Background
Basal cell carcinoma (BCC) is the most common skin cancer in humans, yet it accounts for less than 0.1% of patient deaths due to cancer. Basal cell skin cancer tumors typically appear on sun-exposed skin, are slow growing, and rarely metastasize (0.028-0.55%). BCC usually appears as a flat, firm, pale area that is small, raised, pink or red, translucent, shiny, and waxy, and the area may bleed following minor injury. Tumor size can vary from a few millimeters to several centimeters in diameter. (See Clinical Presentation.)
See the clinical images below of basal cell carcinoma.
A superficial basal cell carcinoma (BCC). Clinically, an erythematous, well-circumscribed macule with minimal scale is present. This tumor is often misdiagnosed as eczematous dermatitis or guttate psoriasis and is often difficult to distinguish clinically from Bowen disease (squamous cell carcinoma in situ). Features that suggest the diagnosis of superficial BCC are the absence of significant white, adherent scale, and a history of the lesion remaining unchanged for several months or years. Treatment options for this tumor include electrodesiccation and curettage, surgical excision, cryosurgery, 5-fluorouracil, 5% imiquimod cream, and superficial radiographic therapy. Electrodesiccation and curettage is the modality most commonly used, with a cure rate of approximately 95%. 
Basal cell carcinoma. 
A 68-year-old patient presenting with an advanced basal cell carcinoma (BCC) of the right periorbital region, frontal view (Images courtesy of M Abraham Kuriakose, DDS, MD) BCC is a nonmelanocytic skin cancer (ie, an epithelial tumor) that arises from basal cells, which are small round cells found in the lower layer of the epidermis. Basal cells invade the dermis but seldom invade other parts of the body. The deoxyribonucleic acid (DNA) of certain genes is often damaged in patients with BCC; therefore, inheritance may be a factor. Most DNA alterations result from damage caused by exposure to sunlight. (See Pathophysiology.)
Body distribution of BCCs is as follows:
- On the head (most frequently on the face[1] ; most common location is the nose, specifically the nasal tip and alae) - 70%
- On the trunk[2] - 25%
The anatomic distribution of BCCs correlates with embryonic fusion planes. Recent data indicate that after adjusting for surface area, BCC occurrence is greater than 4 times more likely on embryonic fusion planes than on other regions of the midface, a finding that supports the possibility of an embryologic role for BCC pathogenesis.[6]
BCC can develop on unexposed areas, and cases of BCC of the prostate have been reported. In some patients, contributing factors are exposure to or contact with arsenic,[5, 7] tar, coal, paraffin,[8] certain types of industrial oil, and radiation. BCC can also be associated with scars (eg, burn complications),[9] xeroderma pigmentosum,[10] , previous trauma.[11] vaccinations, or even tattoos. (See Etiology.)
A skin biopsy (most often a shave biopsy is sufficient) may be necessary to confirm the diagnosis and is often required to determine the histologic subtype of BCC. A punch biopsy may be used to obtain a thick specimen, especially when the clinical suspicion of a BCC is still present after shave biopsy results are negative. (See Workup.)
Neglected tumors can continue to grow and lead to significant local destruction and even disfigurement. Surgery, in almost all cases, is the recommended treatment,[12, 13] with treatments varying on the basis of cancer size, depth, and location. (See Treatment and Management.)
Superficial BCCS have been successfully treated with imiquimod 5% cream,[14, 15, 16, 17, 18, 19] and topical 5-fluorouracil 5% cream may be used to treat small, superficial BCCs.[20] Several studies have shown success in treating small nodular BCCs with imiquimod 5% cream, although this is an off-label indication and patients should be informed of this fact. (See Medication.)
Anatomy
Basal cell carcinomas (BCCs) occur predominantly on the head, with 70% of BCCs occurring on the head (most frequently on the face), 25% on the trunk, and 5% on the penis, vulva,[4] or perianal skin. Very rarely, other organs are affected.
Pathophysiology
Although the exact etiology of BCC is unknown, a well-established relationship exists between BCC and the pilosebaceous unit, as tumors are most often discovered on hair-bearing areas.
Many believe that BCCs arise from pluripotential cells in the basal layer of the epidermis or follicular structures. These cells form continuously during life and can form hair, sebaceous glands, and apocrine glands. Tumors usually arise from the epidermis and occasionally arise from the outer root sheath of a hair follicle, specifically from hair follicle stem cells residing just below the sebaceous gland duct in an area called the bulge.
Signaling pathways
The patched/hedgehog intracellular signaling pathway plays a role in both sporadic BCCs and nevoid BCC syndrome (Gorlin syndrome). This pathway influences differentiation of various tissues during fetal development. After embryogenesis, it continues to function in regulation of cell growth and differentiation. Loss of inhibition of this pathway is associated with human malignancy, including BCC.
The hedgehog gene encodes an extracellular protein that binds to a cell membrane receptor complex to start a cascade of cellular events leading to cell proliferation. Of the 3 known human homologues, sonic hedgehog (SHH) protein is the most relevant to BCC. Patched (PTCH) is a protein that is the ligand-binding component of the hedgehog receptor complex in the cell membrane. The other protein member of the receptor complex, smoothened (SMO), is responsible for transducing hedgehog signaling to downstream genes.[21, 22]
A published article by Zhang et al demonstrates that the ultraviolet (UV)-specific nucleotide changes in 2 tumor suppressor genes, TP53 and PTCH, are both implicated in the development of early-onset BCC.[23]
When SHH is present, it binds to PTCH, which then releases and activates SMO. SMO signaling is transduced to the nucleus via Gli. When SHH is absent, PTCH binds to and inhibits SMO. Mutations in the PTCH gene prevent it from binding to SMO, simulating the presence of SHH . The unbound SMO and downstream Gli are constitutively activated, thereby allowing hedgehog signaling to proceed unimpeded. The same pathway may also be activated via mutations in the SMO gene, which also allows unregulated signaling of tumor growth. How these defects cause tumorigenesis is not fully understood, but most BCCs have abnormalities in either PTCH or SMO genes. Some even consider defects in the hedgehog pathway to be requirements for BCC development.
UV-induced mutations in the TP53 tumor suppressor gene, which resides on band 17p13.1, have been found in some cases of BCC.[24] In addition, frameshift mutations of the BAX gene (BCL2 –associated X protein) have been found in sporadic cases of BCC. A reduction of bcl-2 proteins is observed in the aggressive, infiltrative type of BCC. A recent study also showed an association of a COX-2 gene variant with a lower risk of developing BCC in patients who underwent organ transplantation before age 50 years; this finding suggests a possible role for dysregulation of COX-2 expression in carcinogenesis.
Radiation and immunologic origins
Radiation has proven to be tumorigenic by two mechanisms. The first entails the initiations of prolonged cellular proliferation, thereby increasing the likelihood of transcription errors that can lead to cellular transformation. The second mechanism is direct damage of DNA replication, leading to cellular mutation that may activate proto-oncogenes or deactivate tumor suppressor genes.
Immunologically, the mechanism by which prolonged ultraviolet radiation exposure leads to the development of BCC includes suppression of the cutaneous immune system and immunologic unresponsiveness to cutaneous tumors. This local effect includes a decrease in Langerhans cells, dendritic epidermal T cells, and Thy1+ cells. Furthermore, systemic proliferation of suppressor T cells and the release of immunosuppressive factors (eg, tumor necrosis factor-alpha [TNF-alpha], interleukin 1 [IL-1], prostaglandin [PG], interleukin 10 [IL-10]) are believed to be pathogenic to the development of BCC.
DNA mismatch repair proteins
DNA mismatch repair (MMR) proteins are a group of proteins that physiologically stimulate G2 cell cycle checkpoint arrest and apoptosis. Failure of MMR proteins to detect induced DNA damage results in the survival of mutating cells. MMR proteins have been found to be increased in nonmelanoma skin cancers as compared to normal skin, and there is also some evidence of MMR dysregulation.[25]
Etiology
The exact cause of BCC is unknown, but environmental and genetic factors are believed to predispose patients to BCC.
Radiation exposure
Sunlight, particularly chronic exposure, is the most frequent association with development of BCC; risk correlates with the amount and nature of accumulated exposure, especially during childhood. Patient geographic location affects the risk of developing skin cancer. A latency period of 20-50 years is typical between the time of ultraviolet (UV) damage and BCC clinical onset.
Radiation exposure that contributes to BCC development may include tanning booths and UV light therapy. Both short-wavelength UVB radiation (290-320 nm, sunburn rays) and longer wavelength UVA radiation (320-400 nm, tanning rays) contribute to the formation of BCC. UVB is believed to play a greater role in the development of BCC than UVA, however, and is the primary agent responsible for most skin cancer.[26]
UVB and UVC can modify unsaturated chemical bonds of nucleic acids, which may lead to mutations. UVC does not penetrate the atmospheric ozone layer. The UVA spectrum is absorbed by melanin and, through free-radical transfer, affects cellular deoxyribonucleic acid (DNA). Mutations caused by UV radiation typically include cytosine (C) to thymine (T), or CC to TT, translocation. This process can cause activation of oncogenes or inactivation of tumor suppressor genes, leading to tumor initiation and progression.[27]
The skin can repair superficial damage, but the underlying cumulative damage remains, including DNA damage. The damage worsens with each successive sun exposure, causing a lifetime progression.[28]
Gene mutations
Recent studies demonstrate a high incidence of TP53 gene mutations in BCC. Researchers speculate that ultraviolet sunlight may play an important role in the genesis of this mutation; yet, genetic involvement has been demonstrated on chromosome 9 only in patients with familial basal cell nevus syndrome (Gorlin syndrome). Such mutation involves the patched (PTCH) gene, a tumor suppressor gene.
Inappropriate activation of the hedgehog signaling pathway is found in both sporadic and familial cases of BCC. This results in loss-of-function mutations in tumor-suppressor protein patched homologue 1 (PTCH1) and gain-of-function mutations in sonic hedgehog (SHH), smoothened (SMO), and Gli.
Other radiation exposure
X-ray and grenz-ray exposure are associated with basal cell carcinoma formation.
Arsenic exposure through ingestion
Arsenic has been used as a medicinal agent, predominantly the Fowler solution of potassium arsenite, which was used to treat many disorders, including asthma and psoriasis. Historically, a contaminated water source has been the most common source of arsenic ingestion.
Immunosuppression
A modest increase in the lifetime risk of BCC has been noted in chronically immunosuppressed patients, such as recipients of organ or stem cell transplants and patients with AIDS.
Organ transplant patients must be instructed to limit sun exposure and alerted that skin cancer is a serious problem for them. In fact, immunosuppression and sun damage may cooperate to cause skin cancer. The skin cancer incidence is 10-fold higher in transplant patients than in the general population; up to 65-75% of patients with long-term immunosuppression develop skin cancer. Skin cancers can significantly alter and reduce the transplant recipients’ quality of life; some patients may develop more than 100 skin cancers per year.
Xeroderma pigmentosum
This autosomal recessive disease results in the inability to repair ultraviolet-induced DNA damage. Pigmentary changes are seen early in life, followed by the development of basal cell carcinoma, squamous cell carcinoma, and malignant melanoma. Other features include corneal opacities, eventual blindness, and neurological deficits.
Epidermodysplastic verruciformis
Epidermodysplastic verruciformis is an autosomal recessive disorder characterized by the development of basal cell carcinoma and squamous cell carcinoma from warts (human papillomavirus infection).
Nevoid basal cell carcinoma syndrome
In addition to basal cell carcinoma, this autosomal dominant disorder can result in the early formation of multiple odontogenic keratocysts, palmoplantar pitting, intracranial calcification, and rib anomalies. Various tumors such as medulloblastomas, meningioma, fetal rhabdomyoma, and ameloblastoma also can occur.[29]
Odontogenic keratocysts, palmoplantar pitting, intracranial calcification, and rib anomalies may be seen. Mutations in the hedgehog signaling pathway, particularly the patched gene, are causative.[30]
Go to Nevoid Basal Cell Carcinoma Syndrome to see more complete information on this topic.
Bazex syndrome
Features of Bazex syndrome include follicular atrophoderma (so-called ice pick marks, especially on dorsal hands), multiple basal cell carcinomas, and local anhidrosis (decreased or absent sweating).[28]
Previous nonmelanoma skin cancer
Persons who have been diagnosed with one nonmelanoma skin cancer are at increased risk of developing tumors in the future. The risk of developing new nonmelanoma skin cancers is reported to be 35% at 3 years and 50% at 5 years after an initial skin cancer diagnosis.[31]
Skin type
Albinism has been implicated in BCC. The Fitzpatrick skin-type scale, which ranges from very fair (skin type I) to very dark (skin type VI), categorizes cutaneous sensitivity to ultraviolet radiation. It is based on the individual's tendency to burn and tan and is a good predictor of relative risk among whites. The prevalence of BCC increases in areas of higher altitude and in areas of lower latitude. The incidence of BCC is rising, potentially because of atmospheric changes and the increased popularity of sunbathing.
Rombo syndrome
Rombo syndrome is an autosomal dominant condition distinguished by basal cell carcinoma and atrophoderma vermiculatum, trichoepitheliomas, hypotrichosis milia, and peripheral vasodilation with cyanosis.[32]
Alcohol consumption
A study among adults in the United States reports a strong association between excessive alcohol drinking and higher incidence of sunburn, suggesting a linkage between alcohol consumption and skin cancer.[33]
Epidemiology
The American Cancer Society reports skin cancer as being the most common cancer in the United States, with over 1 million new cases diagnosed and more than 10,000 deaths estimated yearly (ie, approximately 2% of all cancer deaths). The estimated lifetime risk for BCC in the white population is 33-39% for men and 23-28% for women. Basal cell carcinoma incidence doubles every 25 years.
In states near the equator, such as Hawaii, BCC incidence is approaching 3-fold that of states in the Midwest, such as Minnesota. BCC incidence also varies globally. The highest rates of skin cancer occur in South Africa and Australia, areas that receive high amounts of UV radiation.[34] Australia has a trend toward increasing BCC incidence, while Finland has a low reported incidence that is approximately one quarter that in Minnesota; BCC incidence in Finland also appears to be increasing, however, especially among young women.
BCC accounts for 80% of all skin cancers but is the least likely cancer to behave in a malignant fashion and metastasize. BCC differs from squamous cell carcinoma, which accounts for 16% of skin cancers and is more life threatening.
Although BCC is observed in people of all races and skin types, dark-skinned individuals are rarely affected, and it is most often found in light-skinned individuals (type 1 or type 2 skin). Those with type 1 skin are very fair and have red or blond hair and freckles; these individuals always burn and never tan. Those with type 2 skin are fair and burn easily while tanning minimally. Whites of Celtic ancestry have the highest risk for BCC. Incidence is low in blacks, Asians, and Hispanics.[35]
Historically, men are affected twice as often as women. The higher incidence in men is probably due to increased recreational and occupational exposure to the sun, although these differences are becoming less significant with changes in lifestyle. The current male-to-female ratio is approximately 2.1:1.
For tumors involving the periocular skin, Cook et al reported the incidence of BCC to be equal in men and women.[36] In addition, this investigative team found that the age-adjusted incidence rates for all malignant tumors of the eyelid in men and women, respectively, were 19.6 cases and 13.3 cases per 100,000 population per year. The age-adjusted incidence rates for BCC of the eyelid for men and women, respectively, were 16.9 and 12.4 cases per 100,000 population per year.
The likelihood of developing BCC increases with age. With the exception of basal cell nevus syndrome, BCC is rarely found in patients younger than 40 years. Approximately 5-15% of cases of BCC occur in patients aged 20-40 years. Aggressive-growth basal cell carcinoma (AG-BCC) is more frequently noted in patients younger than 35 years than in older individuals. AG-BCC includes morpheaform, infiltrating, and recurrent BCCs.[37]
Data indicate that BCC incidence is far higher (more than 100-fold) in persons aged 55-70 years than in those aged 20 years or younger. Patients 50-80 years of age are affected most often (mean age, 55 y), but the damaging effects of the sun begin at an early age and may not appear for 20-30 years. BCC can develop in teenagers and now appears frequently in fair-skinned patients aged 30-50 years. The median age at diagnosis is 67 ± 2.5 years, and the mean age is 64.4 ± 5.6 years (age range, 20-90 y).
Prognosis
The prognosis for patients with BCC is excellent, with a 100% survival rate for cases that have not spread to other sites. Nevertheless, if BCC is allowed to progress, it can result in significant morbidity, and cosmetic disfigurement is not uncommon.
Although BCC is a malignant neoplasm, it rarely metastasizes. The incidence of metastatic BCC is estimated to be less than 0.1%. Nevertheless, after treatment, which is curative in more than 95% of cases, BCC may develop in new sites.
The most common sites of metastasis are the lymph nodes, lungs,[38] and bones.[39] Typically, basal cell tumors enlarge slowly and relentlessly and tend to be locally destructive. Periorbital tumors can invade the orbit, leading to blindness, if diagnosis and treatment are delayed. BCC arising in the medial canthus tends to be deep and invasive and more difficult to manage; this type of BCC can result in perineural extension and loss of nerve function.
Patients who are diagnosed with BCC have a 35% chance of developing another tumor within 3 years and a 50% chance of developing another (not recurrent) BCC within 5 years. Therefore, regular skin screenings are recommended.[40]
Recurrence
The 5-year recurrence rate is about 5%, but it depends on the histologic subtype and type of treatment; the recurrence rate is less than 1% for primary (previously untreated) BCCs treated with Mohs micrographic surgery. Most reports show that the distance to the closest resection margin is an important predictor of recurrence.
The following is a list of treatments and their 5-year recurrence rates for primary (previously untreated) BCCs:
- Surgical excision - 10.1%
- Radiation therapy - 8.7%
- Curettage and electrodesiccation - 7.7%
- Cryotherapy - 7.5%
- All non-Mohs modalities - 8.7%
- Mohs micrographic surgery - 1%
These rates are probably affected by the fact that clinicians use cryotherapy, curettage, and desiccation mostly on smaller and better-demarcated lesions.
Pieh et al reported a recurrence rate of 5.36% after the first excision of the tumor; the rate increased to 14.7% after the second operation, and the rate reached 50% after the third and fourth operations.[41] The highest recurrence, approximately 60%, was seen with lesions arising from the medial canthus.
Recurrences usually occur 4-12 months after initial treatment, and the risk of developing a second lesion in 3 years is about 44%, which is a 10-fold increase over that of the general population.[42, 43] Tumors on the nose or T-zone of the face have a higher incidence of recurrence. Recurrence is most common on the nose and nasolabial fold, but this observation may be secondary to lack of adequate margins obtained in these areas. Infiltrative, micronodular, and multifocal types are more likely than nodular types to recur.
A recurrence of BCC should be suspected when one of the following conditions occurs:
- Nonhealing ulceration
- Tissue destruction
- Scar that becomes red, scaled, or crusted or enlarges with large adjacent telangiectasia
- Scar that slowly enlarges over time (months)
- Development of papule/nodule within a scar
Histologic types of BCC at higher risk for recurrence include morpheaform (sclerotic), micronodular, infiltrative, and superficial (multicentric). Other conditions that contribute to a higher recurrence rate include recurrent tumors that have been treated previously, large tumors (>2 cm), and deeply infiltrating tumors.
Patient Education
Adequate patient education is essential in the prevention of recurrence and spread of basal cell carcinoma. Patients should avoid possible potentiating factors (eg, sun exposure, ionizing radiation, arsenic ingestion, tanning beds). The regular use of sun-protecting clothing (eg, wide-brimmed hat, long-sleeved shirts, sunglasses with ultraviolet [UV] protection) is recommended when outdoors.
Instruct patients to avoid sun exposure particularly during the middle of the day (ie, 11:00 am to 3:00 pm), which is the most dangerous time. Also, the sun's rays are especially intense in sunny climates and at high altitudes, and UV radiation can also pass through clouds and water. Patients should be instructed to be careful on the beach and in the snow because sand, water, and snow reflect sunlight and increase the amount of received UV radiation.
During the initial consultation, the patient should be counseled regarding the extent of resection, type of reconstructive procedure, and attendant morbidity. High importance should be attached to adequately preparing the patient regarding the cosmetic and functional result of treatment. During posttreatment follow-up, the patient should be counseled regarding sunlight exposure and the risk of developing additional primary skin tumors.
Sunscreen
Regular application and reapplication of sunscreen is recommended prior to sun exposure. Note that SPF ratings correspond to the number of minutes required to get the equivalent of 1 minute of unprotected UVA exposure. People who use sunscreens have a 40% reduction in skin cancer incidence versus nonusers. Two types of sunscreen are available: the first reflect ultraviolet (UV)B and, to a lesser extent, UVA (reflectant); the second absorb UVB into specific chemicals re-emitting insignificant quantities of heat (absorbent).
Educate the patient that use of sunscreens with at least a 15 sun protective factor (SPF) rating is very useful for any skin cancer prevention.[44] Emphasize also that sunscreens must be applied 20-30 minutes before going outside and reapplied about every 2 hours, more often if swimming or sweating; lip balm with an SPF of 15 or higher is useful.
Instruct parents to protect their children's skin with sunscreen or protective clothing to reduce the risk of BCC later in life. It has been estimated that intensive sun protection before age 18 years can reduce nonmelanoma skin cancer by 78%.
Advise parents not to expose children younger than 12 months to direct sunlight and to cover up children aged 12-24 months with a hat, shirt, and a small amount of sunscreen (eg, more than 15 SPF) on the remaining exposed areas. Similarly, for children older than 2 years, instruct parents to consider using sunscreens with an SPF of 15 or more, covering the child's skin with clothing, and, when possible, restricting the child to shaded areas.
Self-examination for skin changes
Educate patients on how to recognize any unexplained changes in their skin, especially changes that last for more than 3-4 weeks. Also, educate patients on how to examine their own skin. The knowledge of mole distribution on the skin is helpful.
Tell the patient to first look at the front and back of his or her body in a full-length mirror, using a hand mirror. The patient also should use the hand mirror to look at the back of the neck and scalp, the back, and the breeches. The patient then should turn and look at each side of the body with the arms raised. Next, the patient should bend the elbows and look carefully at the forearms, the back of the upper arms, and the palms. Instruct the patient to sit down and check the backs of the legs and feet, including the spaces between the toes and bottoms of the feet.
For those without a history of skin cancer, a dermatologic examination (skin cancer screening) is recommended every 3 years for persons aged 20-40 years and every year for persons older than 40 years.
The American Cancer Society recommends a dermatologic examination every 3 years for people aged 20-40 years and every year for people older than 40 years.
For patient education information, see Skin Cancer.
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