Basal cell carcinoma of the skin is the most common malignant tumour in humans. They are typically slow growing and cause local destruction of adjacent tissues. They almost never metastasise. If neglected, they may invade bone or other deep tissues, leading to disfigurement. Ulceration of tumours may lead to infection.
Basal cell carcinomas are usually noticed as a growing lump by the patient or a relative. Some lesions may be pigmented or scar-like, and larger lesions may ulcerate. The superficial variant of basal cell carcinoma may appear as a plaque. Basal cell carcinomas typically occur in sun exposed areas. Perineural invasion may cause pain or numbness in affected nerves.
The classic lesion is a pearly nodule with surrounding telangiectasia. Perineural invasion may lead to parasthesia in the distribution of cutaneous nerves. It can be difficult to differentiate basal cell carcinoma from more aggressive cancer without a biopsy.
Features are dependent on the subtype of basal cell carcinoma.
Classical Microscopic Features
Regardless of their subtype, basal cell carcinomas show similar features to the stratum basalis of the skin. The majority of cells are small with minimal cytoplasm. They are usually arranged in groups, with the outer layer demonstrating peripheral palisading. The cluster of basaloid cells is surrounded by a stromal reaction. Notably, the malignant cells may be separate from the stroma, unlike the basal layer of the skin which is firmly attached to its basement membrane.
Melanocytes may be present in any of the subtypes, causing them to become pigmented.
Superficial Basal Cell Carcinoma
These tumours are most frequent on the trunk and make up 10 - 30% of basal cell carcinomas.
These tumours are typically erythematous, and may have a pearly border. They may extend over a large area of skin but show minimal invasion to deeper layers.
Superficial basal cell carcinoma demonstrates nodules extending through the epidermis into the papillary (but not rete) dermis. They are usually continuous but may show some separate nodules.
Nodular Basal Cell Carcinoma
This is the most common subtype, making up over 60% of BCC. They occur most commonly on the face and neck.
These tumours show the classic features of basal cell carcinoma, presenting as a pearly nodule with surrounding telangiectasia. Rarely they may be flatter lesions or be pigmented.
Nodular BCC is arranged into lobules of malignant cells with peripheral palisading.
Micronodular Basal Cell Carcinoma
A more aggressive type of basal cell carcinoma. They usually occur on the back.
These lesions have a variety of macroscopic appearances, with flat or elevated appearances with or without pigmentation.
Micronodular BCC is arranged in tiny clusters of basaloid cells which infiltrate through the dermis. Connective tissue separates the nodules. The microscopic spread of disease may be far greater than the macroscopic changes suggest.
Infiltrating Basal Cell Carcinoma
The most aggressive of the usual basal cell carcinomas.
This tumour usually appears as a poorly defined plaque.
Strands of basaloid cells may extend through the layers of the skin and into deeper tissues. The other features of basal cell carcinoma (eg. peripheral palisading) are not seen. Microscopic disease may extend far beyond the macroscopic borders of the tumour.