The new AJCC guidelines in practice: utility of the MITF immunohistochemical stain in the evaluation of single-cell metastasis in melanoma sentinel lymph nodes.

The American Joint Commission on Cancer 2010 guideline changes recommend all patients with single-cell melanoma metastasis identified by immunhistochemical staining of sentinel lymph nodes to be classified as N1 stage. In this study we evaluated the utility of adding microphthalmia transcription factor (MITF) to our current sentinel node evaluation protocol. Twenty benign lymph nodes from nonmelanoma patients were stained with MITF, MART-1/Melan-A, and HMB-45. Hematoxylin and eosin-stained and immunohistochemically stained slides were assessed for the frequency of positive staining, pattern and location of staining, and for the presence of pigment within the node. Fifteen of 20 (75%) lymph nodes evaluated showed >1 cell staining with MITF in variable locations and patterns throughout the lymph node. MART-1/Melan-A-positive cells were identified in 4 of 20 cases (20.0%), mostly in the sinusoidal location (3/4). Fifty percent (10/20) of the lymph nodes contained cells that stained positive for HMB-45. Nine of 10 of the HMB-45-positive cells were in a sinusoidal location corresponding to easily discernible pigmented histiocytes. We conclude that MITF stains nonmelanocytic cells in benign lymph nodes 75% of the time. MITF has a much higher rate of positive staining in benign lymph nodes compared with MART-1/Melan-A, which had positive staining only 20% of the time. MITF showed 2 different staining patterns, type A and type B. Both patterns could be difficult to distinguish from true metastasis, and thus adding MITF to sentinel node evaluation panels is not helpful and may be histologically vexing.