Have you ever slipped on some sandals, looked down at your toes, and noticed an unusual dark spot on your toenail? It’s possible you stubbed your big toe or dropped something heavy on your foot that caused bleeding underneath the nail. If so, expect that bruise to change in color from red, to purple, to dark brown or black as the blood is resorbed and a new nail grows out. But if you don’t recall any run-ins with the sofa and the mark doesn’t disappear after a few weeks, you might want to get your toenail checked for a certain—sometimes elusive—type of cancer.

What Is Toenail Melanoma?

You’ve likely heard of melanoma, a skin cancer that strikes the skin’s pigment-making cells, or melanocytes, but maybe not considered that means it could also crop up “under the hood” of your toenails and fingernails within the skin of your nail bed. Yikes.

There are actually numerous kinds of melanoma and this variant, called subungual (a.k.a. underneath-the-nail) melanoma, accounts for only 0.7% to 3.5% of malignant melanomas worldwide. “Subungual melanomas most often affect the big toe or thumbnail, which account for 75% to 90% of all cases,” says Dr. Rachel Westbay, M.D., a board-certified dermatologist at Marmur Medical in New York City.

Dr. Westbay also notes that there’s a higher prevalence of subungual melanomas in people of color, namely Black and Asian people: “It’s the most common types of melanomas diagnosed in deeply pigmented people, probably due to this population’s low incidence of cutaneous melanoma. In contrast to cutaneous melanoma, subungual melanoma does not appear to be related to sun exposure.” In perhaps the most noted example of how serious this form of melanoma can be, reggae icon Bob Marley tragically died when a subungual melanoma on his toe metastasized. The Jamaican singer believed that the dark spot under his toenail had instead been caused by a soccer injury.

When caught early and treated, though, the survival rate for subungual melanoma is high. At the earliest stage, the five-year survival rate is around 97%, and the 10-year survival rate is around 95%. The trick: Monitoring any spots on your nails, noting any changes, and seeing a dermatologist if their appearance seems suspicious.

Distinguishing Subungual Melanoma vs. Toenail Fungus

Your feet don’t get as much mirror time as other parts of your body but they can reveal certain signs of trouble with your health. Toenails (and fingernails), for instance, are one of the stealthy places where cancer can appear on your body.

So what exactly should you be on the lookout for? “In general, subungual melanomas present as a dark spot that is expanding, or a longitudinal pigmented streak that widens or extends to the skin around the nail,” says Erum Ilyas, M.D., a board-certified dermatologist based in King of Prussia, PA, with the Schweiger Dermatology Group. In other words, picture a vertical racing stripe that runs just beneath the nail down to your cuticle. Again, unlike a simple bruise (or subungual hematoma), this dark spot won’t fade away as the nail grows out.

According to Dr. Westbay, another telltale sign is the black or brown spot becoming wider and nonuniform in color. “If the pigment band becomes wider than 3 millimeters (mm), especially near the cuticle; becomes more irregular in pigmentation, such as a mixture of light and dark brown shades; or develops blurred borders extending to the skin around the nail fold [or, bottom of the nail], you have historical indicators of nail melanoma,” she says.

However, not all nail melanomas have a distinct shade. In fact, up to half of nail melanomas either have little pigment or completely lack pigment, says Dr. Westbay. That means any persistent pain, thinning, cracking, bleeding, or nail distortion should be monitored because a colorless nail melanoma could be the culprit.

Strangely enough, says Dr. Ilyas, a lot of patients coming in with concerns that they may have a nail melanoma have actually confused unsightly toenail markings with those that derive from a much more common condition, nail fungus. Both problems can cause discolored and misshapen toenails, but in the case of nail fungus, the toenails become yellowed and thickened—as opposed to thinner—and may crumble at the edges.

What Causes Subungual Melanomas vs. Toenail Fungus?

The exact cause of subungual melanomas is unknown. “Researchers do know, however, that this type of melanoma is different from others because it has no connection to sun exposure,” says Dr. Westbay. Risk factors for the disease include severe injury, damage, or trauma to the nail; having close family members with subungual melanoma; and genetic mutations with known predispositions to skin cancer.

Meanwhile, the cause of toenail fungus is clear-cut. According to the Cleveland Clinic, the infection usually occurs when certain fungi get through a “crack or cut in your toe,” allowing the invaders to embed between the toenail and the tissue just underneath. “Nail fungus, also known as onchyomycosis, develops as a result of a fungus under that nail plate, triggering a buildup of debris under the nail plate, thickening of the nail plate, and a white chalky residue,” explains Dr. Ilyas.

If your doctor is not able to diagnose toenail fungus on sight, they may take some nail clippings to be tested to determine the cause of your nail symptoms. Besides prescription oral and topical treatments, there are some over-the-counter creams like terbinafine (Lamisil) that can help—although treatment for toenail fungus isn’t always needed, per the Mayo Clinic. Talk to your health care provider to see which approach is right for you.

Subungual Melanoma Diagnosis and Treatment

If a dermatologist examines your toenails and finds any markings or physical signs to be concerning, they’ll most likely need a biopsy of the nail bed. “The nail itself is a bit of a window to see the skin underneath,” says Dr. Ilyas. “A biopsy of the skin under the nail plate is needed to see if the lesions fit the criteria suggestive of skin cancer.”

If a melanoma is detected via the biopsy, your dermatologist will determine what next steps are required to remove it. “If the melanoma is confined to the epidermis [top layer of skin], then it may be managed conservatively by wide surgical excision of the entire nail apparatus with margin control,” says Dr. Westbay. If the subungual melanoma has spread beyond the skin just below the nail and further into the toe itself, it is considered invasive. Although “traditionally, this has required complete amputation of the digit,” says Dr. Westbay, don’t panic: There is another approach that can preserve the bone and joints of the affected toe called Mohs micrographic surgery.

In this surgery, a thin layer of tissue at the cancer site is removed and studied under a microscope and examined for signs of cancer. The process is repeated until no signs of cancer are seen in the layer of tissue removed.

In cases of metastatic melanoma, where the cancer has spread beyond the toe to another area of the body, “immunotherapy agents are typically selected,” says Dr. Westbay.

When to See a Doctor

Many of us may take our feet for granted but it’s smart to give them some regular heel-to-toenail attention when we kick off our shoes, scrub them in the shower, or prep them for a pedicure. And if you notice unexplained discoloration in your toenails or symptoms like persistent pain, thinning, cracking, bleeding, or nail distortion, see your health care provider or dermatologist to have them examined. Remember: With early detection, subungual melanoma is highly curable, so stay calm and connect with a qualified dermatologist who can make a treatment plan specifically for you.