What is Basal cell carcinoma

Basal cell carcinoma is the commonest malignant tumor affecting the skin. Clinically, it is a slow-growing locally invasive and locally destructive tumor in which distant metastases rarely occur. Several types are described based on their physical appearance, and there are a variety of clinical classifications. The essential component determined by clinical investigation is the extent of the tumor. Localized tumors generally have a clear cut-off point from tumor to normal tissue and the margins can be well defined. The papulonodular variety fits the classical description of the rodent ulcer with a rolled, pearly edge which often develops central ulceration. The solid type almost appears to grow out of the skin in an exophytic way and frequently has telangiatatic vessels coursing across its surface. The cystic variety can appear as a thin cyst, particularly around the eyelids, and sometimes also has telangiatatic vessels.

In the diffuse type, the pattern of spread is insidious, and defining the tumor margins can be difficult. The infiltrating type is clearly not purely an exophytic growth and infiltration of adjacent tissues can be demonstrated by palpation. The multifocal variety appears to have areas of almost normal looking skin which may represent healing of a previously ulcerated area. These multifocal lesions may be superficial or can infiltrate in depth. The morphoeic type of basal cell carcinoma infiltrates the dermis and produces a dense stromal reaction with stromal fibrosis. This gives the skin a characteristic white plaque-like appearance which is stiff, hence the term morphoea. The difficulty lies in determining the lateral extent of these tumors because of the dense stromal reaction. Metatypical basal cell carcinomas commonly appear as large, ulcerating, often exophytic, lesions. Characteristically they look very similar to squamous cell carcinomas but have a long history. It is the length of history that usually differentiates these tumors. Patients may present with more than one primary basal cell carcinoma which may be associated with a syndrome as discussed previously. Patients who have a basal cell carcinoma show a high incidence of a second lesion compared with the normal population. Second basal cell carcinomas frequently develop in the first year (16 per cent) and the incidence then falls to approximately 10 per cent over the next 4 years.
In addition to the physical appearance of primary basal cell carcinoma, there are three other clinical pictures which are encountered, namely recurrent basal cell carcinoma, aggressive or horrifying basal cell carcinoma, and metastatic basal cell carcinoma.

Recurrent basal cell carcinoma
The clinical appearance of recurrent basal cell carcinoma is very variable and is often dependent on previous treatment. The margins are difficult to determine but recurrence should always be regarded as having a diffuse pattern. Patients at risk of developing recurrence include those who have already presented with recurrent basal cell carcinoma, those with basal cell carcinomas showing an aggressive histological pattern, and lesions arising at cosmetically sensitive sites where tissue is scarce.

Horrifying basal cell carcinoma
This is the most dangerous basal cell carcinoma, characterized clinically by deep invasion and widespread destruction of adjacent tissues. The terminology is somewhat confusing, since it is also termed aggressive basal cell carcinoma, but this latter term applies to the histological appearance rather than the clinical appearance and behaviour. These tumors tend to occur in young individuals; they are large (more than 3 cm) and often appear in the region of the head and neck, particularly the scalp. Inadequate primary treatment is cited as an important factor in the development of horrifying basal cell carcinoma, particularly deep tumor extension following radiotherapy. The diffuse infiltrative type of basal cell carcinoma has been implicated in the development of these horrifying lesions, with a high incidence in morphoeic or metatypical basal cell carcinomas and those with adenoid differentiation. Unusual aetiologies such as arsenic, immuosuppression, and X-ray-induced tumors have also been implicated.
The clinical danger of these tumors, particularly those arising in the head and neck, is their capacity to infiltrate through bone and into the central nervous system causing widespread local destruction.

Metastatic basal cell carcinoma
The development of a distant metastasis from a basal cell carcinoma is exceptionally rare, so much so that it remains worthy of a single case report. The incidence is reported as varying from 0.0028 per cent to 0.55 per cent. Characteristically, patients have a large basal cell carcinoma, usually of long standing, which has proved resistant to treatment at the local site. Metatypical basal cell carcinoma with squamous differentiation has been associated with metastases. Facial basal cell carcinomas tend to metastasize to regional nodes, but a wide variety of organs have also been reported as showing metastasis including lung, liver, brain, heart, and pericardium. The histological variations that occur in basal cell carcinoma continue to fuel the debate as to whether basal cell carcinomas do indeed metastasize or whether these tumors are variants of adnexal tumors.

Pathological features
One of the major problems with basal cell carcinoma is that the cell of origin and the histiogenesis have not been accurately determined. The keratin phenotype of basal cell carcinoma would suggest an origin in the hair follicles. If this is indeed the case, then basal cell carcinoma is a type of adnexal tumor; however, because of its frequent occurrence it has been classified separately. Unlike many other tumors there does not appear to be a premalignant phase for basal cell carcinoma arising de novo. Basal cell carcinomas are composed of islands or nests of basophilic cells which resemble miniature basal cells of the epidermis lying in a connective tissue stroma. Typically the cells pack together in a regular manner to produce peripheral palisading. The cellular islands and nests within the stroma give them different patterns, and this has led to a variety of classifications. From the clinical perspective, it is the pattern and degree of infiltration and the arrangement of cells within the stroma that is most important rather than the degree of differentiation or number of mitoses present.