Amelanotic melanoma papule defies dermoscopy, reflectance confocal microscopy

Key Points

Scottsdale, Ariz. - A case of amelanotic melanoma that upon evaluation with dermoscopy and reflectance confocal microscopy (RCM) appeared to be basal cell carcinoma (BCC) turned out to be invasive melanoma, says a dermatologist who worked on the case.

About a year ago, a 66-year-old man with a history of nonmelanoma skin cancer presented at the Mayo Clinic in Scottsdale, Ariz., with a nonulcerated, pearly pink papule on the mid-back measuring 1 cm and possessing clinical features consistent with BCC, says David L. Swanson, M.D., chief, Section of Medical Dermatology, at the hospital. "The lesion appeared to contain no pigment," he says.

Dermoscopic analysis revealed blue-grey ovoid nests and polymorphous blood vessels. Accordingly, Dr. Swanson says the differential diagnosis for this lesion included amelanotic melanoma, squamous cell carcinoma (SCC) and inflamed seborrheic keratosis, but favored BCC.

"The confocal image showed no features that we find typical of melanoma on RCM, but it had features one would expect to be more typical of BCC," he says.

To perform RCM analysis, Dr. Swanson says a technician attaches a speculum-like adapter equipped with a ring at its tip that affixes to the skin with disposable adhesive.

"The other end of the ring fits into the confocal microscope using a magnet, then the confocal microscope, which is on an articulated arm, images the lesion. Altogether, the process takes about 10 minutes," Dr. Swanson says.

Biopsy results ultimately showed the tumor to be a malignant melanoma at least 1.66 mm in depth (Speetzen LS, DiCaudio DJ, Swanson DL. Poster P1902. American Academy of Dermatology Summer Meeting. August 4-8, 2010. Chicago). "We were surprised," Dr. Swanson says. "In looking at the morphology of the lesion on pathology, we could see how one could have the impression that there were tumor islands. Since the tumor was virtually melanin-free, they didn't look like large pagetoid cells that one sees with melanoma under RCM."

The patient's treating physician excised the amelanotic melanoma and one positive lymph node, Dr. Swanson says.

"The patient appears to be tumor-free at present," he says.