Pilomatricoma

Pilomatricoma is a benign skin tumor derived from the hair matrix. These neoplasms are relatively uncommon and typically occur on the scalp, face, and upper extremities. Clinically, pilomatricomas present as a subcutaneous nodule or cyst with unremarkable overlying epidermis that can range in size from 0.5 to 3.0 cm, but the largest reported case was 24 cm.

Associations
Pilomatricomas have been observed in a variety of genetic disorders including Turner syndrome, myotonic dystrophy, Rubinstein–Taybi syndrome, trisomy 9, and Gardner syndrome. It has been reported that the prevalence of pilomatricomas in Turner syndrome is 2.6%.

Hybrid cysts that are composed of epidermal inclusion cysts and pilomatricoma-like changes have been repeatedly observed in Gardner syndrome. This association has prognostic import, since cutaneous findings in children with Gardner syndrome generally precede colonic polyposis.

Histologic features
The characteristic components of a pilomatricoma include a stroma of fibrovascular connective tissue surrounding irregularly shaped, lobulated islands containing basaloid cells (being darkly stained, round or elongated, with indistinct cell borders and minimal cytoplasm, with nuclei being round to ovoid, deeply basophilic and generally prominent nucleoli), which abruptly or gradually transitions into ghost cells (having abundant, pale, eosinophilic cytoplasm, well defined cell borders and a central clear area, but only faint traces of nuclear material), which in turn may transition into keratinaceous to amorphous necrosis.

The presence of calcifications with foreign-body giant cells is common within the tumors.

Pathogenesis
Pilomatricoma is associated with high levels of beta-catenin caused by either a mutation in the APC gene or a stabilizing mutation in the beta-catenin gene, CTNNB1. A high level of beta-catenin increases cell proliferation, inhibits cell death, and ultimately leads to neoplastic growth.