SAN DIEGO — The incidence of Merkel cell carcinoma has increased by an "astonishing" 95% in the last decade, according to the lead investigator of a national study.
The rare but dangerous skin disease is becoming more common as the population ages, outpacing growth in melanoma, and the increase is likely to continue, Song Youn Park, MD, from the University of Washington in Seattle, told Medscape Medical News. "We think it's going to increase a lot in the coming 10 years."
The most recent estimate of the incidence of Merkel cell carcinoma in the United States was about 1500 cases in 2007 (J Invest Dermatol. 2007;127:2100-2103).
Park and colleagues estimated that there were roughly 2800 cases in the United States in 2010, but she said about 40% of these ended the patients' lives. In comparison, mortality from melanoma is approximately 8%.
"It's rare, and might have been under-recognized," Park said here at the American Academy of Dermatology (AAD) 2018 Annual Meeting. Even dermatologists who have practiced for more than 10 years may never have diagnosed a patient with this cancer, she pointed out. But early diagnosis is important because outcomes are better the earlier it is treated.
To update these numbers, Park and colleagues analyzed data from the Surveillance, Epidemiology, and End Results (SEER) Program of the National Cancer Institute, which captures roughly 28% of the US population. They used the SEER-18 database, which covers the years 2000 to 2013.
The investigators found that the incidence per 100,000 population increased 40% from 0.5 cases in 2000 to 0.7 in 2013.
In absolute numbers, which are increased by the growth in population, as well as the rise in incidence, the rate of Merkel cell carcinoma shot up 95% from 2000 to 2013, the researchers report. In comparison, the total number of solid tumor cases increased by 15%, and the number of melanoma cases increased by 57%.
The aging of the Baby Boom generation is driving this spike in Merkel cell carcinoma, as the cancer becomes more common with age, Park explained. Older adults are more susceptible because they have diminished immunity, including a decline in B- and T-cell function, she said.
Older people are also more likely to take drugs that suppress aspects of their immune system because they have autoimmune diseases, or that help them tolerate organ transplants, an audience member explained after Park's presentation. Park responded that it is hard to predict how much these treatments will affect the increase in incidence of Merkel cell carcinoma in the future.
Unlike the incidence of melanoma, the incidence of Merkel cell carcinoma continues to rise after the age of 85 years. As a result, the researchers calculated that the number of new cases per year would increase from more than 2400 in 2015 to more than 3200 in 2025.
Estimating a 10% increase in the incidence rate by 2025, the researchers calculated that 25% of the increased number of cases per year could be attributed to the growth in population, and 75% could be attributed to the aging of the Baby Boomers.
Many dermatologists could benefit from better education to diagnose the disease, Park said. "Fortunately, there has been increased understanding of disease pathophysiology and treatment," she added. "When it is diagnosed in early stages it has a better prognosis. We now have US Food and Drug Administration–approved immunotherapy drugs that work well in about 60% of patients."
The cancer got its name because it develops in cells that look like Merkel cells, but Park and colleagues do not believe that it originates in these cells, she said. Instead, it appears that the Merkel cell polyomavirus causes the cancer in fibroblasts in about 80% of cases, Park said.
The virus is found in about 60% of adults and is not usually detrimental, but apparently it causes cancer when it undergoes a mutation after after harmful exposures, such as to ultraviolet light, she explained. The other 20% of cases occur in epidermis cells directly as a result of exposure to ultraviolet light without the involvement of a virus, Park said.
Diagnoses can prove challenging because Merkel cell carcinoma's presentation varies, Darrell Rigel, MD, from New York University in New York City, told Medscape Medical News.
Park advised using the mnemonic "AEIOU" to recognize the cancer and biopsy suspected lesions (J Am Acad Dermatol. 2008.58:375-381)
Older than 50 years
Ultraviolet-exposed and fair skin
"It's exciting because we're getting better awareness," Hensin Tsao, MD, PhD, from Harvard University in Cambridge, Massachusetts, told Medscape Medical News.
But with that awareness may come increased reporting, and it is hard to tell how much of the rising number of cases in the SEER database can be attributed to this, as opposed to a true surge in the incidence, he pointed out.
Eventually, however, the effect of increased reporting will plateau and surveillance data will more accurately reflect the true change in incidence, he added.
When the disease is in stage 1 and localized without lymph node involvement, the first-line treatment is surgery, which is sometimes combined with radiation, said Park.
For metastatic disease, antibodies for PD-1 or PD-L1 may be effective. These checkpoint inhibitors reduce the tumors' defenses against the body's own T-cells. So far, one drug in this class, avelumab (Bavencio, Pfizer), has been approved by the US Food and Drug Administration for Merkel cell carcinoma.
"The drug works relatively well in this disease about 50% to 70% of the time," Park said. Other drugs in the class may also be effective, she added.
These treatments are often worth a try in people with metastatic Merkel cell carcinoma, Rigel agreed. "There are not a lot of long-term data, but the fact is that they are surviving better."
This study was funded by the National Institute of Health, the Prostate Cancer Foundation, the Patient Gift Fund, and the Bloom endowment at the University of Washington. Park and Rigel have disclosed no relevant financial relationships. Tsao is a consultant to Epiphany Dermatology.
American Academy of Dermatology (AAD) 2018 Annual Meeting: Abstract F057. Presented February 17, 2018.
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