A 21-year-old had the most dangerous form of skin cancer. It looked harmless, and she had no other symptoms.

• A dermatologist shared the case study of a young patient with a mole that turned out to be skin cancer.
• Aggressive, dangerous skin cancers may be hard to spot, and don’t always match typical warning signs.
• To reduce your risk, keep track of any new moles or marks, especially ones that change rapidly.

A 21-year-old patient showed up at the dermatologist’s office with a small mole on her cheek. It turned out to be melanoma, one of the most aggressive and deadliest forms of skin cancer.

A majority of doctors would likely have missed it, according to Dr. Michael Christopher, who treated the patient. He shared the case on social media and conducted an informal poll (not a scientific study) among fellow doctors on how they’d respond. About 80% said it didn’t look like a cause for concern.

Leaving potential skin cancers untreated can be “extremely dangerous,” Christopher said.

“Many people are at risk, and they believe that they are not,” he told Business Insider. “Regardless of your age or your family history or risk factors, you can develop a skin cancer.”

In the case of his 21-year-old patient, ignoring the mole or waiting to treat it may have allowed the cancer to develop aggressively and potentially spread to other areas of the body.

“She could have gone from stage zero to invasive disease,” Christopher said.

He shared what everyone should know about their risk of skin cancer, and when to see a doctor for a mole, spot, or splotch.

Warning signs of skin cancer

Christopher lives in Arizona, where the sunny weather, high-altitude environment, and outdoor culture create a high risk of skin cancer.

He said many melanoma deaths are linked to misdiagnosis, lack of awareness, or simply not spotting the signs of skin cancer.

Most types of skin cancer are extremely treatable when caught early, but missing the signs or symptoms can allow it to spread rapidly.

“If you develop a new mole, a new spot, you let us know. If something’s painful or tender, itchy, you give us a call,” he said. “Those are big-deal things.”

Remember the most common signs of skin cancer using the ABCDE rule to assess skin marks, moles, or spots.

Christopher said the criteria still aren’t a sure-fire way to diagnose skin cancers: if you’re not sure, see a doctor.

It’s normal to develop new moles, especially in your 20s and 30s. As you get older, however, your risk of skin cancer increases, so it’s important to be more aware of changes to your body.

“A new, rapidly growing spot, even if it’s symmetric, is very important to get looked at,” Christopher said.

He recommends doing a body check every month or two, focusing on hard-to-see areas, such as your scalp, back, butt, and groin.

“Understanding what is there is very important. How long have I had this spot, and is that spot actually changing?” he said.

Skin cancer can spread quickly

One of the biggest concerns with skin cancer is that it may not cause major symptoms until more advanced stages, according to Christopher.

Skin cancers can also progress quickly: squamous cell carcinoma, the second-most common type of skin cancer after basal cell carcinoma, can develop relatively quickly, sometimes within weeks.

“You can get a spot that just showed up, and it is literally two days later, and it develops a nodule,” Christopher said.

Heavy sun damage can increase the risk of developing skin cancer. To protect yourself, buy the right sunscreen, use it regularly, and reapply as needed when swimming or sweating outdoors.

“I’m not saying avoid the sun,” Christopher said. “You just have to be very good with sun protection.”