A long-term hypopigmentary effect of thorium-X on freckled skin.

Sixteen years ago, an area on the forearm of a freckled subject was painted with a solution containing 500 esu of thorium-X per ml. Since then, despite much exposure to solar irradiation, there has been a complete absence of freckles from this area, which stands out in marked contrast to the surrounding skin. Biopsy of the skin from the area was examined by the Dopa technique and by electron microscopy. Melanocytes are present in apparently normal numbers, but in a condition of almost complete inactivity. This depression of melanogenesis is difficult to account for either on the basis of an environmental effect, or of intracellular inhibitory mechanisms.