A Lesion Hits Its Growth Spurt

DISCUSSION
The contents are consistent with those of a somewhat unusual lesion, commonly called pilomatricoma. It is also known as calcifying epithelioma of Malherbe and pilomatrixoma.

This type of cyst is derived from the hair matrix and is commonly seen on the face, neck, scalp, and arms of children and young adults. This patient’s lesion was atypical in its prominence and erythema, at odds with the firm bluish intradermal papule or nodule usually seen in these cases. But the unique contents established the diagnosis with considerable certainty.

All that remained was the excision—which, given the patient’s age and the cosmetic concerns, would require above-average surgical skills. Once removed, the sample will be sent for pathologic examination, which should show anucleate squamous cells (“ghost cells”), benign viable squamous cells with a lining consisting of basaloid cells. Calcifications with foreign body giant cells account for the pathognomic white flecks seen in the extruded material.

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Pilomatricoma’s cause is debatable, but it appears to involve increased levels of beta catenin caused by mutations of the APC gene. This effectively inhibits apoptosis, leading to focal increases in cell growth.

The differential for this type of lesion includes simple acne cyst (unlikely in such a young child), carbuncle (which would have been quite painful and full of pus), or even squamous cell carcinoma.

TAKE-HOME LEARNING POINTS

• Pilomatricomas are benign cysts usually seen on the face, neck, scalp, and arms of children and young adults.
• The typical pilomatricoma (sometimes called calcifying epithelioma of Malherbe) is an intradermal papule or nodule, often displaying a faintly bluish color, that is relatively firm on palpation.
• The contents of a pilomatricoma usually consist of whitish curds or flecks of material that represent calcified tissue mixed with foreign body giant cells.
• Pilomatricoma has little or no malignant potential but is often cosmetically significant.