Discussion

DT is a rare benign skin tumour derived from the outer sheath of the hair follicle. This occurs with a frequency of about 0.003% of all skin tumours.6 This correlates with our 14 years of data; trichilemmoma is uncommon as it comprised 0.006% of all eyelid specimens encountered. Trichilemmoma is more prevalent in men, even though our data showed more females affected (61%).6 Trichilemmoma is most commonly found in the Caucasian population in the sixth decade of life.7 We found a similar racial and age distribution in our series; all of Caucasian race and majority being in their 60s.

Typically, this lesion appears as a slow­-growing, solitary, hard, annular lesion, 5–10 mm in size, with raised border and depressed centre. Not uncommonly, trichilemmoma may also have a papular, nodular, dome-shaped, papillomatous, verrucous or cutaneous-horn appearances. They may be erythematous, skin-coloured, keratotic, smooth and some even have central ulceration. Because of its clinically highly variable appearances, trichilemmoma is often misdiagnosed as other benign or malignant skin lesions such as BCC (nodular or morphoeic forms), SCC, follicular keratosis or cutaneous horn.3 Our cohort of patients displayed various clinical appearances and this is reflected on our variable clinical differential diagnoses (SCC, BCC, papilloma, actinic keratoses and sebaceous cyst). Multiple trichilemmomas that mainly involve the face are features of Cowden’s disease which is a multiple hamartoma syndrome with increased risk of carcinoma of breast and thyroid.8

Histopathological misdiagnosis of trichilemmoma is not uncommon and patients may be subjected to unnecessary extensive surgery and anxiety surrounding the diagnosis of malignant skin condition. Histopathological appearances of trichilemmoma may resemble tricholemmal carcinoma, SCC, trichoepithelioma and inverted follicular keratosis. Illueca et al described the use of CD34 immunohistochemistry for DT to differentiate it from other tumours with desmoplastic differentiation particularly BCC. BerEP4 stains BCC but not DT and CD34 stains DT but not BCC.9 Occasionally, the CD34 staining of DT may be focal. Careful analysis of histological appearances coupled with immunohistochemisty is essential in making the correct diagnosis.

The aetiology of trichilemmoma is not fully understood. Due to similar histological and clinical appearance to verruca vulgaris, it was suggested that trichilemmoma may be caused by human papillomavirus.3 6 10 However, subsequent study did not support this.11 A tumour suppressor gene, PTEN/MMAC1, has been associated with aetiology of trichilemmoma in Cowden’s disease.12

In the published literatures, the management of trichilemmoma is non-specific including shave excision, curretage, 5-fluorouracil and complete excision.13 Depending on the clinical suspicion of the lesion, our standard practice is to perform a biopsy, complete excision or staged excision and reconstruction. Mohs micrographic surgery is an alternative surgical management for trichilemmoma.14 Mohs micrographic surgery is particularly useful for trichilemmoma of the face as it provides maximum tissue-sparing benefit and excellent cosmetic results. No patient in our case series underwent Mohs micrographic surgery and this is due to service provision in our department.

It was previously reported that BCC can occur in association with DT.5 Such cases may represent primary DT with contiguous BCC or primary BCC with tricholemmal differentiation.15 Histopathological analysis across specimens is essential in determining the nature of the lesion. This is illustrated in case 2, where the analysis across specimens showed that majority of lesion was DT (95%) with a minor BCC component. Therefore, case 2 represents a DT with focal transformation to BCC or coexistence with BCC, rather than primary BCC with tricholemmal differentiation. Trichilemmoma is derived from the external sheath cells of pilosebaceous units.16 BCC is a malignant tumour of trichoblast differentiation and possibly of follicular derivation.17 Therefore, coexistence of the two lesions can be explained by their hair follicle origins. DT has also been reported occurring within a naevus sebaceous.18 19

Following the first operation, three patients (8%) had trichilemmoma with associated BCC. In those without coexisting BCC, 12 patients (36%) opted to be discharged with incompletely excised trichilemmoma because of the benign nature of the lesion. A patient (3%) from this cohort, returned two years later with BCC. This highlights that an incomplete excision of trichilemmoma can give a false sense of reassurance. For incompletely excised trichilemmoma, we strongly advocate offering further surgery to achieve clear surgical margins to ensure that potential coexisting local malignancy is not left untreated. In our experience, elderly patients tend to opt for observation instead of multiple surgeries to achieve clear surgical margins for this benign lesion. Out of 36 cases of biopsy-proven eyelid trichilemmoma, we identified four cases with coexisting BCC, thus giving a frequency of 11%. Those who opt for observation should be warned of the risk of coexisting BCC. Similar to patients with BCC, this group of patients should be offered 6-monthly follow-up for three to five years with sequential clinical photography and examination. Alternatively, they can be discharged with advice to report any signs of recurrence of the lesion, skin changes or loss of lashes especially at the site of previous trichilemmoma lesion. This allows for better workload prioritisation by healthcare providers.