Can a black or brown dot under toenails be a sign of skin cancer?
Any dot or streak under a toenail or fingernail that has not always been there should be investigated by a dermatologist, the physician responsible for the health of skin, hair, and also nails.
A brown or black dot or streak under a toenail or fingernail could be a blood clot, fungus infection, or possibly a skin cancer.
If it is a melanoma skin cancer, it can be very serious, unfortunately, and may require surgery and other treatments. My advice is to get the nail checked as soon as possible, for reassurance or treatment.
Although most black dots seen in nail are collections of dried blood; when this is the case the dot will move farther towards the tip of the nail with time. A dermatologist is best to diagnose whether this is the case or there is another harmless cause, such as normal mole or "nevus", fungus or mold. Melanomas do occur in the nail and are very serious, primarily because of delay in diagnosis (Bob Marley died from a melanoma of a toe). What often complicates the picture is that 40% of people that have a melanoma of the nail coincidentally report a prior history of trauma - leading to a false sense of security. Brown or black color that extends onto the cuticle, or Hutchinson's sign, is a strong indicator that the spot is likely to be indeed a melanoma and needs urgent attention.