Doctors in Australia suspected a 30-year-old woman had lymphoma – a cancer of the lymphatic system – but after they operated, they discovered her symptoms were caused by something completely unexpected. Her enlarged lymph nodes were actually the result of a reaction she had to a 15-year-old tattoo.

The odd case is detailed in a new paper published in the Annals of Internal Medicine.

The woman came into a medical clinic reporting small lumps under her arms that she had noticed for two weeks. According to the report "she had no fever, night sweats, weight loss, or pulmonary symptoms."

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Several scans left doctors suspecting the patient had lymphoma, a type of cancer that affects the immune system.

However, when they removed a swollen lymph node from her armpit, they found it was enlarged not because of cancer cells but because it was filled with black pigment.

The doctors concluded that the woman likely had a hypersensitive reaction to a tattoo she had gotten on her back 15 years earlier.

"The lymph system helps to filter our blood and helps to identify infections so our immune system can react and fight the infection, but they filter lots of other things, too, and in this case, they were filtering the pigment," Dr. Melissa Piliang, a dermatologist at Cleveland Clinic, told CBS News. Piliang was not associated with the case report.

The woman had two tattoos – a large black-ink tattoo on her back that had been there for 15 years and one on her left shoulder that was two and a half years old.

Doctors believe it was her back tattoo that caused the issues, since the swelling occurred in both armpits.

Why the patient experienced the reaction so many years later appears to be a mystery.

The doctors found reports of similar cases of delayed lymph node swelling caused by tattoo pigment, including one that occurred 30 years after tattooing.

However, Piliang says these instances are rare. "What's much more common is allergic reactions to tattoos in the skin," she said. Infections are also common.

That's why it's important to do research before choosing a place to get a tattoo to ensure it meets all safety standards and to closely follow directions for care.

"It's important that you go to a professional tattoo artist," she said, "somebody who runs a clean and reputable business who follows sterile procedures and understands what that means."