Picomets: Assessing single and few cell metastases in melanoma sentinel lymph node biopsies

Lymph node involvement is a significant prognostic factor for melanoma. Both number of positive nodes and disease burden within a lymph node affects survival. However, the significance of few tumor cells within a single node and subsequent optimal management remains without consensus. We investigated the implications of minimal nodal disease on clinical outcomes.

We reviewed 752 patients who underwent lymph node sampling at time of primary melanoma resection at our institution over 15 years. We deemed patients who had 1 node with 1 to 4 atypical cells staining positive for either Melan-A or Sox-10 as having "picomets." We examined the initial clinicopathological features, subsequent management, and outcomes.

Thirty-three patients (4%) met criteria for having picomets. The most common number of positively staining atypical cells was 1 (n = 13). Nodal staging at initial pathology review varied, and overall stage ranged from IA to IIIC. Four patients underwent further therapy, none of whom had recurrent disease. Of the 29 patients undergoing observation/surveillance only, 5 had disease recurrence (17%).

Although patients with picomets had better outcomes than historical stage matched cohorts, a small subset had recurrent disease. Staging patients with picomets as "N0" may not reflect the true negative prognostic significance of picomets. A larger population of patients meeting picomets criteria is needed to draw further conclusions.