Melanosis

By Peter Galvin, MD

Melanoma is a type of skin cancer that is caused by melanocytes, which are skin cells that produce melanin, a skin pigment, growing uncontrollably. This uncontrolled growth is caused by a genetic mutation in the melanocytes. These mutations are often caused by exposure to ultraviolet radiation, either from the sun or tanning lamps. When the body detects any tumor, including a melanoma, the immune system sends various kinds of white blood cells to fight the invader. Included in these white cells are macrophages that ingest foreign materials, including abnormal melanocytes and free melanin.

An 84-year-old man with a history of melanoma of the forehead complicated by metastases to the scalp presented to the dermatology clinic in Liverpool, Australia. Seven months before this presentation, he had been started on two immunological agents which resulted in the shrinking of the metastatic scalp nodules. These nodules were replaced by blue-gray spots on his scalp (see the photo). These spots were flat, not nodular (bumpy). Also noted on his scalp was an unhealed ulcer from unrelated trauma. A few of the nodules were biopsied as it was unclear whether these spots represented more metastatic disease.

What the biopsies revealed was scattered melanin-laden macrophages without evidence of melanoma or abnormal melanocytes. A diagnosis of tumoral melanosis was made. Tumoral melanosis is a pigmented skin lesion that results when melanin released from melanoma cells is taken up by macrophages in the skin. These macrophages can then travel locally (and occasionally distantly) via the bloodstream to be deposited elsewhere in the skin. The fact that the skin lesions were flat, not nodular, supported tumoral melanosis but biopsy is needed to differentiate it from metastatic melanoma. Patients with tumoral melanosis need to be evaluated by regular dermatologic surveillance, and any new lesions must be biopsied. Of course, anyone diagnosed with melanoma, even if it is an early, low-grade tumor completely removed by surgery, should undergo regularly scheduled skin surveillance for the rest of their life because their risk of having another melanoma is much higher.

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