A plantar fibroma is a fibrous soft tissue mass (lump) in the arch of the foot. It is part of the plantar aponeurosis which is a band that extends from the heel to the toes on the bottom of the foot. A fibroma is benign and may or may not cause pain. Fibromas may slowly enlarge over time and there can be multiple. They can be on one foot or both. The exact cause cannot typically be determined. Trauma and some systemic diseases can prone one to developing them.