Mucinous carcinoma
Primary cutaneous mucinous carcinoma (PCMC) is a very rare malignant adnexal tumor of the skin, usually showing eccrine or apocrine sweat gland differentiation. Its biology is distinctive: it grows slowly and quietly, produces large pools of extracellular mucin, and has a high local recurrence rate (30–40%), yet it spreads to distant sites far less often than mucinous carcinoma that has metastasized to the skin from an internal organ.
How the Extracellular Mucin Matrix Builds Up (MUC2 / MUC5AC)
- Overproduction of mucin glycoproteins: PCMC tumor cells secrete large amounts of high-molecular-weight glycoproteins (mainly MUC2 and MUC5AC) into the dermal stroma.
- A protective "shield" for the tumor: The mucin pools act as an artificial extracellular matrix that walls tumor nests off from immune attack (T cells, NK cells) and limits blood vessel ingrowth. This helps explain why the tumor grows so slowly yet so persistently.
- Invasion by mucin dissection: Hydrostatic pressure pushes mucin through the fibrous septa, splitting the surrounding stroma and creating microscopic extensions that reach well beyond the clinically visible edge of the tumor.
Hormone Receptors (ER/PR) and Differentiation
- As in breast carcinoma, roughly 50–80% of PCMCs show strong expression of estrogen receptor (ER) and progesterone receptor (PR).
- Estrogen signaling helps maintain the glandular, acinar pattern of differentiation and keeps the Ki-67 proliferation index low (<10%), while supporting long-term survival of the tumor cells.
Histopathology and Immunohistochemistry (IHC) Criteria
Microscopic Features (H&E)
The tumor sits in the dermis and subcutaneous fat and shows the following classic microscopic features:
- Large extracellular mucin lakes: Pools of faintly basophilic or neutral mucin that stain with Alcian blue (pH 2.5) and diastase-resistant PAS (periodic acid–Schiff).
- Floating cellular islands: Clusters, nests or cords of small to medium-sized basaloid epithelial cells suspended within the mucin.
- Fibrous septa: Thin bands of fibrous tissue carrying small feeding vessels divide the mucin into many small compartments.
- Cytology: Tumor cells are polygonal or round with uniform nuclei and few atypical mitoses. Small tubular or cribriform glandular structures may be seen in the center of the cell islands.
Immunohistochemistry (IHC) Panel for Differential Diagnosis
Distinguishing primary PCMC from mucinous carcinoma that has metastasized from an internal organ (especially the colon and rectum, breast, lung or appendix) is MANDATORY, because it determines the entire prognosis and treatment plan.
| IHC Marker | Primary Cutaneous Mucinous Carcinoma (PCMC) |
|---|---|
| CK7 | STRONGLY POSITIVE (+++) |
| CK20 | NEGATIVE |
| CDX2 | NEGATIVE |
| SATB2 | NEGATIVE |
| ER / PR | POSITIVE (2+ to 3+) |
| GATA3 | Weakly positive or negative |
| TTF-1 | NEGATIVE |
| p63 / SMA | Patchy positivity at the periphery (myoepithelial cells) |
Imaging and High-Frequency Ultrasound
- High-frequency skin ultrasound:
- A clearly anechoic or hypoechoic mass, reflecting its acellular mucinous content, crossed by thin hyperechoic bands (fibrous septa).
- Little color Doppler signal within the mucin pools; vascular signal appears only in the peripheral septa.
- Whole-body PET/CT and screening endoscopy: Routinely required before a definitive diagnosis of PCMC is made, to exclude a primary tumor in the gastrointestinal tract or breast.
Clinical Presentation
- Typical sites: About 45–50% of cases occur around the eye (upper or lower eyelid, inner corner of the eye). Other sites include the scalp, face, neck and armpit.
- Appearance:
- A solitary nodule or papule, soft or firm, smooth and shiny, pale pink, reddish-gray or faintly bluish. It can resemble a hidrocystoma or a basal cell carcinoma (BCC).
- Usually 0.5–3.0 cm across, growing very slowly over months to years.
- Symptoms: Usually painless and not itchy. On the eyelid it may only cause drooping (ptosis) or obstruct vision.
Our Multimodal Treatment Protocol at Haute-Derma
Because the tumor favors the eyelids and face and spreads beneath the stroma through mucin dissection, Haute-Derma follows a standardized protocol that combines screening to exclude metastasis, Mohs micrographic surgery and hormonal management:
Systemic Screening to Exclude Metastatic Disease (Mandatory)
Before a skin lesion can be called primary PCMC, the patient must complete a full systemic work-up:
- Total colonoscopy: To exclude metastatic mucinous carcinoma of the colon, rectum or appendix.
- Mammography and breast MRI: To exclude mucinous breast carcinoma.
- Whole-body PET/CT: To look for FDG-avid lesions in the lungs, ovaries or stomach.
Mohs Micrographic Surgery (MMS): The Gold Standard
- Why MMS is recommended:
- The eyelid predilection means as much healthy tissue as possible must be preserved to avoid lid margin distortion and incomplete eyelid closure (lagophthalmos).
- Mucin dissection makes the clinical margins unreliable; conventional wide local excision (WLE) has recurrence rates as high as 30–40%.
- How we perform it at Haute-Derma:
- The tumor is removed in thin, saucer-shaped layers.
- Each layer is color-mapped and 100% of the microscopic margin is examined on frozen sections.
- Intraoperative Alcian blue staining is added when needed to detect hidden mucin pools containing tumor cells at the margin.
- Excision continues until both deep and lateral margins are completely clear of tumor cells and extracellular mucin.
Adjuvant Hormonal Therapy and Long-Term Follow-Up
- Adjuvant endocrine therapy:
- For recurrent tumors that cannot be fully excised, or tumors with strong ER/PR expression (+++), tamoxifen or an aromatase inhibitor (anastrozole/letrozole) may be considered to slow tumor progression.
- Long-term surveillance:
- Because PCMC can recur late, after 3–5 years, patients need clinical review and high-frequency ultrasound of the surgical site every 3 months for the first 2 years, then every 6 months until year 5.
