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Dermatofibromas (fibrous histiocytomas) are red-to-brown, firm nodules, ranging in diameter from a few millimeters to 2 centimeters. These benign lesions are often located on the lower extremities and tend to persist indefinitely. Lateral pressure on the lesion produces dimpling of its surface.1 Larger lesions may be concerning for dermatofibrosarcomaprotuberans, which is a rare, more invasive lesion. Biopsy is diagnostic in both cases and may be necessary to distinguish the lesions. Biopsy of a dermatofibroma reveals prominent fibrocytes and coarse bundles of collagen in a haphazard fashion, frequently arranged in short fascicles that interweave (“storiform pattern”). There is often hyperplasia and hyperpigmentation of the overlying epidermis.2Dermatofibrosarcoma protuberans is usually located on the trunk, often has a blue hue, and may be mistaken for a vascular lesion.3 It is locally invasive and may only rarely metastasize. Wide local excision or Mohs surgery is indicated, and preoperative MRI may help delineate the extent of the lesion before surgery.4

Mastocytosis is a group of disorders in which increased numbers of mast cells infiltrate tissues and organs, especially the skin. Symptoms result from degranulation of mast cells. In contrast to adult forms of the disease, mastocytosis in children often spontaneously resolves. In the most common form of cutaneous mastocytosis, urticaria pigmentosa, a varying number of brown-to-orange macules, papules, or plaques cause a cobblestone or orange peel–like appearance over any part of the skin surface (Fig. 363-1). Spontaneous wheals, vesicles, or paroxysms of pruritus are not uncommon. The solitary mastocytoma presents as a single orange-brown nodule or plaque that may also urticate and blister with trauma, although severe pruritus is uncommon. Diffuse cutaneous mastocytosis is a rare, severe form in which there is marked and widespread infiltration of the skin with mast cells. The skin is thickened, often with a yellow-orange hue, and widespread blistering is often present. Bone, liver, spleen, gastrointestinal tract, and other organs may also be involved. Extracutaneous disease, more common in adults than in children, may be associated with flushing and tachycardia, hypotension, syncope, apnea, headache, vomiting, diarrhea, and/or abdominal pain.5 Mutations in the c-kit gene have been demonstrated in adult patients with mastocytosis, but affected children do not tend to have this mutation.6 The diagnosis of mastocytosis can usually be confirmed clinically by inducing an urticarial wheal-and-flare reaction by stroking a lesion (Darier sign); however, this sign is not entirely specific to mastocytosis. Geimsa or Toluidine blue-stained skin biopsies demonstrate increased numbers of mast cells.7 The combination of H1 and H2 antihistamines is used to decrease the frequency and severity of episodes of mast cell degranulation. Patients with severe gastrointestinal symptoms may be helped by oral disodium cromoglycate. Most children with urticaria pigmentosa follow a benign self-limited course with improvement or resolution occurring during the first decade. Exposures to medications or physical factors that induce mast cell degranulation, including aspirin, alcohol, morphine, codeine, thiamine, scopolamine, polymyxin B, and very hot or cold ...

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