Skin cancer can be difficult to spot and may not cause symptoms until later stages when the disease has spread. BSIP/UIG via Getty images 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.…