If you haven't specifically gone out of your way to become educated about skin cancer, you probably don't know much more about the subject than the average layperson. I don't mean that to be harsh, just helpful.

I base this on almost 20 years of seeing 6,000 dermatology patients a year, virtually all of whom were referred from providers outside of dermatology, mostly primary care. I know the kind of derm training PAs get, because I'm heavily involved in its delivery, and I'd be the first to admit that it's not nearly enough. And having started out in primary care all those years ago gives me a good feel for the challenges that primary care providers face. I know firsthand the unspeakable tragedy of missing a melanoma, but I have also had the experience of snatching a patient right out of the jaws of death by finding the black mark everyone else had overlooked.

Far from being esoteric facts you could find in any textbook, I have specifically confined my remarks below to information you can assimilate and use today.

1. Not all skin cancers are caused by the sun.

We speak and act as though basal cell carcinoma, squamous cell carcinoma and melanomas are the whole skin cancer story. The fact is that a great number of cancers can present on the skin that have nothing to do with the sun.

For example, many other cells in the skin, including those below, can undergo malignant transformation with little or no help from the sun:

• Cells of the sebaceous glands
• Merkel cells
• Eccrine gland cells
• Smooth muscle cells (leiomyosarcoma)
• Cells of the fibrous dermis (dermatofibrosarcoma protuberans)
• Lymph vessels (lymphangiosarcoma)
• Endothelial cells from the lining of blood vessels (endothelial sarcoma, or Kaposi's sarcoma)
• Adipocytes (liposarcoma)

Any internal cancer can metastasize to the skin, including cancers of the breast, colon, lung, prostate, kidney and liver. Cancers of the hematopoietic system can show up on the skin, including leukemia (both eosinophilic and mast cell) and lymphoma (T-cell, B-cell and others).

Here's the point: skin cancer doesn't always look the way you think it should, so the range of possible explanations for that odd lesion you see are vast and potentially dangerous. Have some respect for the differentials. If you don't know what it is, biopsy or refer to someone who can.

2. Not all skin cancers present as solid tumors.

A number of very serious cancers can show up as a rash. Paget's disease (both mammary and extramammary), mycosis fungoides (T-cell lymphoma) and metastatic breast cancer come to mind in this regard. Stay awake, or these will catch you napping.

3. Mycosis fungoides (MF) is one of the few skin cancers that can wax and wane over several years.

It is a very good rule of thumb in medicine that most cancers are headed inexorably onward and upward in terms of growth. Once MF gets going, it will do the same, but first it can wax and wane in size and extent over a decade or more. Even though the incidence of MF is low, it is by no means rare, and it is just one more example of why one should be liberal with biopsies. MF usually presents as large (3 to 8 cm), round to oval, slightly scaly patches of skin on the lower trunk. Sometimes these lesions have a faintly purpuric vascular look. MF favors men older than 50.

4. The words "safe" and "tan" don't belong in the same sentence.

"Safe tan" is an oxymoron. There is no such thing. The purpose of sunscreens is to avoid tanning. If you tan, you didn't use enough sunscreen, or you didn't apply it soon enough or often enough. I'm quite aware that the social norm in this country is to keep a "healthy tan" year-round, but when we diagnose a melanoma on a young person, attitudes among family and friends change rapidly.

That's what's happened in Australia, which leads the world in incidence of melanoma. Very few families there haven't been touched by this cancer. It's no longer cool to be tan in Australia. With the rising rates of melanoma in the United States we'll see that come to pass here, too, but only after a lot of people have died.

5. Melanomas are puzzling in a number of ways.

Intuition would suggest that a dangerous skin cancer like melanoma would be a lump sitting well above the skin line, while the truth is that most melanomas are essentially flat. And one would think a cancer that is indisputably sun-caused would be confined to sun-exposed skin, but the fact is that they can and do show up anywhere, even where the sun seldom shines, even in the eye. Trust me, most patients and providers don't know this.

Here are a couple of other odd things about melanoma: unlike basal cell carcinoma and squamous cell carcinoma, melanoma is not age-related. The incidence of melanoma peaks at about age 40, while the other carcinomas are more common as the patient reaches his or her 70s. And while basal and squamous cell carcinomas are usually related to the cumulative effects of a lifetime of excessive and poorly tolerated sun exposure, melanoma seems to be more related to episodic sunburns, especially in childhood. Thus, statistically, melanoma is more likely in the company secretary who burns every year at the company picnic than in the construction worker who is exposed to the sun every day.

6. Very few birthmarks in children need to be removed.

Melanoma occurrences in children younger than 12 are literally a one-in-a-million thing, so the odds are on kids' sides, but recent studies have shown that all but the largest (greater than 20 cm) pigmented congenital nevi are safe. Besides size, other factors that might influence the decision to remove or biopsy a congenital nevus could include change (in color, size or texture, for example), family history of melanoma or other subjective cause for alarm.

7. If you're in primary care and you're not finding several melanomas a year, you're not paying attention.

I tell my students and residents that they can save as many lives with their eyes as with a stethoscope. Don't know what to look for? Then think about who you're looking at - that is, how likely it is that the person will develop melanoma based on skin type and history of poorly tolerated sun exposure. Any black, mostly flat lesion bigger than a pencil eraser is worth examining more closely. At least look at as much skin as you practically can. More lives are lost from not looking than from not knowing.

Pay particular attention to the skin below the waist on women. For some unknown reason, melanomas are significantly more common there on women than on men.

Why all this attention to melanoma? Well, for one thing, the incidence of melanoma is growing faster than for any other kind of cancer. For people born in the 1930s, the melanoma incidence was about one in 1,500. For children being born now, their lifetime risk will be about one in 75. The decrease in the ozone layer only accounts for a small part of this alarming trend. The good news is that while 85% of melanoma patients died from their disease in the 1930s, now 85% survive because we catch them earlier, when they are smaller, before they have had a chance to metastasize. Even so, melanoma still accounts for 8,000 deaths a year in the United States, the vast majority of which could have been prevented at some point by an alert provider or patient.

Melanoma is dangerous precisely because all our efforts to treat it after the fact come to very little. If it can't be prevented, it must be detected and removed before it becomes thick enough to start showering tumor cells into dermal vasculature.

8. Not all squamous cell carcinomas (SCCs) are caused by the sun.

Why is this significant? For one thing, I just finished telling you to watch for skin cancers on older, sun-damaged patients, and that's true for most SCCs too, but many SCCs are caused by things other than sun exposure. For example, SCCs can be caused by human papillomavirus (especially on the genitals), by chronic arsenic exposure, by post-radiation sequelae (e.g., the older patient whose acne was treated with radiation) or in chronic wounds such as osteomyelitis or leg ulcers. These atypical SCCs, along with the usual sun-caused types on locations such as the temples, dorsal hands, scalp or near any orifice, are more prone to metastasize and thus need larger surgical margins on excision.

9. Biopsy of skin cancer will not cause it to spread.

Studies comparing outcomes of melanomas that were incisionally biopsied with those that were removed with clear margins showed no difference in sequelae or survival rates.

Why is this significant? Because many primary care providers tell me they are afraid to biopsy suspected melanomas for fear of spreading the cancer. This is just not a reasonable fear. If you are going to biopsy a suspected melanoma, it is true that it would be best to excisionally biopsy it - that is, remove the whole lesion - but only because it gives the pathologist all the tissue needed not only to make the diagnosis, but also to stage the cancer in terms of tumor depth.

If you can't remove the whole lesion, then incisionally biopsy it, taking the darkest, most irregular portion of the lesion. If you can't do that, then obtain multiple (two or three) punch biopsies from different sections of the lesion.

10. Risk factors for sun-caused skin cancer include a Southern accent.

If you're already in the South, this makes no sense. But if you're in the North, particularly in an area that gets relatively little sun, you need to listen for Southern accents, which tell you that person lived in the South long enough to acquire the accent and to receive enough sunlight to develop a skin cancer. In other words, don't assume the patient has lived in a low sunlight area all his or her life. Ask about military service in World War II or Vietnam, or occupational exposure. And pay attention to skin type, by which we in derm mean, "How does the patient tolerate the sun?" Sometimes, such as when a patient has red hair, blue eyes and a very fair complexion, it's obvious they are at higher risk.