A lump in the arch of the foot is a symptom of a type of skin cancer known as melanoma, states Medical News Today. A lump in the foot's arch is usually caused by a noncancerous nodule known as plantar fibroma, says the American Orthopaedic Foot & Ankle Society.
Melanoma is the rarest yet the most dangerous type of skin cancer. Foot melanoma is possibly caused by viruses, exposure to carcinogens or genetics. It starts in the epidermis of the skin and is usually treatable if recognized early, notes Medical News Today. Plantar fibroma is usually found in the center of the foot. It is caused by cysts, nerve tumors, tendon ruptures or genetics, and is commonly treated with an in-sole for relieving pressure on the foot and surgery, according to the American Orthopaedic Foot & Ankle Society,