SAN FRANCISCO – A new definition of giant congenital nevi should help physicians identify infants with supersized moles that are at risk for melanoma, neurocutaneous melanocytosis, liposarcoma, and other tumors.
For many years pediatric dermatologists sized congenital nevi as small, medium, or large, with "large" comprising nevi that are estimated to be 20 cm or greater in diameter when the patient has grown into adulthood. Then, physicians started referring to some of the large nevi as "giant" congenital nevi, with no universal agreement on what defines a giant nevus, Dr. Sheila Fallon Friedlander said at the Women’s & Pediatric Dermatology Seminar sponsored by Skin Disease Education Foundation (SDEF).
Courtesy Dr. Sheila Fallon Friedlander
After years of subjective diagnoses, a new definition for giant congenital nevi has been decided upon.
"Here’s our new definition that you’re going to be seeing in the literature," said Dr. Friedlander of the University of California, San Diego. Congenital nevi with diameters less than 1.5 cm after the patient is fully grown are considered small, nevi with diameters of at least 1.5 cm but less than 20 cm are medium size, and large nevi are at least 20 cm but less than 40 cm in diameter. Giant congenital nevi are those with diameters of 40 cm or greater when the patient is fully grown.
"When you are examining a baby in a nursery, how does that help you? That’s an adult-type figure," she noted. To estimate the grown size of a congenital nevus, measure the nevus on the baby and multiply by a scaling factor that’s based on different growth rates for different parts of the body. Lower extremities grow the most, so multiply the size of nevi located there by 3.3. Multiply the size of a nevus on the torso or an upper extremity by 2.8, and a nevus on the head by 1.7.
For example, a congenital nevus 7 cm in diameter on a baby’s leg qualifies as a large congenital nevus (7 x 3.3 = 23.1 cm), "so you need to worry more" about risks for that baby, she said.
Estimates of risk have been evolving with the terminology. Most studies have lumped large and giant categories together and referred to them as large congenital nevi. The true risks in studies of registry data are clouded by the fact that many families choose to excise or partially excise large congenital nevi.
Between 1% and 11% of patients with large congenital nevi will develop malignant melanoma or neurocutaneous melanocytosis, a 52-1,000 times greater risk than for someone without the lesion, according to the results of a New York University study of 160 patients (Pediatrics 2000;106:736-41). Large congenital nevi also are associated with increased risks for liposarcoma, rhabdomyosarcoma, and (in some body locations) tethered spinal cord syndrome.
In the New York University study, 14 of 15 patients who developed melanoma or neurocutaneous melanocytosis had lesions greater than 50 cm in diameter, which now would be considered giant. The risk is highest for lesions on the torso. The melanoma often can occur deep in the dermal-epidermal junction, making them hard to find, Dr. Friedlander said. The melanoma also can occur underneath the lesion in the CNS or mesentery.
Although melanomas in the general population are unlikely to develop until adolescence or adulthood, in children with large congenital nevi who develop melanoma, 70% of the cancers appear by age 10 years, and 50% appear by age 5. "So this is something that you can’t put off. You really need to deal with it," she said.
Dr. Sheila F. Friedlander
Studies report that 7% of patients with large congenital nevi will develop neurocutaneous melanocytosis, an aberrant migration of melanocytes in leptomeninges that creates a risk for ventricular obstruction. The presence of more than 20 satellite lesions around a large congenital nevus increases the risk for neurocutaneous melanocytosis five-fold and warrants an MRI evaluation, she said.
"Five years ago I was taught that posterior axial location may have a higher risk for neurocutaneous melanocytosis, but recent data shows that it’s the satellite lesions rather than the location" that matters, Dr. Friedlander said.
A posterior axial location is, however, a risk factor for cutaneous melanoma in patients with large congenital nevi. Nevus size is a risk factor for both melanoma and neurocutaneous melanocytosis, said Dr. Friedlander; "20 cm is bad, and 40 cm is worse."
Dr. Friedlander disclosed that she has been a speaker, consultant, or researcher for Pierre Fabre, Onset Therapeutics, Johnson & Johnson, and Galderma.
SDEF and this news organization are owned by Elsevier.