A woman was told a tiny mark on her cheek was a clogged oil gland. Doctors later discovered it was a cancerous tumor.

• Alison O'Neill told Good Morning America her cancerous tumor was initially diagnosed as a clogged oil gland.
• Doctors diagnosed O'Neill with angiosarcoma, a rare cancer that forms in blood vessel lining.

A woman went in to her dermatologist get an unsightly bump removed from her face — she had been told that it was harmless, but she wanted it done anyway.

She is grateful she did: after the minor operation, her doctor discovered the bump was actually a rare, cancerous tumor.

Alison O'Neill, a 49-year-old woman from Arizona, told Good Morning America she went to a dermatologist in 2017 after noticing a "tiny little mark" on her right cheek. The doctor told O'Neill her mark was likely a clogged oil gland.

The mark grew over time, so O'Neill said she went to get it removed for cosmetic reasons in 2020. A biopsy revealed the spot was actually a cancerous tumor that causes angiosarcoma.

Angiosarcoma is a rare type of cancer that forms in the lining of the blood vessels and lymph vessels, which collect and dispose of bacteria and viruses. Per Mayo Clinic, angiosarcomas can occur in any place in the body, but typically occur in the skin on the head or neck.

Renowned fashion designer Virgil Abloh died last year of an angiosarcoma on his heart.

Angiosarcoma on the skin appear as a raised, swollen area that looks like a bruise and grows bigger over time. Researchers are not sure what causes angiosarcomas, but identified certain risk factors like exposure to radiation or chemicals.

O'Neill told GMA she is now in remission from angiosarcoma following surgery to remove the tumor and surrounding area, radiation, facial reconstruction surgery, and scar lightening procedures.

O'Neill shared her story with GMA to motivate other women to speak up they feel they aren't getting adequate medical care. Women are more likely than men to suffer from misdiagnoses, and female patients tend to wait longer for a cancer diagnosis, Insider's Anna Medaris previously reported.

"You have to advocate for yourself when going through any medical journey," O'Neill told GMA. "I describe the last two years as being like crawling through mud. It is incredibly difficult to regain your health and you have to work really, really hard at it."