Basal cell carcinoma

(Date of publication 24 September 2004)

Most members of the public have probably never even heard of basal cell carcinoma (BCC), or 'rodent ulcer' - yet it is the commonest of all cancers. Approximately one third of primary cancers affect the skin, and of these the vast majority are BCC, which develops from the cells at the base of the epidermis. Fortunately, lesions rarely metastasise, but they are locally invasive and can become quite large, as in the examples here. Chronic exposure to sunlight is the major cause, so these carcinomas generally appear on areas such as the face, ears, neck, scalp and shoulders. The basic account of BCC at the Skin Cancer Foundation site includes the five warning signs for the condition:

This video outlines the major types of BCC - it uses Real Player and lasts about 90 seconds - but more are mentioned at eMedicine. The statistics on this page reinforce just how common the condition is; the lifetime risk for white males is 33 - 39%, and for white females is 23 - 28%. Dark-skinned people are rarely affected. It is thought that ultraviolet radiation leads to mutation of tumour-suppressor genes, particularly in the elderly, as there is a long latency period of 20 - 50 years.

Once you have read this article and looked at the associated images, you may wish to see further illustrations of the different types of lesion. The most common nodular form appears here as a translucent papule, and the corresponding histological section demonstrates islands of rather uniform tumour cells invading the dermis. Borders of the morpheaform type are ill-defined both superficially, as in this example, and internally; tumor cells can be seen penetrating deeply into the underlying tissue in this photomicrograph. This latter type of BCC has a growth pattern that produces strands of cells rather than round nests and is often aggressive, making recurrence more common.

As was mentioned at a couple of these sites, some inherited conditions are closely associated with basal cell carcinoma. Xeroderma pigmentosum is a rare genetic defect in the mechanisms that repair DNA following exposure to ultraviolet radiation. Cumulative and irreversible damage results from exposure to all forms of ultraviolet radiation, especially sunlight, giving rise to blistering or freckling, premature ageing of the skin, blindness and neurological complications. The risk of skin cancer or pre-cancerous tumours is increased more than 1000-fold; hardly surprising considering that the severity of skin damage in this photograph is not unusual in sufferers.

Basal cell nevus syndrome, also rare, is inherited as an autosomal dominant trait and gives rise to multiple defects of the skin, nervous system, eyes, endocrine glands and bones. It is characterised by the development of basal cell carcinomas, particularly around the eyes and nose, at or about the time of puberty. This condition is caused by mutations in the PTCH (patched) gene found on chromosome arm 9q, which is important for the correct patterning and development of many embryonic tissues, such as the neural tube, pharyngeal arches and limb buds.

Death from BCC is extremely rare. Although radiotherapy may be given to elderly and debilitated patients, and cryotherapy can achieve cure rates close to 90%, treatment is primarily by surgery, the objective being to destroy or remove the tumour so that no malignant tissue can proliferate further. This may be done by curettage and electrodesiccation, a quick, simple technique that can achieve cure rates in excess of 90%, although the specimen cannot be examined for margin control. Surgical excision is a more time-consuming and costly method than curettage, and also requires the sacrifice of normal tissue.

However, the highest cure rates (98 - 99% for primary BCC) are obtained with Mohs micrographic surgery. In this technique the surgeon also acts as pathologist, using a microscope to trace and remove all traces of malignancy. Once the visible tumour has been excised, he or she removes an additional thin layer of tissue from the site, creates a map or drawing of this tissue as a guide for locating any remaining cancer cells, and then examines the tissue microscopically. If any tumour cells are discovered, the map is used to remove another layer of tissue from around the malignant cells only, allowing preservation of surrounding normal tissue. This process is repeated until the removed tissue layer is clear.

A more detailed explanation of Mohs micrographic surgery, including the preparation of frozen sections and the colour coding used to orientate individual pieces of excised tissue, can be found on the University of Texas site. A surprisingly large cavity may be left following the elimination of even an apparently small tumour, as here, but the aesthetic results of reconstructive surgery can be remarkably good - look at the 'before' and 'after' pictures in the middle of this page.

Looking ahead, some exciting new treatments for BCC are currently being investigated. One of these is photodynamic therapy, in which tumour cells are rendered sensitive to certain wavelengths of laser light by the absorption of a drug. Subsequent exposure to the light causes the photosensitising drug to produce a form of oxygen that kills nearby cells. In addition, the drug may damage blood vessels in the tumour, thereby restricting the supply of nutrients, and activate the immune system. Retinoids, related to vitamin A, may have a useful role in the prevention and treatment of BCC. The greatest potential benefit by far, however, lies in education; persuading the public of the dangers of unprotected sun exposure and the need for early diagnosis of any abnormal skin lesions.

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A tour around basal cell carcinoma

A tour around basal cell carcinoma