Dermatofibroma (fibrous histiocytoma) is a common benign skin lesion.
PATHOGENESIS AND EPIDEMIOLOGY
Debate remains about whether dermatofibromas are neoplastic lesions or reactive processes secondary to cutaneous injury. Lesions are most common in young adults, with a slight female preponderance.
Dermatofibromas are red-to-brown, firm nodules, typically less than 1 cm in diameter, that occur most often on the lower extremities and tend to persist indefinitely. Lateral pressure on the lesion produces dimpling of its surface.
The differential diagnosis for a dermatofibroma includes a melanocytic nevus, blue nevus, Spitz nevus, and a scar. Larger lesions may be concerning for dermatofibrosarcoma protuberans, which is a rare, locally invasive malignant neoplasm with a small risk of metastasis. It is usually located on the trunk, often has a blue hue, and may be mistaken for a vascular lesion. Biopsy may be required to distinguish benign dermatofibroma from invasive dermatofibrosarcoma protuberans. Biopsy of a dermatofibroma reveals prominent fibrocytes and coarse bundles of collagen in a haphazard fashion in the dermis, frequently arranged in short fascicles that interweave (“storiform pattern”). There is often hyperplasia and hyperpigmentation of the overlying epidermis. In contrast, dermatofibrosarcoma protuberans is composed primarily of monomorphous spindle-shaped cells, often invading into the fat and deeper tissues.
Because dermatofibromas are benign, no treatment is required; however, pruritic or unaesthetic lesions can be treated with excision, cryotherapy, intralesional steroid injection, or pulsed dye laser therapy. Complete excision including removal of underlying subcutaneous fat reduces the risk of recurrence but may leave a scar more prominent than the original lesion. For dermatofibrosarcoma protuberans, wide local excision or Mohs surgery is indicated, and preoperative magnetic resonance imaging may help delineate the extent of the lesion before surgery.
Mastocytosis is a group of disorders in which increased numbers of mast cells infiltrate tissues and organs, especially the skin. Cutaneous mastocytosis may be localized or disseminated and is sometimes subdivided into (1) maculopapular cutaneous mastocytosis (MPCM), which is commonly known as urticaria pigmentosa; (2) solitary or multiple mastocytomas; and (3) diffuse cutaneous mastocytosis (DCM).
PATHOGENESIS AND EPIDEMIOLOGY
Mastocytosis is caused by overexpansion of mast cells. Symptoms result from mast cell degranulation, which releases substances including histamine, tryptase, serotonin, and tumor necrosis factor. Activating mutations in the c-kit gene are common in both adult and pediatric mastocytosis; most adult patients carry the KIT D816V mutation, whereas patients with pediatric-onset mastocytosis typically express other KIT mutations.
The most common forms of cutaneous mastocytosis are solitary or multiple isolated mastocytomas (MPCM), in which a varying number of brown-to-orange macules, ...