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Squamous cell carcinoma (SCC) is the second most common cutaneous malignancy. It may arise de novo or from pre-cancerous lesions such as actinic keratosis (AK) and Bowen’s disease.
Cumulative exposure to ultraviolet (UV) radiation is the leading cause, hence the majority of lesions are seen on sun-exposed sites in patients who have fair skin and are more than 60 years old. Seventy to 80% of SCCs occur on the head and neck, especially the lower lip, ears and scalp. Other common sites are the dorsal surfaces of the hands, forearms and lower legs. SCC on non-exposed skin does occur, but is much less common. Patients who are immunosuppressed after an organ transplant are estimated to be at a 65 - 250-fold greater risk of developing SCC.
History
SCCs are recognized by patients as growths that have developed over a period of several weeks or months. Keratoacanthomas (KAs) develop at a similar rate and cannot be distinguished by the history alone. Lesions are usually asymptomatic, but may be tender. Bleeding often occurs because SCCs invade dermal blood vessels. KAs are epidermal lesions that do not invade the dermis and so do not normally bleed, a useful clinical distinction.
Clinical appearance
Most SCCs are ulcerated nodules, although some present as expanding ulceration and others as plaques. A consistent physical sign in SCC is induration, which is thickening at the edge or base of a lesion that represents infiltration of the dermis by tumour cells. Induration is not a feature of benign and premalignant epidermal lesions. When SCC occurs on sun-exposed sites, surrounding solar damage, including solar lentigines and AK, is often present. SCC occurring in immunosuppressed patients tends to be more aggressive and yet can appear deceptively banal.
Nodules
Most SCCs begin as skin-coloured to erythematous papules and plaques that develop into nodules. Lesions display differing degrees of keratinization, which is a feature of better differentiated tumours and manifests as the presence of scale, a central keratin plug or a keratin horn. Fifteen per cent of all keratin horns arise from an SCC. Ulceration usually occurs as lesions increase in size, giving a pink, infiltrated, raised edge that may be mistaken for the pearly rolled edge of a nodulo-ulcerative basal cell carcinoma (BCC).
Poorly differentiated tumours do not keratinize much, appearing as eroded and friable erythematous nodules. Exudate and fibrin may form a crust at sites of ulceration, occasionally concealing infection and pus. This should be carefully removed to permit better visualization of the lesion. Advanced tumours may invade so deeply that they become fixed to deeper structures.
Ulcers
SCC can present as an ulcer. The edge is irregular, indurated and usually everted, whereas the base is erythematous and friable. This appearance is more common on the scalp of balding men, lower legs, and in Marjolin’s ulcers (SCC that arises in longstanding scars and benign ulcers). SCC complicates 0.2% of venous leg ulcers, but may also masquerade as a venous leg ulcer. Warning clues include the presence of an isolated expanding ulcer that is refractory to treatment and the absence of venous hypertensive changes in the surrounding skin. It is important to be aware that SCC can mimic or complicate benign lesions in this way, as diagnosis is often delayed.