Thin malignant melanomas and recurrence potential.

Of 846 patients with stage I malignant melanoma that was less than 0.76-mm thick who were followed up for two to 31 years, 61 (7.2%) developed a recurrence. For those patients who did not initially undergo an elective lymph node dissection, the majority of first recurrences were at regional lymph nodes. Attempts have been made to identify those patients at risk of relapsing. Axial lesions, particularly those on the scalp, had the highest recurrence rate, with 15% of all thin scalp lesions recurring compared with only 4% of all thin extremity lesions. Three histological features proved to be useful prognostic indicators when analyzed by single-factor analysis. Evidence of ulceration in the primary lesion increased the recurrence rate from 6.7% to 26.1%. While only 4.3% of lesions displaying low mitotic activity recurred, this rate rose to 23.8% for those lesions of a high mitotic grade. Only 5% of Clark's level II lesions recurred, compared with about 12% of lesions at either level III or IV. Evidence of regression in thin lesions had no deleterious effect on prognosis. This study defines a small subset of patients who may benefit from elective lymph node dissection.