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What is a Merkel cell?
Merkel cells are found in the skin (see diagram below). Although the exact function of Merkel cells is unknown, they are thought to be touch receptors.
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Schematic of the location of Merkel cells within the skin (McKee. Pathology of the skin. 2nd ed. Mosby: London, 1996. pg 1.13). |
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What is Merkel cell carcinoma
MCC, sometimes referred to as a neuroendocrine carcinoma of the skin, arises
from the uncontrolled growth of Merkel cells in the skin. It is a rare skin
cancer with roughly 1000 cases per year in the United States, making it about
40 times less common than melanoma. MCC has the potential to be lethal, and
thus prompt aggressive treatment is warranted.
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Carcinoma skin appearance
MCC does not have a distinctive appearance (see photograph below). It usually develops on sun-exposed skin (e.g., head, neck, arms) as a painless, firm, flesh-colored to red or blue bump. Frequently, patients seek advice from their doctor because the bump is growing rapidly or the overlying skin is breaking down. Most MCCs are diagnosed when a skin biopsy is performed to rule out another sun-induced skin cancer or a cyst. In the vast majority of cases, both the doctor and the patient are surprised by the diagnosis of MCC.
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Merkel cell carcinoma on the lip of a 92-year-old man (Nghiem, 2001). |
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What are the causes?
The exact causes of MCC are not known. Factors strongly associated with the development of MCC are:
- Age over 65 years
- Fair skin
- History of extensive sun exposure
- Chronic immune suppression (e.g., kidney or heart transplantation or HIV)
The best available information on characteristics of patients with MCC comes from a study of 1,034 patients summarized in the table below (Agelli, 2003). The average (median) age of the patients with MCC was 74 years. The most common sites of involvement were head followed by arms (upper limb). At the time of diagnosis, half of the patients had disease localized to the skin, while the other half had MCC that was no longer confined to the skin.
Total Number of Patients = 1034 |
| Median Age (years)
74 |
| Age groups (years) |
No. |
(%) |
| < 65 |
247 |
(23.9) |
| 65-74 |
281 |
(27.2) |
| >/= 75 |
506 |
(48.9) |
| Race |
No. |
(%) |
| White |
968 |
(93.6) |
| Black |
12 |
(1.2) |
| Other |
37 |
(3.6) |
| Unknown |
17 |
(1.6) |
| Anatomic site |
No. |
(%) |
| Head |
499 |
(48.3) |
| Trunk |
117 |
(11.3) |
| Upper limb |
199 |
(19.3) |
| Lower limb |
165 |
(16.0) |
| Other |
54 |
(5.2) |
| Stage at diagnosis |
No. |
(%) |
| Localized |
507 |
(49.0) |
| Regional |
281 |
(27.2) |
| Distant |
81 |
(7.8) |
| Unstaged |
165 |
(16.0) |
Patient characteristics. Adapted from Agelli,
2003. |
Age & Merkel cell carcinoma
The number of MCC cases increases with advancing age. The average age for developing MCC is 74 years, with 75% of patients over the age of 65 years. The incidence of MCC (see graph below) is somewhat greater in men (squares) than in women (circles) for each age group. Thus, advancing age and male gender are risk factors for MCC.
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Frequency of MCC by age & sex: men (square) & women
(circle). Adapted from Agelli, 2003.
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Sunlight & Merkel cell carcinoma
It is believed that ultraviolet radiation from the sun plays a significant role in the development of MCC (see schematic below). This cancer is found most commonly on sun exposed areas of the body (e.g., head, neck, arms) in older Caucasian individuals, who may also have other sun-induced skin cancers. There is more MCC in sunny climates (Hawaii) as opposed to areas with less sun (Connecticut) (see graph below). Thus, a history of extensive sun exposure is a risk factor for MCC.
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Frequency of MCC of the head in Caucasians by UV exposure (Agelli, 2003) |
Immune function & Merkel cell carcinoma
MCC is associated with a profoundly weakened immune system, such as in patients with HIV, or in patients who have received an organ transplant who are on medications that suppress the immune system. The risk of developing MCC is increased 8-fold in HIV patients (Engles, 2002), and 10-fold in organ transplant patients (Penn, 1999). Long-term suppression of the immune system (for many years) appears to be a risk factor for MCC. It is sensible to eat well, exercise, and get plenty of sleep to promote your own immune health. We know of no other way to increase immune function beyond that of maintaining good health.
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