MOHS MICROGRAPHIC SURGERY
SOURCE: Dept. of Otolaryngology, UTMB Grand Rounds
DATE: February 21, 1996
RESIDENT PHYSICIAN: Kyle L. Kennedy, M.D.
FACULTY: Byron J. Bailey, M.D.
SERIES EDITOR: Francis B. Quinn, Jr., M.D.
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Mohs micrographic surgery was first conceived approximately 50 years ago and has undergone various modifications of the original technique since that time. It has become an integral component in the treatment of cutaneous malignancies, particularly difficult to treat tumors. This presentation will discuss Mohs surgery with regard to the following:
Emphasis will be placed on its use in the management of basal cell carcinoma, although many of the same concepts apply to other tumors with similar behavior patterns.
By the end of this discussion, it should become apparent that four of the most important characteristics of Mohs surgery in the treatment of cutaneous head and neck neoplasms are these:
In excess of one half million new cases of cutaneous malignancy are diagnosed each year in the United States alone. Certain subsets of the general population have been shown to be at increased risk for the development of these tumors. Risk factors include advancing age, fair skin, light hair, blue or green eyes, substantial sun exposure, frequent and/or severe sunburn. Other predisposing factors include exposure to certain chemicals (e.g. arsenic), ionizing radiation with radiodermatitis, burn scars, chronic ulcers, immunosuppression from various causes such as transplantation or leukemia/lymphoma, and certain genetic conditions (e.g. xeroderma pigmentosum, nevoid basal cell carcinoma syndrome).
The basic premise in the adequate treatment of cutaneous malignancy remains complete extirpation of the tumor in question. Routine treatment modalities include field therapy techniques such as electrodessication and curettage, cryosurgery, radiation therapy, and excisional surgery. These methods involve removal or treatment of not only the tumor itself but of a significant portion of normal adjacent tissue in an attempt to achieve clear margins. The extent of resection is based upon known tumor growth patterns and biological behavior as they are currently understood. These modalities often achieve high cure rates for selected tumors. The confines of the head and neck, particularly the face, make preservation of normal tissue a tremendous consideration during tumor removal in order to maintain adequate function and cosmesis. Mohs micrographic surgery is a technique which allows meticulous tumor excision and high cure rates with maximal preservation of normal surrounding tissues. This discussion will focus principally on the technique of Mohs micrographic surgery and its indications in the management of cutaneous malignancies.
Perhaps the most significant modification to Mohs' original technique was the excision of fresh tissue specimens under local aneshesia in the fresh-tissue technique popularized by Mohs himself and later Tromovitch in the 1960s. This eliminated much of the discomfort previously experienced by the patients, allowed multiple stages to be performed in a single day with the possibility of immediate closure of the wound, and achieved cure rates similar to those of the initial chemosurgical technique. This is the method most commonly employed by micrographic surgeons today in the removal of cutaneous malignancies.
The cutaneous micrographic surgeon first examines the tumor and attempts to delineate its clinical boundaries. Local anesthetic typically with a vasoconstrictive agent is used to infiltrate the area. Following the attainment of adequate local anesthesia, the lesion is then debulked of gross tumor with an instrument such as a scalpel or curette. With the scalpel oriented at 45 degrees to the skin surface, the margin of the excision site is then beveled toward the base of the excision site. A thin, saucer-shaped portion of tissue is removed. Precise orientation of the tissue specimen is maintained with respect to the excision site as the site is carefully mapped. The specimen is inverted and divided into pieces of an appropriate size to allow adequate sectioning and subsequent microscopic evaluation. The edges of these pieces are color coded to aid in orientation. A cryostat is used to produce horizontal frozen sections of the specimen approximately 5-7 micrometers thick. The sections are taken from the deep aspect of the specimen to its peripheral epidermal margins and are subsequently stained with hematoxylin and eosin or toluidine blue. Microscopic examination of the sections is then performed typically by the micrographic surgeon, and any residual tumor which is discovered is then mapped to the excision site. The surgeon then returns to the patient with further excision performed in only those areas demonstrating the presence of residual tumor. The excision, processing, and evaluation steps are repeated until all sections are free of tumor.
When excision is complete, wound repair is contemplated depending upon size and location. An occlusive dressing is applied, and repair is usually performed within 1 to 10 days. Immediate reconstruction of the defect is possible with histologic proof of complete tumor excision. Some wounds are allowed to heal by secondary intention with primary closure for acceptable defects. Larger and more complex lesions are repaired or reconstructed by various means including split or full thickness skin grafts, as well as local, regional, distant, or free flaps.
Indications Many different types of cutaneous malignancy are amenable to treatment with Mohs micrographic surgery. The vast majority of these are basal cell carcinomas and squamous cell carcinomas. However, there are other more rare forms of cutaneous neoplasia such as verrucous carcinoma, dermatofibrosarcoma protuberans, and various adnexal tumors which bear some mention as well. The discussion will principally involve the use of Mohs micrographic surgery in the management of basal cell carcinomas, in that these are the most frequently encountered tumors. Many of the same priciples apply to the treatment of the other neoplasms as well. Mohs surgery is most successful for those cutaneous neoplasms with the essential features of a contiguous growth pattern, low risk for metastasis, and histopathology which lends itself to thorough microscopic evaluation.
Indications for the use of Mohs micrographic surgery in the treatment of basal cell carcinoma (BCC) of the head and neck include the following:
Relative contraindications to the use of Mohs surgery would then include previously untreated primary BCCs with the opposite characteristics including low-risk location, small size, favorable histology, and less aggressive clinical characteristics. The ability to distinguish individual tumors with regard to their location, size, histology, and clinical characteristics is essential in choosing the appropriate therapy.
Bleeding probably represents the most important potential complication of Mohs surgery. Therefore, Mohs surgery is contraindicated in those patients with coagulopathy or in those patients receiving therapeutic anti-coagulation. Patients should also be carefully screened for other underlying serious medical conditions as when contemplating any type of surgery.
The following is a discussion of the above-named indications which should develop an appreciation of why Mohs surgery is an effective method of treatment for these particularly difficult to treat lesions.
Basal cell carcinoma accounts for approximately 90% of cutaneous malignancies. It is not one disease but rather presents as a variety of histopathologic forms with different biologic behaviors. While the majority of lesions are small, slowly growing nodular tumors with discrete borders, some forms of basal cell carcinoma are substantially more aggressive and warrant more aggressive treatment. These include morpheaform or sclerosing, infiltrating, and keratotic basal cell carcinoma, which is also known as basosquamous or metatypical carcinoma.
The typical basal cell carcinoma is a small nodular tumor which remains somewhat superficial in its growth pattern. The tumor margins are usually well-circumscribed, and ulceration may occur and appear as nodulo-ulcerative lesions. There is an indolent progression of tumor spread with contiguous projections of tumor which have a propensity to extend along fascial planes, embryonic fusion planes (e.g. intersection of nasal ala and nasolabial fold), periosteum, perichondrium, or nerve sheaths. This behavior allows the tumor to invade locally with the ability to do so being increased in certain high-risk locations. Metastasis is rare.
Tumor location is important in predicting possible tumor behavior and in selecting appropriate therapy. Local tumor invasion and aggressiveness is more significant in certain areas of the face such as the midface, periorbital region, temple, and periauricular region including preauricular area and postauricular sulcus. The combination of these areas is often known as the H-zone of the face. These constitute high-risk areas of considerable functional and cosmetic significance in which a technique such as Mohs micrographic surgery, with its tissue-sparing capability, is a distinct advantage. These areas also involve the potential for increased local invasion since they contain embryonic fusion planes as well as periosteal and perichondrial pathways along which tumor may spread.
Increasing tumor size has been noted to increase the difficulty of excision and to decrease the cure rates for basal cell carcinoma. Larger tumors have also been shown to have more significant subclinical extension and more aggressive behavior with the likelihood of metastasis possibly being increased as well. There is a higher probability of tumor invasion into surrounding vital structures. Tumors which have been present for an extended period of time are also noted to have an increased incidence of aggressive behavior.
Various histologic patterns have been identified which are less favorable in terms of their degree of subclinical extension, aggressive behavior, and patterns of recurrence. These include morpheaform or sclerotic basal cell carcinoma, infiltrative basal cell carcinoma, and basosquamous or keratinizing basal cell carcinoma.
Morpheaform or sclerosing basal cell carcinoma may grossly appear as a white plaque-like lesion with poorly demarcated borders and often resembles scar tissue. Histologically, there are long strands of poorly differentiated basal cells deeply invading a markedly sclerotic dermis. These tumors present significant potential for subclinical extension.
Infiltrative basal cell carcinoma presents the histologic appearance of small nests of tumor cells with irregular spiky projections into the surrounding tissues. There may also be loss of cellular differentiation and loss of the peripheral palisading seen in the more common nodular forms of basal cell carcinoma.
Keratinizing basal cell carcinoma is also known as basosquamous or metatypical carcinoma. These lesions demonstrate varying degrees of squamous differentiation and have characteristics of basal cell and squamous cell carcinoma, which may make it difficult to determine the prognosis and biologic behavior of these tumors. Focal areas of keratinization may occur. Potential exists for extensive local invasion, and metastasis may occur. Most investigators currently consider these tumors to be a form of basal cell carcinoma with aggressive behavior patterns.
While most basal cell carcinomas are nodular with distinct clinical margins and indolent growth, there are those with aggressive clinical behavior, ill-defined borders, and a multicentric pattern. This lack of contiguous spread with extensive subclinical spread make resection more difficult and increase the likelihood of recurrence.
Tumors may have a more progressive clinical course in an immunosuppressed host or if overall health is poor. Tumor surveillance may be impaired in those individuals immunosuppressed as a result of organ transplantation or from other malignancies such as leukemia or lymphoma. The autosomal recessively inherited disorder xeroderma pigmentosum or the autosomal dominantly inherited disorder nevoid basal cell syndrome may also result in tumors exhibiting unusually aggressive behavior, increasing the difficulty of treatment.
The use of Mohs micrographic surgery in the treatment of recurrent basal cell carcinoma has been shown to offer significantly improved five year survival rates when compared with further treatment using the original therapeutic modality. The five year survival rate for Mohs surgery is on the order of 96% or better compared to approximately 50% for the other forms of therapy. Recurrent tumors are also known to often demonstrate histologic patterns which are more invasive and difficult to treat than that of the original tumor.
When the margins of excision are noted to contain foci of residual basal cell carcinoma, there has been shown to be a substantial increase in the rate of recurrence. There is evidence that standard surgical excision techniques are often inadequate in a large percentage of patients. Residual tumor is often left behind and an amount of normal tissue is often excised which is in great excess of that necessary for cure. Therefore, the precise control and microscopic evaluation of tumor excision with maximal conservation of normal adjacent tissue which is achievable with Mohs surgery is highly desirable under these circumstances.
Other forms of cutaneous neoplasia in the head and neck region often amenable to treatment with Mohs surgery include those mentioned below.
Squamous cell carcinoma accounts for perhaps 10% of malignant cutaneous neoplasms. When compared with basal cell carcinoma, squamous cell carcinoma is typically a more aggressive lesion which caries a higher risk of regional and distant metastasis and death. Various series have shown metastatic rates from approximately 3% up to 16%. The rate of metastasis in recurrent lesions rises to anywhere from 25% to 45%. Tumors considered to be high-risk have some of the following characteristics: large size, poor cellular differentiation, deeply invasive growth, immunosuppressed patient, recurrence following prior treatment, perineural invasion, and those tumors arising in burn scars, chronic ulcers, or previously irradiated skin.
As with basal cell carcinoma, adequate treatment of squamous cell carcinoma consists of complete removal of the tumor. Most small, localized lesions may be treated with traditional methods of field therapy. However, those high-risk lesions mentioned above are best treated with Mohs micrographic surgery. High-risk lesions treated with Mohs surgery have survival rates of approximately 95% or slightly better which exceeds that of other modalities. Recurrent lesions treated with Mohs surgery have survival rates of approximately 90%, also better than survival rates achievable with other methods of therapy.
The potential for perineural invasion is increased in cutaneous squamous cell carcinoma over that seen with basal cell carcinoma. The presence of perineural involvement with tumor substantially increases the chance for metastasis and local recurrence. The tumor may extend along nerves and thereby enter cranial foramina with resultant intracranial involvement. Mohs surgery may be of benefit in tracking tumor growth in those lesions with perineural growth.
Squamous cell carcinoma arising on the mucosa of the lip may be more aggressive than that arising in keratinized epithelium. However, the potential for regional nodal metastasis is less than that found in other squamous cell lesions involving oral mucosa. Mohs micrographic surgery is useful in adequately excising the tumor while preserving the uninvolved adjacent tissues. This should hopefully lessen somewhat the complexity of reconstruction necessary following more routine forms of removal of squamous carcinoma of the lip.
Nodal involvement in cutaneous squamous cell carcinoma may necessitate a multidisciplinary approach to the management of these tumors. Local tumor control with reconstruction after excision, nodal dissection for control of regional disease, and radiation therapy to the involved field may all be important considerations.
Other lesions of the head and neck are also amenable to excision under the microscopic guidance of Mohs surgery, with the highest cure rates for these tumors often being obtained with this method of treatment. These neoplasms have in common the characteristics upon which the foundation of successful Mohs micrographic surgery are based: contiguous growth patterns with local invasion and low metastatic potential.
Keratoacanthomas are benign but often rapidly growing tumors with an exophytic crater-like appearance and a central keratin plug. The larger, more aggressive lesions may be locally destructive, particularly in certain locations. Of importance with regard to this lesion is the ability to distinguish it from squamous cell carcinoma, especially when the keratoacanthoma is in its earlier stages of development and appears as a keratotic papule.
Dermatofibrosarcoma protuberans is a low-grade malignancy of dermal origin which may display extensive local tissue invasion and a propensity for local recurrence following treatment with routine methods.
Sebaceous carcinoma often presents as a benign-appearing nodular lesion of the periocular region. However, this lesion may be quite aggressive, with high rates of local recurrence and the potential for metastasis. Its common presentation in the periocular region makes adequate excision with optimal preservation of normal tissues a major concern, and Mohs surgery is often of benefit.
Atypical fibroxanthoma is a benign lesion which often appears on the sun-exposed areas of the head and neck in older individuals and may mimic more ominous lesions.
Adnexal tumors are of many varieties and represent a rare group of neoplasms which may be quite malignant. They arise from vascular, neural, and connective tissues as well as hair follicles and sweat and sebaceous glands.
From the previous discussion, it is apparent that the advantages of Mohs surgery include:
Not all tumors may be cured with the use of Mohs surgery. The technique is not optimal under all circumstances, and treatment failures may occur for various reasons. A lesion may be so large as to preclude complete excision or may extend to involve important adjacent structures which may prevent removal of all tumor cells. A discontinuous tumor growth pattern or inattention to meticulous detail during the procedure may result in residual tumor being left behind. Adequate histologic evaluation of the tumor specimen might be hampered by tumor location or inflammatory reaction and additional means of histologic examination may become necessary such as permanent sections. Recurrent disease may occur with incomplete excision or an inadequate histopathologic specimen, and adjunctive therapy may be necessary to effect a cure.
Many commonly encountered cutaneous malignant lesions such as basal cell carcinomas or squamous cell carcinomas may be treated adequately with standard modalities such as electrodessication and curettage, cryosurgery, radiation therapy, or surgical excision. For those primary lesions which are large, unfavorable histologically, in high-risk locations, or appear clinically aggressive and for those tumors which are recurrent or incompletely excised, Mohs micrographic surgery should be considered. Contiguous tumor spread and low metastatic potential are essential features, and the tumor should lend itself to thorough histopathologic evaluation.
Mohs surgery offers the advantages of evaluation of entire tumor margins, meticulous tumor excision with high cure rates and optimal conservation of adjacent normal tissues. This satisfies the primary cutaneous oncologic treatment objective of complete tumor removal while minimizing the impact on function and cosmesis of the involved area of the head and neck. These objectives may be attained in a cost-effective manner with the possibility of immediate reconstruction of the resultant surgical defect. The procedure is tedious and sometimes prolonged. The technique requires the expertise of specially trained personnel, and should always be considered in the management of the difficult to treat cutaneous neoplasms.
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