PA34 A trichilemmal cyst on dark skin mimicking dermatofibrosarcoma protuberans and nodular melanoma

Abstract A 14-year-old boy with Fitzpatrick skin type VI presented with a nontender lump over his right anterior thigh. This was first noticed by his parents 4 years prior as a 3-mm smooth, skin-coloured nodule that gradually increased in size. There was no history of trauma or insect bites or a family history of skin cancer or lesions. The patient has a language disability disorder but is otherwise well with no regular medications. Two weeks before presenting, the patient had fallen on the lesion, and it became significantly tender, larger and darker with oozing of pus and blood. The patient also reported noticeable weight loss during this period. He had completed a 7-day course of co-amoxiclav, which had no effect. Physical examination demonstrated a 20 × 15-mm darkly pigmented nodule, with a flaccid bulla surrounding its base. Dermoscopy showed structureless areas with increased melanization. Although no lymphadenopathy was found on examination, concern was raised for a potential nodular melanoma. The patient was sent for an urgent magnetic resonance imaging scan, which revealed a part solid, part cystic nodule within the subcutaneous tissue. It had no clear invasion through the fascia but showed perilesional subcutaneous oedema. With these clinical features and imaging results, dermatofibrosarcoma protuberans (DFSP) was considered as a potential differential diagnosis. This case history and imaging was discussed in the sarcoma multidisciplinary team, which could not rule out DFSP and recommended excision. The patient underwent an excisional biopsy with 4-mm margins under general anaesthesia. Histology showed a well-circumscribed cyst lined by stratified squamous epithelium, with focal scattered duct-like structures. The lumen contained brightly eosinophilic keratin, inflammatory and necrotic debris, and focal calcification – all features consistent with a trichilemmal cyst (TC). No evidence of dysplasia or malignancy was noted. TCs, also known as pilar cysts, are benign, keratin-filled cysts originating from the external root sheath of hair follicles. They are most commonly found on the scalp, and can be mistaken for an epidermoid cyst or dermatofibroma. Clinicians should be aware that TCs can occasionally mimic nodular melanoma and DFSP, particularly in individuals with darker skin tones (Fitzpatrick skin types V and VI), as demonstrated in our case. Such differentials require urgent excision and histological investigation, with Mohs micrographic surgery being indicated in certain cases. However, it is even more critical to recognize that DFSP often presents as a benign-appearing lesion and is frequently misdiagnosed. Maintaining a high index of suspicion and thoroughly evaluating any lesion with atypical features – regardless of initial clinical impressions – is essential to avoid missing a potential diagnosis of DFSP.