Lethal "thin" malignant melanoma. Identifying patients at risk.

Thin melanomas can metastasize and be lethal. The purpose of this review was to identify negative risk factors in patients with melanomas less than 0.76 mm thick. Six hundred and eighty-one (681) such patients are reviewed in this study. Of those referred without metastatic disease (583 patients), metastases developed in 4.8% after a mean followup of 3.6 years. Of those referred with metastatic disease (98 patients), mortality was 35% after a mean followup of 5.9 years. Male patients (p less than 0.04) and patients with axial primaries (p less than 0.05) were at an increased risk of metastasis. Severe histologic regression was present in 40% of the primary lesions that metastasized and in only 17% of similar lesions that did not (p less than 0.001). Increased age was associated with increased local skin metastases, but not with increased nodal or distant metastases. A prognostic model was designed, using two clinical risk factors (axial primary site and male sex) and two histologic risk factors (Clark's Level IV and severe histologic regression). The prognostic model identified a low-risk population--women with extremity primaries--with an actuarial risk of metastasis at 10 years that was less than 3%. Patients with either (1) both clinical risk factors or (2) one clinical risk factor and one histologic risk factor were identified as high-risk patients. Their actuarial risk of metastasis was 11% at 5 years and 22% at 10 years (p = 0.0084). Identifying high-risk and low-risk patients with thin melanomas may improve guidelines for the application of adjuvant therapies to this population.