Are All Melanomas Dangerous?

Melanoma is a cancer of the pigment-producing cells of the body, called melanocytes. It is a relatively common malignancy; 1/54 Canadian men and 1/74 women are expected to develop melanoma in their lifetimes.1 So it is unfortunately all too often that I find myself in the position of telling a patient of mine that they have been diagnosed with melanoma. Understandably, most people are shocked and terribly upset by this news. They've read that melanomas are the most dangerous form of skin cancer, and indeed they are. What I think is not as well represented in the media is that the vast majority of melanomas are not fatal. In fact, most carry an excellent prognosis.

The single most important factor determining the prognosis associated with a melanoma at the time of diagnosis is its thickness (doctors use the term vertical height).  Melanomas that are less than 1 mm (0.04") in thickness are considered to be thin, those that are 1.0-4.0 mm thick are considered to be intermediate, while those that are 4.0 mm or more in thickness are considered to be thick melanomas.

melanoma-in-situ

Staging is a system doctors use to determine how severe a cancer is, and how far it has spread in the body.  A melanoma's stage depends, in large part on its vertical height (thickness).  Melanomas that are stage 0 are known as in situ melanomas; they are confined to the epidermis (the top layer of the skin) and have not become invasive

Invasive melanomas are grouped based on their thickness, and on whether they have spread beyond the skin.  Those that are less than or equal to 1 mm in thickness are divided into stage 1a and 1b.  Stage 1b melanomas are either ulcerated (the top layer of skin above the melanoma is not intact), or they have evidence of mitoses (dividing cells) when the pathologist looks at them under the microscope.  Melanomas that are stage 1a are less than or equal to 1 mm in thickness but have neither ulceration nor mitoses.  A full description of staging in melanoma can be found here.

It is estimated that 137,310 new cases of melanoma will be diagnosed in the US this year (similar statistics are not available for Canada).2 Of these, 63,440 (46%) are expected to be in situ (noninvasive), while 73,870 are expected to be invasive.2 A 2010 review of the SEER database from 1988-2006, found that, of all the melanomas diagnosed that were invasive (had gone deeper than the epidermis into the dermis), 70% were less than or equal to 1 mm in thickness.3 These numbers tell us that the vast majority of melanomas are diagnosed at an early stage.

These statistics are important because of the dramatic impact that a melanoma’s thickness has on prognosis.  Most thin melanomas will not have spread beyond the skin.  Oncologists usually frame the prognosis of a given cancer by looking at the ‘5-year survival rate’.  This is calculated by dividing the percentage of people diagnosed with a certain type of cancer who are alive 5 years after their diagnosis, by the percentage of the general population of corresponding age and sex that are alive after 5 years.  This number gives an indication of how likely someone is to die as a result of their cancer.

The 5-year survival rate for melanoma-in-situ (stage 0 melanoma) is 100%. 4 That means that, 5 years after diagnosis, people diagnosed with melanoma-in-situ are just as likely to be alive as those who have not been diagnosed with this type of cancer. The prognosis for thin invasive melanomas is also very good; the 5-year survival rate for melanomas that are stage 1a is 97% while it is 92% for those that are are 1b. 5 A description of survival rates in melanoma stratified by stage can be found here. The prognosis remains good for many invasive melanomas that are greater than 1 mm in height, but declines with increasing thickness, and particularly with any evidence of spread of the skin cancer beyond the skin. The prognosis is generally better for thinner growths because, in most cases, they have been removed with surgery before they have had the chance to spread to lymph nodes or other organs.

Studies have shown that 84% of melanomas are diagnosed when they are localized to the skin, and that the 5-year survival associated with these skin cancers is 98.3% 6 While favourable statistics are not a guarantee that someone diagnosed with a thin melanoma will do well, they do provide a high degree of reassurance that that is likely to be the case. Although a discussion of thicker melanomas, or those that have spread beyond the skin, is beyond the scope of this post, many people whose melanomas are diagnosed at a more advanced stage will also do well.

A diagnosis of melanoma should always be taken seriously; the purpose of this post is not to minimize the severity of this type of cancer. Melanomas that have metastasized have the potential to be very aggressive, and those that have spread to organs beyond the lymph nodes unfortunately often carry a very poor prognosis. Although it is less common, thin invasive melanomas do in some cases spread, and, because they are so much more common than thick lesions, they actually account for a larger number of deaths. 7

It is my hope that this post can give some comfort and reassurance to someone who has recently been diagnosed with melanoma.  The wealth of scientific information we have accumulated tells us that melanomas that are confined to the skin (as most are) carry an excellent prognosis, and the majority can be cured with surgery.  If you have recently been diagnosed with this type of skin cancer, please, do not panic; speak with your dermatologist about what your diagnosis means. Remember that, in many cases, the odds are heavily in your favour.

 

References

1 Canadian Cancer Society. Canadian Cancer Statistics 2015. Toronto: Canadian Cancer Society; 2015.

2 American Cancer Society. Cancer Facts & Figures 2015. Atlanta: American Cancer Society; 2015.

3 Criscone VD, Weinstock MA. Melanoma thickness trends in the United States, 1988-2006. J Invest Dermatol 2010;130:793-7.

4 American Cancer Society. Melanoma Skin Cancer Detailed Guide. Atlanta: American Cancer Society, 2015.

5 American Cancer Society. Melanoma Skin Cancer Detailed Guide. Atlanta: American Cancer Society, 2015.

6 National Cancer Institute. SEER Stat Fact Sheets: Melanoma of the Skin. Bethesda: National Cancer Institute, 2015.

7Whiteman DC, Baade PD, Olsen CM. More people die from thin melanomas (< 1mm) than from thick melanomas (>4 mm) in Queensland, Australia. J Invest Dermatol 2015;135(4):1190-3.

 

Michelle Levy

Dr. Michelle Levy is a board-certified dermatologist specializing in medical and aesthetic dermatology. A graduate of the University of Toronto's Faculty of Medicine, Dr. Levy provides a full spectrum of dermatologic services in Toronto, Canada. Education: M.D., University of Toronto, 1999 Residency in Dermatology, University of Toronto, 1999-2004 Employment History: Self-employed, North York, Ontario, 2005-Present Medcan. Consultant Dermatologist. 2007-Present