A woman who had previously battled melanoma was terrified when she saw a new brown mark on her heel. Although the mark looked very similar to a type of skin cancer that appears on the foot, doctors found it was actually a harmless spot caused by touching walnuts.

The woman was in her 60s and had previously been diagnosed with melanoma on her back. One day, while examining her skin, she saw a new brown spot on her left foot that had features of skin cancer — it was a discrete, tan and asymmetrical patch of skin.

The mark "looked quite a lot like it might be a melanoma," said Dr. Garth Fraga, a pathologist at the University of Kansas Medical Center, who was involved in the woman's case. In particular, the mark looked like a type of skin cancer called acral lentiginous melanoma (ALM), which typically appears on the foot as a large, flat, dark-colored lesion, Fraga said. [You and the Sun: 10 Burning Questions]

The woman tried to wash off the mark, but it would not fade, and this further "alarmed her to a possible acral lentiginous melanoma," said Dr. Deede Liu, a dermatologist at the University of Kansas Medical Center who treated the patient. Indeed, Liu was concerned that the patient might have melanoma, she said. The woman did not have any other signs of staining on her hands or feet, Liu said.

The doctors preformed a biopsy, and Fraga looked at the tissue under a microscope. But he didn't see an increase in melanocytes — the cells that produce brown pigment — that is typically seen in ALM, Fraga said. "It looked just like normal skin," Fraga said.

On an image of the lesion, Fraga said it looked like something was accumulating in the sweat ducts, and he suggested that the skin may have been exposed to dye. The woman remembered that she had been shelling black walnuts, which contain a substance called tannin that is used to make ink, and can dye the skin.

Because the doctors did not specifically ask the patient if she had been exposed to dyes or other substances that stain the skin when she was first evaluated, their "initial evaluation failed to reveal information that may have allowed for earlier exclusion of ALM," the researchers wrote in the July 30 issue of the journal JAMA Dermatology. "Our case serves as a reminder to clinicians of the importance of [taking] a thorough history," they said.

Liu said that doctors should ask about possible sources of staining when patients are evaluated for a new, pigmented skin lesion.

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