The skin is a complex organ that covers the complete surface of the human body. [13] Skin cancer is the most common type of cancer with UV-exposure being the most prevalent risk factor and the incidence is rising. There are multiple therapeutic options and plastic surgeons are receiving an increasing number of referrals for reconstruction of patients with complicated soft tissue lesions. Therefore knowledge of complications, cosmetic outcomes and recurrence rates is required. [15] [23]

The skin is the largest organ and it accounts for 15 % of the total body weight. It has three main vital functions:
•    Protection (against external physical, chemical and biological aggressions)
•    Regulation (body temperature, peripheral circulation and fluid balance)
•    Sensation (as it includes an extensive network of nerve cells) [13]

The macroscopic structure of the skin depends on the location in the body and the mechanical stress. Two types of skin can be found: meshed skin and ridged skin.

Meshed skin covers approximately 95 % of the body surface and includes appendages (e. g. sweat glands, sebaceous glands, hair).

Ridged skin makes up 4 % of the body surface and extends over the bottom of the feet and the palm of the hands. It includes only small sweat glands, is hairless and more resistant to pressure. [31] [2]

Furthermore tension vectors across the skin can be identified dependent of the volume and movement of the underlying structures. As a quantitative measurement of the skin tension is difficult, skin lines are used as a surrogate parameter for skin tension. The most common ones are Langer’s lines and relaxed skin tension lines. Langer’s lines run parallel to the main collagen bundles in the dermis. Relaxed skin tension lines are visible as wrinkles when the skin is pinched and released without local tension. The wrinkles occur in a 90° angel to the underlying muscle fibers. Over many parts of the body Langer’s lines and relaxed skin tension lines run the same, but in mechanically complex regions they differ from each other. [29] [4]

Microscopically the skin is arranged in three layers including the epidermis, dermis and hypodermis.

The epidermis is the most superficial stratum of the skin and consists of a multilayered keratinized squamous epithelium. The squamous epithelium is organized in five different layers, that are determined by the degree of the differentiation of the keratinocytes. From inside to outside the following zones can be found:

Moreover there are melanocytes, merkel cells and Langerhans cells in the epidermis. Melanocytes are located in the stratum basale, they provide melanin and store it in melanosomes. Merkel cells are also located in the stratum basale and serve as pressure receptors. Langerhans cells are based in the stratum spinosum and are antigen presenting cells.  [22] [2] [13]

Just below the epidermis the dermis can be found. It consists mainly of connective tissue and the vascular and nervous plexuses are running through it. The dermis is arranged in two zones:

Besides vessels and nerve endings the dermis contains several cells including mast cells, dermal dendrocytes and macrophages. Furthermore also hair follicles are located in the reticular dermis. [22] [13]

The innermost layer is described as hypodermis and consists of loose connective tissue. It contains the epifascial vascular system as well as the cutaneous nerves. The subcutaneous adipose tissue is important for thermoregulation, insulation, nutritional and energy storage. Furthermore it serves as protection from mechanical stress. [13] [22]

As in every other tissue the skin can also develop tumors. It is important to know whether the skin tumors are benign or malignant. If cells pass the basal membrane and spread to other tissues, tumors are considered malignant. [8] There is a large number of different benign skin lesions. this chapter is intended to provide an overview of the most common ones.

Benign neoplasms of the skin can develop from different tissues or cells of the skin e. g. the epidermis or the dermal appendages.  The diagnosis is usually made with the help of a clinical examination and the patient’s history. For any lesions with uncertain diagnosis a biopsy should be scheduled to rule out malignancy. Some of these neoplasms may be an indicator for internal disease or genetic syndromes. [12] [14]

With regard to the literature there are different classifications. The following table shows a histological classification of common benign skin tumors. [8] [14] [12]

Although a routine removal is not necessary due their benign nature,  patients may wish excision due to aesthetic or symptomatic distress. There are multiple forms of therapy available, including excision, cryotherapy, curettage, laser therapy and pharmacotherapy. Treatment is based on the type of tumor and its location. For lipomas, dermatfibromas, keratoacanthomas and peridermoid cysts excision is the treatment of choice. Furthermore treatments for cherry angiomas and sebaceous hyperplasia are laser therapy and electrodesiccation. Seborrheic keratoses is often treated with cryotherapy or shave excision. [21] [12]

Epidemiology

Clinical presentation

Diagnostic
: direct-light microscopy: visible keratin cysts [8]

Therapy
: as seborrheic keratosis is a benign skin tumor a treatment is not necessary. Nevertheless many patience wish a removal of the tumor for cosmetic reasons. [8]

Epidemiology: most common type of cuteaneous cyst [12]

Clinical presentation

Diagnostic
: epidermal cysts can be diagnosed via clinical examination. After excision diagnosis is confirmed via histopathological examination.

Therapy
: a treatment is not necessary. If desired, the definitive treatment is a complete excision of the cyst. Excision should be performed when the lesion is not inflamed. (It is suggested to wait 4 – 6 weeks after the inflammation has resolved) [12]

Epidemiology

Clinical presentation

Diagnostic
: trichilemmal cysts can be diagnosed via clinical examination. After excision diagnosis is confirmed via histopathological examination.

Therapy
: a treatment is not necessary. If desired, the definitive treatment is a complete excision of the cyst. [12]

Sometimes epidermoid cysts and trichilemmal cysts are referred to as sebaceous cysts. This is a misnomer as they both arise from entrapment of surface epithelium into the dermis. [14]

Epidemiology

Clinical presentation

Diagnostic
: fibromas can be diagnosed via clinical examination.

Therapy
: a treatment is not necessary. Surgical excision is possible, in case of painful or aesthetic disturbing fibromas with scissors only, often even placing sutures is unnecessary.[8]

Skin cancer is the most common type of cancer. It usually occurs in sun-exposed areas of the skin. As the incidence is inversely proportional to the melanin content of the skin, fair-skinned persons have a higher risk.

Basal cell carcinoma is the most common malignant skin cancer. (About 8 out of 10 skin cancers) It has a low mortality but can cause significant local destruction. [15]

Cell growth is regulated by the intracellular hedgehog signaling pathway. A permanent activation of this pathway can cause tumor formation. The most common mutations leading to basal cell carcinoma development are inactivation mutations of PTCH1 gene or activating mutations of SMO gene. [15]

As basal cell carcinomas have a low metastatic potential the therapeutic target is local control.

First line therapy is surgical excision, as a total removal of the carcinoma has the best prognosis. (95 % 5-year cure rate) There are different options for surgical excision:

Squamous cell carcinomas occur when growth of the keratinocytes of the epidermis is out of control. It is the second most common skin cancer. (Squamous cell carcinomas represent 20 % of skin cancer) [32]

Malignant melanomas are the third most common type of skin cancer (they represent approximately 3 % - 5 % of skin cancer) and they are caused by malignancy of melanocytes. [1] [23]

Mutations of the genome (most common BRAF mutation) can activate signaling pathways leading to uncontrollable cell growth. Two-thirds of melanomas are de novo formations, one-third of melanomas develops out of preexisting lesions. [24]

*Stages according to AJCC 2016

In case of melanomas with a thickness > 1 mm a sentinel lymph node biopsy should be performed.

Surgical therapy of locoregional metastases is recommended if there is no evidence for distant metastases and if a complete removal (R0-resection) is possible. [20]

Satellite and in-transit metastases can be treated with radiotherapy to achieve local tumor control. [20]

In patients with multiple, rapidly recurring metastases (concerning only one arm / leg) indication for isolated limb perfusion should be reconsidered. [20]

Resection of distant metastases should be considered if R0 resection is technically possible, if it is not leading to an unacceptable loss of function and if other therapeutical options remain unrewarding. [20]

As malignant skin tumors may concern multiple organs the respective specialties should be involved in the therapy. To provide an optimal treatment a close cooperation especially between dermatologists and plastic surgeons is necessary. 

Therefore the following standard was established at authors hospital:

Microscopically controlled surgery is defined as a tumor excision with subsequent complete visualization of the excision margins without diagnostic gaps. [3]

Usually a conventional histological examination is used to evaluate the tumor and its excision margins. Conventional histological examination may simulate a R0 resection as not the whole tumor border is integrated in the histological processing. The fewer sections are made through the excised material, the greater the risk that tumor extensions will not found. As a conclusion histological methods that examine the complete tumor margins are significantly more sensitive. The superiority of the microscopically controlled surgery compared to conventional histology has been well documented in large comparative overviews. [3] [30]

When processing the edges of a excised tumor the aim is to represent its three-dimensional edges in two-dimensional histological sections. There are a different options of histologically processing the excised tumor, the paraffin section method and the cryostat procedure.  In the context of the paraffin section method a vertical incision of the lateral tumor border is made and the tumor margin is separated as a strip. In this way the three-dimensional outer edges are brought into two-dimensionality of a histological section plane. Another option of microscopically controlled surgery is to remove the tumor in the form of a bowl and process it in the cryostat procedure. Compared to conventional histological processing, the effort of a 3D histology in the paraffin section method is not necessarily higher, whereas there is significantly more effort of a 3D histology in the cryostat process.

The sensitivity of microscopically controlled surgery to detect tumor extensions is very high. Therefore, the distance to healthy tissue during excision can be kept small, if necessary. This is conductive for good aesthetic results after reconstructive surgical procedures. However, it should be noted that with small excision margins more follow-up surgeries are necessary on average. This has to be considered in each individual case. But in total, large studies show very low recurrence rates after excision of skin tumors and examination of the excision margins with 3D histology. [3] [28] [6] [18] [7]

Cave: 3D histology can only be used in soft tissue tumors. Processing of osseous structures is only possible after decalcification. [3]

Resection of malignant skin tumors may lead to extensive skin- or soft tissue lesions that cannot be closed by a primary wound suture. There are numerous methods for plastic reconstruction depending on the size and location of the tissue defect. The following chapter provides an overview of general reconstruction principles and different techniques. 

The choice of the method for plastic reconstruction depends on many different influencing variables that may be different for each patient and defect. Wounds of the same location and same size may be treated unequally in different patients depending on varying factors. Nevertheless, there a few general principles when it comes to surgical reconstruction that should always be considered. [27]

Preoperative evaluation of the defect, especially etiology, location, extension in width and depth. Regarding the depth it is important to analyze if there are any functional structures (such as muscles, tendons, bones) are exposed. Furthermore, in terms of the location also the surrounding tissue has to be assessed in regard to skin-color, hairiness and texture. It is important to consider the specific characteristics of the respective anatomic region. In particular in the face (where malignant skin tumors are located often) it is essential to preserve functional structures for e. g. closing the eyes, closing the mouth and free nasal breathing. [27]

In terms of plastic reconstruction it is also important to pay attention to the individual characteristics of each patient, especially age, comorbidities and the patient request have to be considered. [27]

Principle of the “reconstructive ladder”: This describes the principle of escalating reconstruction procedures adapted to the tissue defect. As a general rule, the aim is to reconstruct a tissue defect using the simplest possible method that leads to the best outcome.  The patient’s individual situation always has to be considered in the decision process as age and risk factors may change the decision towards a more simple or complex method. [27]

For every plastic reconstruction the principle “replace like with like” (established by Sir Harold Gillis) should be applied. This means, whenever it is possible to recreate an area with tissue similar to the surrounding tissue. [10] [27]

Furthermore, the experience and the personal preference of the surgeon should be included in the decision-making process. [27]

The following images show a few exemplary reconstructions of aesthetically sensitive facial areas affected by malignant skin tumors.

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