A 20-year-old woman in Australia was left without a thumb after she claims a nail-biting habit – formed as a coping mechanism for bullying – led to a rare form of cancer.

Courtney Whithorn, who said her habit was so bad that she bit her thumb nail off completely in 2014, noticed that her thumb had started to turn black over the last four years, The Sun reported.

Whithorn was reportedly diagnosed with acral lentiginous subungual melanoma (ALM), which appears on the palms of hands, soles of feet, or under nails. According to the Aim at Melanoma Foundation, ALM may be difficult to recognize as it can be mistaken for a stain or a bruise, and can occur on seemingly healthy skin.

Most cases of AML on the nails occur either on the big toe or thumb. ALM can occur in anyone, and does not appear to be related to sun exposure.

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Many patients report trauma or injury to the affected area where ALM is diagnosed, but the link remains unclear. Whithorn said she was shocked when she discovered that her habit might be linked to her diagnosis.

“When I found out that biting my nail off was the cause of the cancer it shattered me,” she told The Sun. “My hand was just constantly in a fist because I didn’t want anyone to see it — not even my parents. I got a bit freaked out when my skin started to go black so I showed them for the first time this year. I can’t even explain how self-conscious I was. I always had fake nails to hide it because it was so black. It was like paper whenever it grew back.”

Whithorn said she first consulted a doctor for cosmetic reasons and was referred to two plastic surgeons, who suggested a skin graft but recommended a biopsy before operating.

She was then sent to a specialist for further testing when the results came back unclear, The Sun reported.

“They did more tests and when those results came back, I was told that it was a malignant melanoma, which was very rare to have there, especially for someone my age and at that size,” she told The Sun. “I was obviously very shocked I couldn’t believe it all. My mom just burst into tears.”

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Whithorn has since undergone four surgeries, with the last resulting in an amputation of her thumb. Whithorn said she entered the surgery without knowing whether she would wake up without a thumb.

She said she now will be under surveillance for five years to see if the cancer returns.

“Without my boyfriend or family I honestly don’t know how I would have got through all this,” she told The Sun, while adding that she wants others to think twice about bullying their peers.

“I just wish I was as confident and as outspoken as I am now back then,” she told The Sun.