2.3.3.1 Introduction
1.Basal cell carcinoma
Basal cell carcinoma（BCC）is one of the common skin cancers，with low malignancy，and its metastasis is extremely rare.It occurs more commonly in the people above middle age with fair skin.
（1）Etiology and pathogenesis：The incidence of BCC increases with increasing sun exposure and age.In addition，a fair complexion，a positive family history of BCC，chronic arseniasis，radiation and immunosuppression are risk factors of BCC.
（2）Clinical features and classfication.
①Nodular BCC：As the most common subtype，nodular BCC also known as classic BCC，accounts for 50％-80％of all BCCs.It favors the head and neck.The lesion typically presents as nodule.As the lesion becomes larger，ulcer and bleeding can occur on the tumor.
②Superficial BCC：Superficial BCC is the second most common subtype of BCC，accounting for approximately 10％-30％of tumors.The average age of onset is younger than other subtypes.Superficial BCC always presents as single or multiple erythematous plaque with superficial flat growth，which have clear thin rolled borders and scale on it.This type of BCC is most commonly found on the trunk and distal extremities，whereas head and neck are less common to be found.
③Pigmented BCC：This subtype is similar to nodular BCC，but in addition，the lesion contains melanin.
④Fibroepithelial BCC：It is also known as fibroepitheliomas of Pinkus.This uncommon subtype has a predilection for the trunk，which presents as an elevated，skin-colored or erythematous sessile plaque.
⑤Infundibulocystic BCC：This subtype of BCC presents as well-circumscribed pearly papules，which is commonly found on the head and neck of the elderly.(https://www.daowen.com)
⑥Morpheaform BCC：It clinically presents as white sclerotic plaque with smooth surface and poorly defined borders.It resembles the plaque of morphea，and telangiectasia is present.
（3）Diagnosis and differential diagnosis：According to the clinical presentation and histopathological features，BCC is easy to be diagnosed.It is frequently mistaken for BCC in squamous cell carcinoma，hypertrophic actin keratosis，keratoacanthoma，Bowen's disease，Paget's disease，and seborrheic keratosis.
2.Squamous cell carcinoma
Squamous cell carcinoma（SCC）is the second most common form of skin cancer after basal cell carcinoma.It is a malignant tumor of keratinocytes originating within the epidermis or its appendages，which commonly occurs on the areas that have had sun exposure，especially the head and neck.SCC favors the people aged 60 years or over.
（1）Etiology and pathogenesis：Long-term sun exposure is the major risk factor of SCC，UV damages DNA，which induces gene mutations in keratinocytes and subsequently malignant change happens.Radiation and chemical carcinogens such as 3，4-benzpyrene and arsenic can promote the development of SCC.Long-term HPV infection，primarily HPV 16，18，31，and 35，play a role in SCC that develops on genitalia.Chronic non-healing ulcers，lesions of discoid LE can develop into SCC.In addition，immunosuppression（e.g.，organ transplantation）enhances the risk of SCC.
（2）Clinical features：SCC commonly occurs on sun exposed sites，mostly the head and neck，and non-sun-exposed areas are less favored.It is more likely to be found in people over 60 years.The typical appearances present as a non-healing，usually rapidly growing，and skin colored nodule with indurated base and sometimes with adherent surface，crust or central ulceration.Lesions are painful and bleed on contact.
（3）Diagnosis and differential diagnosis：Diagnosis biopsy should be performed when lesions with indurated base and central ulceration that grow rapidly and bleed on contact occur on the head and neck.Skin biopsy is the main evidence to diagnose SCC.In the early stages，it is difficult to distinguish SCC from hypertrophic actin keratosis.In addition，SCC also may be confused with basal cell carcinoma，keratoacanthoma，and sporotrichosis.