Question: How common are basal cancerous cells?

Basal-cell carcinoma




Basal-cell carcinoma (BCC), also known as basal-cell cancer, is the most common type of skin cancer.[2] It often appears as a painless raised area of skin, which may be shiny with small blood vessels running over it; or it may present as a raised area with ulceration.[1] Basal-cell cancer grows slowly and can damage the tissue around it but is unlikely to spread to distant areas or to result in death.[7]

Risk factors include exposure to ultraviolet light, having lighter skin, radiation therapy, long-term exposure to arsenic, and poor immune-system function.[2] Exposure to UV light during childhood is particularly harmful.[5] Tanning beds have become another common source of ultraviolet radiation.[8] Diagnosis often depends on skin examination, confirmed by tissue biopsy.[2][3]

It remains unclear whether sunscreen affects the risk of basal-cell cancer.[9] Treatment is typically by surgical removal.[2] This can be by simple excision if the cancer is small; otherwise Mohs surgery is generally recommended.[2] Other options may include application of cold, topical chemotherapy, laser surgery, or the use of imiquimod.[10] In the rare cases in which distant spread has occurred, chemotherapy or targeted therapy may be used.[10]

Basal-cell cancer accounts for at least 32% of all cancers globally.[7][11] Of skin cancers other than melanoma, about 80% are basal-cell cancers.[2] In the United States, about 35% of white males and 25% of white females are affected by BCC at some point in their lives.[2]

Signs and symptoms
Individuals with a basal-cell carcinoma typically present with a shiny, pearly skin nodule. However, superficial basal-cell cancer can present as a red patch similar to eczema. Infiltrative or morpheaform basal-cell cancers can present as a skin thickening or scar tissue – making diagnosis difficult without using tactile sensation and a skin biopsy. It is often difficult to visually distinguish basal-cell cancer from acne scar, actinic elastosis, and recent cryodestruction inflammation.



Dermoscopy showing telangiectatic vessels
Basal-cell carcinoma

Cause
About two thirds of basal-cell carcinomas occur on sun-exposed areas of the body. One-third occur on areas of the body that are not exposed to sunlight, emphasizing the genetic susceptibility of basal-cell cancer.
Pathophysiology


Basal-cell carcinomas are currently considered to have origin from the folliculo-sebaceous-apocrine germ, also known as trichoblast. The differential diagnosis with trichoblastic carcinoma, a rare malignant form of trichoblastoma, can be challenging.[12] Alternatively, one argument is that basal-cell carcinoma is trichoblastic carcinoma.[13] Overexposure to sun leads to the formation of thymine dimers, a form of DNA damage. While DNA repair removes most UV-induced damage, not all crosslinks are excised. There is, therefore, cumulative DNA damage leading to mutations. Apart from the mutagenesis, overexposure to sunlight depresses the local immune system, possibly decreasing immune surveillance for new tumor cells.
Basal-cell carcinomas can often come in association with other lesions of the skin, such as actinic keratosis, seborrheic keratosis, squamous cell carcinoma.[14] In a small proportion of cases, basal-cell carcinoma also develops as a result of basal-cell nevus syndrome, or Gorlin Syndrome, which is also characterized by keratocystic odontogenic tumors of the jaw, palmar or plantar (sole of the foot) pits, calcification of the falx cerebri (in the center line of the brain) and rib abnormalities. The cause of this syndrome is a mutation in the PTCH1 tumor suppressor gene located in chromosome 9q22.3, which inhibits the hedgehog signaling pathway. A mutation in the SMO gene, which is also on the hedgehog pathway, also causes basal-cell carcinoma.[15]

Diagnosis
To diagnose basal-cell carcinomas, a skin biopsy is performed for histopathologic analyses. The most common method is a shave biopsy under local anesthesia. Most nodular basal-cell cancers can be diagnosed clinically; however, other variants can be very difficult to distinguish from benign lesions such as intradermal naevus, sebaceomas, fibrous papules, early acne scars, and hypertrophic scarring.[16]
Classification
Basal-cell carcinoma can be divided into three groups, based on the growth patterns.
Superficial basal-cell carcinoma, formerly referred to in-situ basal-cell carcinoma, is characterized by a superficial proliferation of neoplastic basal-cells. This tumor is generally responsive to topic chemotherapy, such as imiquimod, or fluorouracil.
Infiltrative basal-cell carcinoma, which also encompasses morpheaform and micronodular basal-cell cancer, is more difficult to treat with conservative methods, given its tendency to penetrate into deeper layers of the skin.
Nodular basal-cell carcinoma includes most of the remaining categories of basal-cell cancer. It is not unusual to encounter heterogeneous morphologic features within the same tumor.
Histopathologic classification
The histopathologic classification includes:[17][18]:646–650
Nodular basal-cell carcinoma (also known as "classic basal-cell carcinoma") most commonly occurs on the sun-exposed areas of the head and neck.[17]:748[18]:646
Cystic basal-cell carcinoma is morphologically characterized by dome-shaped, blue-gray cystic nodules.[18]:647
Cicatricial basal-cell carcinoma (also known as "morpheaform basal-cell carcinoma," and "morphoeic basal-cell carcinoma") is an aggressive variant with a distinct clinical and histologic appearance.[17]:748[18]:647
Infiltrative basal-cell carcinoma is an aggressive type characterized by deep infiltration.[18]:647
Micronodular basal-cell carcinoma is characterized by a micronodular growth pattern.[18]:647
Superficial basal-cell carcinoma (also known as "superficial multicentric basal-cell carcinoma") occurs most commonly on the trunk and appears as an erythematous patch.[17]:748[18]:647
Pigmented basal-cell carcinoma exhibits increased melanization.[17]:748[18]:647 About 80% of all basal-cell carcinoma in Chinese are pigmented while this subtype is uncommon in white people. 
Rodent ulcer (also known as a "Jacob's ulcer") is a large skin lesion of nodular basal-cell carcinoma with central necrosis.[17]:748[18]:647
Fibroepithelioma of Pinkus most commonly occurs on the lower back.[17]:748[18]:648
Polypoid basal-cell carcinoma is characterized by exophytic nodules (polyp-like structures) on the head and neck.[18]:648
Pore-like basal-cell carcinoma resembles an enlarged pore or stellate pit.[18]:648
Aberrant basal-cell carcinoma is characterized by the formation of basal-cell carcinoma in the absence of any apparent carcinogenic factor, occurring in odd sites such as the scrotum, vulva, perineum, nipple, and axilla.[18]:648
See also:
Nevoid basal-cell carcinoma syndrome

Cystic basal-cell carcinoma
Micronodular basal-cell carcinoma
Superficial basal-cell carcinoma
Micrograph of a fibroepithelioma of Pinkus

Prevention
Basal-cell carcinoma is a common skin cancer and occurs mainly in fair-skinned patients with a family history of this cancer. Sunlight is a factor in about two-thirds of these cancers; therefore, doctors recommend sunscreens with at least SPF 30. However, a Cochrane review examining the effect of solar protection (sunscreen only) in preventing the development of basal-cell carcinoma or cutaneous squamous cell carcinoma found that there was insufficient evidence to demonstrate whether sunscreen was effective for the prevention of either of these keratinocyte-derived cancers.[19] The review did ultimately state that the certainty of these results were low, so future evidence could very well alter this conclusion. One-third occur in non-sun-exposed areas; thus, the pathogenesis is more complex than UV exposure as the cause.
The use of a chemotherapeutic agent such as 5-Fluorouracil or imiquimod can prevent development of skin cancer. It is usually recommended to individuals with extensive sun damage, history of multiple skin cancers, or rudimentary forms of cancer (i.e., solar keratosis). It is often repeated every 2 to 3 years to further decrease the risk of skin cancer.
Treatment
The following methods are employed in the treatment of basal-cell carcinoma (BCC):
Surgery
Standard surgical excision
This can be with either frozen section histology, or paraffin-embedded fixed-tissue pathology. It is the preferred method for removal of most BCCs. A dermatoscope can help an experienced surgeon accurately identify the visible tumour that the naked eye can not see.[20][21]
The cure rate for this method, whether performed by a General Surgeon, Otolaryngologist, Head & Neck Surgeon, Plastic Surgeon, Maxillofacial Surgeon or dermatologist is totally dependent on the surgical margin. The narrower the free surgical margin (skin removed that is free of visible tumor) the higher the recurrence rate.[22][23][24][25] If a 4mm free surgical margin is obtained around a small tumor (less than 6mm), or a wider 6mm free surgical margin is obtained around a larger tumor (greater than 6mm), the cure rate is very high—95% or better.[26][27][28][29][30][31] However, for cosmetic reasons, many doctors take only very small surgical margins 1–2mm,[32] especially when operating on the face. In such a case, a pathology report indicating the margins are free of residual tumour is often inaccurate, and recurrence rates are much higher (up to 38%).[28][32][33][34]
A weakness with standard surgical excision is the high recurrence rate of basal-cell cancers of the face, especially around the eyelids,[33] nose, and facial structures.[35] There is no clear approach for treating basal-cell carcinoma around the eye.[36] A diagram on page 38 of the National Comprehensive Cancer Network publication demonstrate the area of high risk of recurrence as being most of the face with the exception of the central cheek and upper forehead.[26] On the face, or on recurrent basal-cell cancer after previous surgery, special surgical margin controlled processing (CCPDMA—complete circumferential peripheral and deep margin assessment[26])[37] using frozen section histology (Mohs surgery is one of the methods) is required.[38][39]
With surgical margin controlled frozen section histology, a surgeon can achieve a high cure rate and low recurrence rate on the same day of the excision.[40] However, most standard excisions done in a plastic surgeon or dermatologist's office are sent to an outside laboratory for standard bread loafing method of processing.[41] With this method, it is likely that less than 5% of the surgical margin is examined, as each slice of tissue is only 6 micrometres thick, about 3 to 4 serial slices are obtained per section, and only about 3 to 4 sections are obtained per specimen (see figure 2 of reference[42]).
When in doubt, a patient should demand that either Mohs surgery or frozen section histology with either margin control (ccpdma) or thin serial bread-loafing is utilized when dealing with a tumour on the face.[26] The pathologist processing the frozen section specimen should cut multiple sections through the block to minimize the false negative error rate. Or one should simply process the tissue utilizing a method approximating the Mohs method (described in most basic histopathology text books or described in this reference [38]) during frozen section processing. Unfortunately, these methods are difficult when applied to frozen sections; and they are very tedious to process. When not utilizing frozen section, the surgeon might have to wait a week or more before informing the patient if more tumour is left, or if the surgical margin is too narrow.[43] A second surgery must be performed to remove the residual or potential residual tumour once the surgeon informs the patient of the positive or narrow surgical margin on the surgical pathology report.
A meta-analysis of randomized controlled trials supports surgery as the most effective treatment of BCC of the skin.[44] There is no clear research comparing the effectiveness of Mohs micrographic surgery versus surgical excision for BCC of the eye.[36]
Mohs surgery
Mohs surgery (or Mohs micrographic surgery) is an outpatient procedure, which was developed by Frederic E. Mohs in the 1940s,[45] in which the tumor is surgically excised and then immediately examined under a microscope. It is a form of pathology processing called CCPDMA. The base and edges are microscopically examined to verify sufficient margins before the surgical repair of the site. If the margins are insufficient, more is removed from the patient until the margins are sufficient. It is also used for squamous-cell carcinoma; however, the cure rate is not as high as Mohs surgery for basal-cell carcinoma. The 2008 study found MMS to be a good option for both primary and high-risk recurrent BCCs.[45]
Cryosurgery
Cryosurgery is an old modality for the treatment of many skin cancers. When accurately utilized with a temperature probe and cryotherapy instruments, it can result in very good cure rate. Disadvantages include lack of margin control, tissue necrosis, over or under treatment of the tumor, and long recovery time. Overall, there are sufficient data to consider cryosurgery as a reasonable treatment for BCC. There are no good studies, however, comparing cryosurgery with other modalities, particularly with Mohs surgery, excision, or electrodesiccation and curettage so that no conclusion can be made whether cryosurgery is as efficacious as other methods. Also, there is no evidence on whether curetting the lesions before cryosurgery affects the efficacy of treatment.[46] Several textbooks are published on the therapy, and a few physicians still apply the treatment to selected patients.[47]
Electrodesiccation and curettage
Electrodesiccation and curettage (EDC, also known as curettage and cautery, simply curettage)[45] is accomplished by using a round knife, or curette, to scrape away the soft cancer. The skin is then burned with an electric current. This further softens the skin, allowing for the knife to cut more deeply with the next layer of curettage. The cycle is repeated, with a safety margin of curettage of normal skin around the visible tumor. This cycle is repeated 3 to 5 times, and the free skin margin treated is usually 4 to 6mm. Cure rate is very much user-dependent and depends also on the size and type of tumor. Infiltrative or morpheaform BCCs can be difficult to eradicate with EDC. Generally, this method is used on cosmetically unimportant areas like the trunk (torso). Some physicians believe that it is acceptable to utilize EDC on the face of elderly patients over the age of 70. However, with increasing life expectancy, such an objective criterion cannot be supported. The cure rate can vary, depending on the aggressiveness of the EDC and the free margin treated. Some advocate curettage alone without electrodesiccation, and with the same cure rate.[48]
Chemotherapy
Some superficial cancers respond to local therapy with 5-fluorouracil, a chemotherapy agent. Topical treatment with 5% Imiquimod cream, with five applications per week for six weeks has a reported 70–90% success rate at reducing, even removing, the BCC [basal-cell carcinoma]. Both Imiquimod and 5-fluorouracil have received FDA approval, and topical IMQ is approved by the European Medicines Agency for treatment of small superficial basal-cell carcinoma.[45] Off label use of imiquimod on invasive basal-cell carcinoma has been reported. Imiquimod may be used prior to surgery in order to reduce the size of the carcinoma. One can expect a great deal of inflammation with this treatment.[49] Chemotherapy often follows Mohs surgery to eliminate the residual superficial basal-cell carcinoma after the invasive portion is removed. Some advocate the use of imiquimod prior to Mohs surgery to remove the superficial component of the cancer.[50] Removing the residual superficial tumor with surgery alone can result in large and difficult to repair surgical defects. One often waits a month or more after surgery before starting the Imiquimod or 5-fluorouracil to make sure the surgical wound has adequately healed. Some people advocate the use of curettage (see EDC below) first, followed by chemotherapy. These experimental procedures are not standard care. The 2008 study reported that topical IMQ appears effective in the treatment of primary small superficial BCCs but only "may possibly" have a role in the treatment of primary nodular BCC.[45]
Approved in 2012, vismodegib is used to treat an advanced form of basal-cell carcinoma.[51]
Immunotherapy
Immunotherapy research suggests that treatment using Euphorbia peplus, a common garden weed, may be effective.[52] Australian biopharmaceutical company Peplin[53] is developing this as topical treatment for BCC. Imiquimod is an immunotherapy but is listed here under chemotherapy.
Radiation
Radiation therapy can be delivered either as external beam radiotherapy or as brachytherapy (internal radiotherapy). Although radiotherapy is generally used in older patients who are not candidates for surgery, it is also used in cases where surgical excision will be disfiguring or difficult to reconstruct (especially on the tip of the nose, and the nostril rims). Radiation treatment often takes as few as 5 visits to as many as 25 visits. Usually, the more visits scheduled for therapy, the less complication or damage is done to the normal tissue supporting the tumor. Radiotherapy can also be useful if surgical excision has been done incompletely or if the pathology report following surgery suggests a high risk of recurrence, for example if nerve involvement has been demonstrated. Cure rate can be as high as 95% for small tumor, or as low as 80% for large tumors. Usually, recurrent tumors after radiation are treated with surgery, and not with radiation. Further radiation treatment will further damage normal tissue, and the tumor might be resistant to further radiation. Radiation therapy may be contraindicated for treatment of nevoid basal-cell carcinoma syndrome. A meta-analysis of randomized controlled trials supports radiotherapy as one of the most effective treatments for BCC.[44] A 2008 study reported that radiation therapy is appropriate for primary BCCs and recurrent BCCs, but not for BCCs that have recurred following previous radiation treatment.[45]
Photodynamic therapy
Photodynamic therapy (PDT) is a new modality for treatment of basal-cell carcinoma, which is administrated by application of photosensitizers to the target area. When these molecules are activated by light, they become toxic, therefore destroy the target cells. Methyl aminolevulinate is approved by EU as a photosensitizer since 2001. This therapy is also used in other skin cancer types.[54] The 2008 study reported that PDT was a good treatment option for primary superficial BCCs and reasonable for primary low-risk nodular BCCs but a "relatively poor" option for high-risk lesions.[45]
Prognosis
Prognosis is excellent if the appropriate method of treatment is used in early primary basal-cell cancers. Recurrent cancers are much harder to cure, with a higher recurrent rate with any methods of treatment. Although basal-cell carcinoma rarely metastasizes, it grows locally with invasion and destruction of local tissues. The cancer can impinge on vital structures like nerves and result in loss of sensation or loss of function or rarely death. The vast majority of cases can be successfully treated before serious complications occur. The recurrence rate for the above treatment options ranges from 50 percent to 1 percent or less.
Epidemiology
Basal-cell cancer is a very common skin cancer. It is much more common in fair-skinned individuals with a family history of basal-cell cancer and increases in incidence closer to the equator or at higher altitude. There are approximately 800,000[55] new cases yearly in the United States alone. Up to 30% of Caucasians develop basal-cell carcinomas in their lifetime.[56] In Canada, the most common skin cancer is basal-cell carcinoma (as much as one third of all cancer diagnoses), affecting 1 in 7 individuals over a lifetime.[57]
In the United States approximately 3 out of 10 caucasians develop a basal-cell carcinoma during their lifetime.[56] This tumor accounts for approximately 70% of non-melanoma skin cancers. In 80 percent of all cases, basal-cell carcinoma affects the skin of head and neck.[56] Furthermore, there appears to be an increase in the incidence of basal-cell cancer of the trunk in recent years.[56]
Most sporadic BCC arises in small numbers on sun-exposed skin of people over age 50, although younger people may also be affected. The development of multiple basal-cell cancer at an early age could be indicative of nevoid basal-cell carcinoma syndrome, also known as Gorlin's Syndrome.[58]