the lymph node parenchyma. Individual immunoperoxidase-positive cells were also identified within the parenchyma of lymph nodes without capsular nevus (9 LNs with MART1 and 3 LNs with MelanA). The false-positive rate is 5.1% for MART1 and 2.4% for MelanA. In conclusion, MART1- or MelanA-positive cells may be present in lymph nodes from patients without melanoma. Therefore, MART1- and MelanA-positive cells in SLNs from melanoma patients, without corresponding atypia or hematoxylin and eosin findings, should be interpreted with caution.