Sentinel lymph node biopsy unnecessary in most cases of thin melanoma

Key Points

Washington - Sentinel lymph node biopsy (SLNB) for melanoma is not intended to be a therapeutic procedure, but it provides excellent information for staging the disease, an expert says. However, he says, the use of SLNB in thin melanomas is somewhat controversial.

"Some hospitals use 0.76 mm as the cutoff point for performing SLNB. Other hospitals use 1.0 mm," says Gary L. Peck, M.D., director emeritus, Melanoma Center, Washington Cancer Institute, Washington Hospital Center. However, he says, SLNB should be considered for the thinner melanomas (<1.0 mm) if adverse histologic or other characteristics are present.

SLNB study

"We wanted to look for predictive factors on initial melanoma biopsies that might favor SLNB being positive," Dr. Peck says. Researchers believed that a tumor's mitotic rate would be an important factor, as prior research has suggested, he adds.

Dr. Peck says, however, that no more than 20 percent of pathology reports for the 472 patients studied included any mention of mitotic rate. Because this test was not routinely performed, "We did not have enough data to make a significant evaluation of mitotic rate as a risk factor for SLN positivity." Although the College of American Pathologists guidelines recommend routine analysis of mitotic rate in melanomas, "We were surprised at how few pathologists did this," he says.

In some pathology reports, Dr. Peck says pathologists wrote that microscopic description had been performed, but gave no details. "Some pathology reports didn't give any microscopic description - just the final diagnosis. There was tremendous variance in quality among pathology reports." Because the Washington Cancer Institute is a referral center, he says, researchers had to rely mainly on initial biopsies performed at outside facilities.

Among the 472 patients studied, 330 had tumors more than 1.0 mm thick (mean Breslow depth: 2.68 mm). Of these tumors, 52 had positive SLNB (mean BD: 3.4 mm), and 278 were negative (mean BD: 2.55 mm; P=0.021). Conversely, 142 patients (30.1 percent) had thin melanomas (mean BD: 0.77 mm). Three of these thin melanomas had positive SLNB (mean BD: 0.81 mm). Overall positive SLNB rate was 55 of 472 (11.7 percent).

Regarding Breslow depth, "The thicker the tumor, the more likely the SLNB will be positive," Dr. Peck says. Other factors that tended to predispose toward a positive SLNB included Clark level of IV or V, nodular subtype and ulceration. Ulceration status approached statistical significance, Dr. Peck says, while the other factors reached statistical significance. Although it's difficult to explain why the nodular subtype had a higher risk of positive SLNB, Dr. Peck notes that these tumors tend to be thicker earlier and more aggressive than other subtypes.

"Also, we found that patients aged 60 or older had a higher rate of SLNB positivity (15.2 percent) compared with patients under 60 (9.7 percent)," Dr. Peck says. "Other studies have shown the opposite."