Fibroma, desmoplastic: A rare type of primary bone tumor that is characteristically composed of well-differentiated cells that produce collagen. Desmoplastic fibromas are discovered most often in the first three decades of life, in the mandible (the femur and pelvis are also favored sites). Although benign, these tumors infiltrate locally and may cause pain and swelling or fluid accumulation. Treatment is surgical removal, but desmoplastic fibromas may recur. Also known as collagenous fibroma.