INTRODUCTION Also known as a calcifying epithelioma of Malherbe, pilomatrixoma is a benign tumor of the hair cortical cells. The lesion tends to occur in children and young adults, with 75% less than 10 years of age. The head and upper extremities are the most common sites of involvement with a significant proportion occurring in the periorbital region, particularly the upper eyelid and brow. Most lesions are misdiagnosed as epidermoid and dermoid cysts, and are unsuspected until histopathologic examination.
CLINICAL PRESENTATION Usually solitary, these slowing growing lesions appear as a solid or cystic, mobile, subcutaneous nodule with normal overlying skin. They are firm, irregular, masses with the overlying skin showing a pink or blue discoloration. In 15% to 18% of cases the lesion can be associated with pain or inflammation. Stretching the overlying skin taut may reveal a whitish, multifaceted tumor, the so-called "tent sign." Dilated vessels may be seen on the tumor surface.
HISTOPATHOLOGY Pilomatrixomas are composed of islands of basophilic cells surrounding eosinophilic shadow cells. The basophilic cells have little cytoplasm, indistinct cell borders, hyperchromatic nuclei, and abundant mitoses. Eosinophilic shadow cells represent mummified basophilic cells and have more cytoplasm and distinct, though faint, cell borders. There may be abrupt transition from the basophilic cells to the eosinophilic shadow cells, or there may be an intermediate zone of transitional cells that develop progressively more eosinophilic cytoplasm while the nucleus becomes pyknotic. Foci of calcification are very common. Pilomatrixomas are dermal lesions that often extend into the subcutis.
DIFFERENTIAL DIAGNOSIS Differential diagnosis includes epidermoid and dermoid cysts, sebaceous cyst, squamous cell carcinoma, and vascular lesions.
These lesions do not regress. Treatment is with surgical excision including the overlying skin. Incomplete removal is associated with recurrence.
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