A 25-year-old woman thought the decade-old tan streak on her nail was a bruise. It was actually a sign of rare skin cancer.

• A decade-old tan streak on a woman's thumb nail turned out to be a rare form of skin cancer.
• Doctors told the 25-year-old patient it was a harmless mole and were reluctant to do a biopsy.

A 25-year-old woman had a mark under her nail for a decade that doctors said was a harmless mole, but turned out to be a rare form of skin cancer, TODAY reported.

Maria Sylvia shared the story in a series of TikTok videos, the first of which has so far been viewed more than 31 million times.

Sylvia said a tan streak appeared under her nail in high school, around 2012, and she assumed it was a bruise related to playing sports. A dermatologist said it was most likely a mole in her nail bed, with approximately 1% chance it might be cancer, Sylvia told TODAY.

"They basically told me to wait and watch and see if it got darker, grew wider," she said.

In late 2021, Sylvia said she had the mark rechecked after a coworker suggested it looks similar to images of a cancerous condition — but even then, she had to insist on a biopsy after another dermatologist wasn't concerned.

The tan streak turned out to be a subungual melanoma, a form of skin cancer that develops in the nail bed of the fingers or toes. While uncommon, it can be difficult to identify and can be aggressive if left untreated, dermatologist Dr. Adam Friedman told TODAY.

Sylvia's cancer was isolated to the nail bed, but it took two surgeries to remove it, and finding someone to treat the rare condition was a challenge, she said. While she won't be able to regrow a nail on her thumb and will require physical therapy to move it again, Sylvia said she was happy sharing her story might help others.

"I've had people reach out to me saying they were so grateful for me to share awareness," she said.

Skin cancer can show up in surprising places, and early detection is key

When spotted early, skin cancer is often treatable, including melanoma, which is a less common but more dangerous form, according to Cleveland Clinic.

However, it can be difficult to spot. Doing a thorough check involves scanning your whole body — including the toenails and bottoms of the feet — and even then, you might need an expert eye in case you miss something, Insider's Hilary Brueck reported.

A few cues that a skin mark may be suspicious include if it is asymmetrical, discolored compared to the surrounding skin and other moles, and changes over time. No one sign is proof of cancer, but since cancer cells develop irregularly, a patch of skin that isn't uniform may be a red flag, and worth getting checked out.

Experts at the Skin Cancer Foundation recommend getting checked out at least once a year, and more often if you have risk factors such as fair skin, red hair, or spend a lot of time in the sun.