Question:

A 60-year old man presented with a 2-month history of a nodule on the hand. It had first presented as a small papule on the dorsum of the hand which had rapidly increased in size in the past few weeks. He was otherwise asymptomatic and in good health and enjoyed playing golf.

Answer: B. Keratoacanthoma

Keratoacanthoma (KA) is a relatively common tumour of squamous cells which can resolve spontaneously without treatment. However, it has to be distinguished from squamous cell carcinoma with which it bears some clinical similarity. Males are more commonly affected (male: female ratio: 3:1). Keratoacanthoma is most common in middle age, in contrast to squamous cell carcinoma and basal cell carcinoma. Chronic sun exposure is the most common finding. Individuals exposed to tar and mineral oil and transplant recipients on immunosuppression are also at increased risk of developing KA.

Keratoacanthoma first presents as a firm, red or flesh-coloured papule which rapidly enlarges over a few weeks to form a well-defined dome-shaped nodule up to 10-20 mm in diameter. It most often occurs on the central part of the face, dorsum of the hand, wrist and forearm (sun-exposed areas). The lesion is dome-shaped, symmetrical and there is a keratin-filled crater at the centre. Lesions may reach 50 mm or more in diameter. After about 2 to 8 weeks, spontaneous resolution occurs as the lesion recedes over 2 to 8 weeks towards its base, leaving a pitted scar. Recurrences may occur after spontanous resolution, especially in large lesions. Clinically, the main differential diagnosis is squamous cell carcinoma. However, the younger age group and history of rapid growth provide a clue to the diagnosis. Other differential diagnoses include viral wart, actinic keratosis and molluscum contagiosum.

Histologically, there is an exoendophytic proliferation of eosinophilic squamous epithelium with a central keratin-filled crater. Well-differentiated squamous epithelium is found at the edge of the crater. There are variable atypical mitoses and inflammation at the base of the proliferation.

Although the lesion may resolve spontaneously, excision is recommended for KA to exclude squamous cell carcinoma and also to avoid excessive scarring. Waiting for spontaneous resolution is hazardous as the lesion may be an invasive squamous cell carcinoma and there have been rare cases of KA progressing to squamous cell carcinoma. For patients who refuse excision, radiotherapy as well as topical 5-fluorouracil ointment have been used.