Skin Adnexal (Appendage) Tumours — 10 Marks

Definition

Classification (by line of differentiation)

Key Individual Lesions (exam-relevant detail)

• Apocrine/eccrine differentiation; classically on the forehead and scalp.
• Multiple coalescing nodules over time can produce a hat-like ("turban") growth.
• Histology: islands of basaloid cells fitting together like jigsaw puzzle pieces, occasional ducts, hyaline basement-membrane material rimming the islands.
• Familial cylindromatosis: autosomal dominant, germline inactivating mutation in CYLD (a deubiquitinating enzyme that negatively regulates NF-κB). Same gene defect can produce trichoepithelioma and spiradenoma in the same family (Brooke-Spiegler syndrome).

• Follicular differentiation; perinasal/facial papules and small nodules.
• Histology: buds of basaloid cells resembling primitive hair follicles.
• Can clinically and histologically mimic basal cell carcinoma — an important differential.

• Eccrine differentiation; multiple small tan papules, classically around the lower eyelids.
• Name from Greek "syrinx" = pipe/tube, reflecting small duct-like structures on histology.

• Occurs predominantly on palms and soles (sites rich in eccrine glands).

• Benign sebaceous differentiation.
• Clinically important marker lesion: associated with Muir-Torre syndrome (a variant of Lynch syndrome, due to germline DNA mismatch-repair gene defects) — presence should prompt screening for internal malignancy (colorectal, endometrial, other visceral cancers).

• Follicular differentiation.
• Multiple trichilemmomas are a cutaneous marker of Cowden syndrome (germline PTEN tumour-suppressor mutation), which carries increased risk of breast, endometrial, and other internal cancers.

Clinical Significance (why adnexal tumours matter beyond dermatology)

1. Diagnostic mimicry — many benign adnexal tumours (e.g., trichoepithelioma) closely resemble basal cell carcinoma clinically and histologically, requiring careful histopathological distinction (BCC shows peripheral palisading, retraction artifact, and mitotic/apoptotic activity; trichoepithelioma is more symmetric with papillary mesenchymal bodies and lacks significant mitotic activity).
2. Genodermatosis markers — several adnexal tumours are cutaneous flags for underlying germline cancer-predisposition syndromes:
   • Trichilemmomas → Cowden syndrome (PTEN)
   • Sebaceous adenomas → Muir-Torre/Lynch syndrome (MMR genes)
   • Cylindromas/trichoepitheliomas/spiradenomas → familial cylindromatosis/Brooke-Spiegler syndrome (CYLD)
3. Site predilection aids clinical diagnosis — cylindroma (scalp/forehead), syringoma (eyelids), eccrine poroma (palms/soles).
4. Malignant transformation — though most are benign, malignant counterparts exist for every category (sebaceous carcinoma, trichilemmal carcinoma, eccrine porocarcinoma, malignant cylindroma), so histological confirmation is essential.

Suggested Answer Structure for 10 Marks

1. Definition and classification principle (2 marks)
2. Tabulated classification by differentiation with examples (3 marks)
3. Description of 2-3 key lesions with histology (3 marks)
4. Clinical significance / syndromic associations (2 marks)