Clinical recognition

Usually presents on the scalp as a firm, lobulated, exophytic, cystic or ulcerated nodule/tumour.
Often develops within or adjacent to a long-standing pilar cyst; a history of sudden enlargement is common.
Typical patients are middle-aged or elderly adults, with a female predominance in many series.
The lesion may be tender, inflamed, foul-smelling, bleeding or secondarily infected, which can obscure the underlying tumour.
Large, recurrent, fixed, ulcerated or very rapidly enlarging lesions should be regarded as higher risk until histology proves otherwise.

Pathology

The defining pattern is trichilemmal keratinisation: abrupt keratinisation without an intervening granular layer, reflecting outer-root-sheath differentiation.
Benign lesions are usually well circumscribed and pushing; atypical lesions show cytological atypia and mitotic activity but may remain circumscribed.
Malignant proliferating trichilemmal tumour shows infiltrative growth, marked atypia, high mitotic activity, necrosis and/or deep invasion.
The histological boundary with well-differentiated cSCC can be difficult; specialist dermatopathology review is appropriate for atypical or malignant cases.
Report wording should communicate whether the lesion is benign, atypical/uncertain or malignant, and whether margins are clear.

Inflamed or ruptured pilar cyst and epidermoid cyst.
Cutaneous squamous cell carcinoma or keratoacanthoma, particularly on the scalp.
Trichilemmal carcinoma, pilomatrix carcinoma and other follicular adnexal tumours.
Cutaneous metastasis or ulcerated BCC in an older patient with a scalp tumour.
Abscess should be considered, but do not incise and drain a persistent solid scalp mass without a plan for histology.

Complete surgical excision with histological margin assessment is preferred for suspected proliferating trichilemmal tumour.
Simple cyst excision may be insufficient if the lesion is large, recurrent, ulcerated, infiltrative or histologically atypical.
There is no universally standardised margin width; tailor excision to size, site, histological grade and MDT advice.
Malignant or recurrent lesions warrant skin-cancer MDT discussion, regional-node examination and consideration of imaging if deep invasion, fixation, neurological symptoms or nodal concern is present.
Mohs/slow Mohs may be considered in selected anatomically constrained cases where margin control is important, depending on local expertise and pathology support.

Benign completely excised lesions have a low risk of recurrence, but recurrence is possible if incompletely removed.
Atypical and malignant lesions have higher recurrence risk; malignant lesions can metastasise to regional nodes or distant sites, although this is uncommon overall.
Follow-up should be risk-adapted: review the scar and regional nodes after malignant or atypical histology, and ensure rapid reassessment for recurrent growth.
Document the exact histological diagnosis because “pilar cyst”, “proliferating trichilemmal cyst” and “malignant proliferating trichilemmal tumour” imply very different levels of concern.
Re-excision should be considered when margins are positive or close in an atypical/malignant lesion.