Plasma L-dopa in the diagnosis of malignant melanoma.

We determined the concentration of L-dopa in the plasma of 98 patients with biopsy-proven melanoma, a dermatological neoplasm that is characterized biochemically by abnormal tyrosine metabolism. For 21 patients previously diagnosed as having melanoma but who were clinically free of disease (stage I), the mean concentration of L-dopa in plasma, 1.01 (SD 0.12) micrograms/L, was not significantly different from that of 32 normal controls, 1.23 (SD 0.16) micrograms/L. However, L-dopa was increased significantly (p less than 0.001) in the plasma of all of 65 patients with active disease (stage II), 2.08 (SD 0.46) micrograms/L, and was highest in 12 patients with stage III malignant melanoma, 8.40 (SD 3.50) micrograms/L. The development of metastases in four patients with stage II melanoma was accompanied by an increase in the concentration of plasma L-dopa. These studies suggest that measurement of plasma L-dopa may be useful in the diagnosis of melanoma.