Pilomatricoma
Pilomatricoma, also known as calcifying epithelioma of Malherbe, is a distinctive benign adnexal tumor that differentiates toward hair matrix cells. Its hallmark is the transformation of matrix cells into ghost (shadow) cells, accompanied by calcium deposits and, secondarily, bone formation.
CTNNB1 Mutations and the Wnt/Beta-Catenin Pathway
- Key molecular mechanism: Most pilomatricomas (75% or more) carry a somatic gain-of-function mutation in exon 3 of the CTNNB1 gene, which encodes beta-catenin.
- Beta-catenin buildup: The mutation removes the serine/threonine sites normally phosphorylated by GSK-3beta, so beta-catenin is not tagged with ubiquitin or broken down by the proteasome. It accumulates to high levels in the cytoplasm and moves into the nucleus.
- Effect on gene transcription: In the nucleus, beta-catenin partners with the transcription factor LEF-1 (lymphoid enhancer-binding factor 1), switching on a primitive hair matrix differentiation program that drives germinative cell proliferation and abnormal keratinization.
How Ghost Cells and Calcification Develop
- Formation of ghost/shadow cells: Basaloid cells in the center of the tumor lobules stop making protein and undergo abrupt keratinizing cell death (apopto-keratinization). They lose their nuclei, leaving a pale round space where the nucleus was, but keep their cell outline and cytokeratin framework.
- Giant cell inflammation and calcification: The mass of anucleate ghost cells acts as a foreign body, provoking granulomatous inflammation with foreign-body giant cells. The alkaline microenvironment favors precipitation of calcium hydroxyapatite, and osseous metaplasia appears in 15–20% of long-standing lesions.
Histopathological Criteria
Microscopic Appearance (H&E)
The lesion sits in the deep dermis or subcutis and is made up of cyst-like tumor lobules with a well-defined fibrous capsule:
- Basaloid cell zone (basaloid cell zone): Usually at the edge of the lobules. Small cells with basophilic cytoplasm, dark nuclei and a high nucleus-to-cytoplasm ratio; mitoses are scattered but not atypical.
- Transitional and ghost cell zone (ghost/shadow cell zone): In the center of the tumor. Round or polygonal cells with pale eosinophilic cytoplasm whose nuclei have disappeared entirely, leaving a pale central nuclear outline.
- Stromal reaction and calcification: Numerous multinucleated foreign-body giant cells surround the ghost cells, with irregular coarse calcium deposits staining deep purple and thickened fibrous connective tissue.
Imaging and High-Frequency Ultrasound
- High-frequency skin ultrasound:
- "Eclipse" pattern (eclipse sign): Highly characteristic: a well-defined hypoechoic mass in the deep dermis or subcutis with a thin hyperechoic rim and strong posterior acoustic shadowing from central calcification.
- Color Doppler: No blood flow inside the calcified mass; vessels are seen only in the surrounding stromal rim.
- Dermoscopy:
- Uniform bluish-white homogeneous areas.
- Opaque yellow-white streaks or clods representing calcium deposits beneath thinned epidermis.
- Radiating telangiectatic vessels around the papule.
Clinical Features
- Who gets it: Most common in children and teenagers (under 20), and among the most frequent benign skin tumors removed in children. It is slightly more common in females than males.
- Typical locations: Head, face and neck (>50–65%), followed by the arms and upper trunk. It is very rare on the legs.
- What it looks and feels like:
- A lump or nodule with a stone-like consistency, lying under the skin or in the deep dermis, 0.5–3.0 cm across.
- Tent sign (tent sign / lorgnette sign): When the overlying skin is stretched between two fingers, angular, uneven contours show through, like tent poles pushing up fabric, because of the calcified mass below.
- Seesaw sign (teeter-totter sign): Pressing on one end of the lump makes the other end pop up, because the tumor is not attached to the deeper tissues.
Treatment and Cosmetic Micro-Excision at Haute-Derma
At Haute-Derma, our goal in treating pilomatricoma is to remove the entire fibrous capsule together with the calcified core, minimizing the chance of rupture and prolonged granulomatous inflammation, while giving children and young people the best possible cosmetic scar.
Ultrasound Mapping and Surgical Planning
- 50 MHz high-frequency ultrasound: Pinpoints the boundary between the tumor capsule and the subcutis and underlying muscle fascia, and shows how firmly the overlying epidermis is attached.
- Planning a cosmetic incision: The incision is marked along natural skin tension lines (Langer's lines) or facial expression lines to hide the scar as well as possible.
Micro-Capsular Enucleation (Micro-Capsular Enucleation)
Technique
- Layered local anesthesia: Lidocaine 1% with epinephrine 1:200,000 is injected into the sub-capsular plane, using hydro-dissection to lift the tumor away from healthy tissue.
- Minimal incision (mini-incision): The cut is only one-half to two-thirds the diameter of the tumor.
- Dissection along the capsule: Fine microsurgical scissors gently separate tissue along the fibrous capsule (CD34+). The firm tumor then slips out through the incision without the calcified core breaking apart.
- Managing tethered skin: If the top of the tumor is stuck to a thinned or atrophic epidermis, a very small ellipse of skin (mini-ellipse) attached to the dome is removed with it so no matrix cells are left behind.
Cosmetic Closure and Histological Examination
- Closing the dead space (dead-space closure): Fine absorbable polyglactin 910 sutures (6-0) close the subcutaneous cavity left by the tumor, preventing a hematoma or seroma.
- Buried skin closure (subcuticular suture): A hidden 7-0 polypropylene suture, finished with sterile adhesive strips (Steri-Strips).
- Routine pathology: Every removed specimen is sent to the pathology lab for H&E staining and an IHC panel (beta-catenin, LEF-1, Ki-67) to rule out the malignant variant, pilomatrical carcinoma.
