SCOTTSDALE, ARIZ. (FRONTLINE MEDICAL NEWS) – Thin melanomas account for most melanoma deaths, even though T4 lesions have the worst prognosis, according to an analysis of melanoma data from the National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) program.

In 2015, a study from Queensland, Australia,reported that more patients died from T1 melanomas than did from T4 melanomas ( J Invest Dermatol. 2015 Apr;135:1190-3 ). But a similar analysis in the United States was not available, so Dr. Shoshana Landow , a dermatologist at the Providence (R.I.) Veterans Affairs Medical Center,and her associates extracted SEER 13data for invasive melanomas between 1992 through 2003. These registries covered Atlanta, Connecticut, Detroit, rural Georgia, Hawaii, Iowa, Los Angeles, New Mexico, San Francisco-Oakland, San Jose-Monterey, Seattle-Puget Sound, Utah, and the Alaska Native Tumor Registry, according to the SEER website.

Among 105,264 recorded melanomas during this period, 37,210 tumors were in situ, the investigators reported in a poster presented at the annual meeting of the Society for Investigative Dermatology. They excluded those cases, as well as 4,789 cases that involved more than one primary melanoma, 8,637 that extended beyond the skin at diagnosis, 5,308 with no record of thickness, and one that was not known to be primary.

That left 49,319 invasive melanomas, of which 35,509 (72%) were T1 (0.01-1.00mm), 7,879 (16%) were T2 (1.01-2.00 mm), 3,948 (8%) were T3 (2.01-4.00 mm), and 1,983 (4%) were T4 (more than 4.00 mm). A total of 17% of the T1 lesions were 0.01-0.25 mm thick at diagnosis, while 42% were 0.26-0.50 mm thick, 25% were 0.51-0.75 mm thick, and 16% were 0.76-1.00 mm thick.

Ten years after diagnosis, 3,660 (7.4%) patients had died of melanoma, according to the study. These deaths included 1,072 T1 patients, 974 T2 patients, 985 T3 patients, and 629 T4 patients. But while T4 lesions accounted for the fewest number of deaths and made up only 4% of all invasive melanomas, the 10-year mortality rate for T4 lesions was nearly 32%, vs. 25% for T3 lesions, 12% for T2 lesions, and 3% for T1 lesions.

Most (42%) T1 lesions were 0.26-0.50 mm thick, 25% of T1 lesions were 0.51-0.75 mm thick, and the remaining T1 lesions were nearly evenly split between the thinnest (0.01-0.25 mm) and the thickest (0.76-1.00mm) categories, the researchers also reported. When considering only the T1 lesions, 10-year mortality rates were 3% for the 0.01-0.25 mm category, 1.9% for the 0.26-0.5 mm category, 3.7% for the 0.51-0.75 mm category, and 5.8% for the 0.76-1.00 mm category.

Thus, the thinnest invasive melanomas had a higher 10-year death rate than did the next-thinnest category, a finding that “demands explanation” and was not caused by ulceration, the researchers noted.

“The greatest total number of deaths was from T1 melanomas and the smallest number from T4, despite the fact that prognosis worsened as melanoma thickened from T1 to T4,” they concluded in their poster. “Our findings highlight the heavy death toll of thin melanomas.”

The authors did not specify funding sources, and had no disclosures.