Trichoblastic carcinoma

Clinical features

• Slow-growing, often well-circumscribed dermal/subcutaneous nodule.
• Most common on the head and neck of older adults; occasionally trunk and limbs.
• May arise within or adjacent to a long-standing trichoblastoma.
• Often misdiagnosed clinically as cyst, BCC, lipoma or epidermoid cyst.
• Median age 60–70; M:F roughly equal.

Histology & differential

• Lobular dermal proliferation of basaloid germinative cells with peripheral palisading and follicular germ-like differentiation.
• Stromal induction with "papillary mesenchymal bodies" — clusters of fibroblasts adjacent to basaloid nests.
• Distinguishing features from benign trichoblastoma:
  • Significant cytological atypia.
  • Brisk and atypical mitoses.
  • Infiltrative growth pattern beyond the original lesion.
  • Tumour necrosis.
  • Lymphovascular or perineural invasion in higher-grade lesions.
• Distinguishing from BCC:
  • Trichoblastic differentiation more pronounced (papillary mesenchymal bodies, follicular germ, sometimes hair-bulb-like structures).
  • Lack of overlying epidermal connection in many cases.
  • BerEP4 positive in both — not discriminating.
  • CK20+ Merkel cells present in trichoblastoma / trichoblastic carcinoma but absent in BCC (variable).
  • PHLDA1 strongly positive in trichoblastoma / trichoblastic carcinoma but typically negative in BCC.
  • Despite these markers the distinction remains contentious.
• Trichoblastic carcinosarcoma — biphasic tumour with epithelial trichoblastic component and a malignant mesenchymal (sarcomatous) component; aggressive behaviour.

Management

• Wide local excision with at least 5–10 mm margins; Mohs micrographic surgery for facial / cosmetically critical sites.
• Imaging staging in high-grade or large lesions.
• Sentinel lymph node biopsy not routinely performed; nodal metastasis is uncommon.
• Adjuvant radiotherapy for incomplete margins, perineural invasion or recurrent disease.
• Systemic therapy for advanced / metastatic disease — limited evidence; case reports of hedgehog pathway inhibitor activity (in lesions with PTCH1 mutation).
• Trichoblastic carcinosarcoma — sarcoma MDT involvement, multimodality therapy.

Prognosis

Generally indolent — local recurrence is the main concern (10–20%); regional or distant metastasis is rare (<5% in most series). The aggressive carcinosarcoma variant has a worse prognosis. Long-term surveillance for at least 5 years with annual full skin examination is appropriate.