Chapter 53 Mohs surgery
1. What is Mohs surgery?
In 1936, Dr. Frederic Mohs, of the University of Wisconsin, developed a precise tumor extirpation technique that involved the controlled, serial, microscopic examination of tissue that had been chemically fixed with zinc chloride paste. The excised tissue was systematically mapped, frozen sections were examined, and the process was then repeated at foci of residual malignancy until a completely tumor-free plane was reached. The goals of Mohs surgery are to completely remove the tumor and to maximize tissue conservation.
2. Is Mohs surgery still performed with the zinc chloride chemical paste?
Very rarely. The technique has evolved to use fresh frozen tissue methods. In the 1970s, the use of frozen sections alone in Mohs surgery was shown to have comparable cure rates to the use of zinc chloride paste. The elimination of zinc chloride paste allowed Mohs surgery to take place in a single day and avoided the pain associated with paste application. In addition, the use of frozen sections made it possible for reconstruction of the Mohs defect to occur the same day as tumor removal.
3. When is Mohs surgery indicated for basal cell and squamous cell carcinoma?
Mohs micrographic surgery is especially effective in treating basal cell carcinomas (BCC) and squamous cell carcinomas (SCC) of the face and other cosmetically sensitive areas, because it can eliminate the cancer while sparing surrounding normal skin. It is also ideal for the removal of recurrent skin cancers. In these tumors, cancer cells persist in areas of scar tissue, and the clinical margins of the recurrent tumor are often indistinct. Cure rates are 99% for primary basal cell cancers and 95% for recurrent tumors. Other indications for Mohs’ surgery include
• BCC with aggressive histopathologic features, such as morpheaform or sclerotic (desmoplastic), micronodular, superficial spreading, and infiltrative growth patterns, subtypes which often extend beyond visualized margins
• Primary BCC or SCC with poorly defined borders, especially those present in locations known to have high recurrence rates (nasolabial fold, nasal ala, medial canthus, pinna, and postauricular sulcus)
4. Is Mohs surgery appropriate for all basal and squamous cell carcinomas?
BCC and SCC are epidemic in the United States. Standard treatments, including excisional surgery, electrodesiccation and curettage, cryosurgery, and radiation therapy, have cure rates in selected series near 90%. Mohs surgery should usually be limited to indications outlined in Question 3, due to the time and expense required by the procedure.
5. In addition to basal cell and squamous cell carcinoma, what other cutaneous tumors can be treated with Mohs surgery?
Almost any type of cutaneous tumor that grows in a contiguous fashion can be treated with Mohs surgery. The technique has been found to be particularly effective for aggressive cutaneous tumors that often recur after conventional excision. Cutaneous tumors that have been demonstrated to have high cure rates with Mohs surgery (90% to 100%) include
6. Is Mohs surgery appropriate therapy for malignant melanoma?
The use of Mohs surgery for melanoma varies between practitioners. Many dermatologic surgeons perform Mohs surgery on lentigo maligna, as the poorly defined margins of the tumor make it a good candidate for the technique. Some surgeons perform Mohs surgery on melanocytic tumors with fresh frozen sections alone, while others perform special stains such as Melan-A (MART-1) to better highlight the atypical melanocytes. The use of Mohs for invasive melanoma remains more controversial. Multiple studies have reported 5-year cure rates with Mohs surgery for invasive melanoma as being equivalent or better than those seen with standard wide local excision. However, some practitioners feel that fresh frozen tissue does not allow adequate assessment of melanocytic tumors. Modifications of the Mohs technique, such as “slow Mohs” with rush permanent paraffin sections, has been advocated as a way to perform tissue sparing surgery without fresh frozen sections.
7. How is Mohs micrographic surgery performed today?
Mohs surgery is performed in the outpatient setting. Patients are instructed to plan for the procedure to last an entire day. After informed consent is obtained, the tumor site is identified, measured, and photographed. Local anesthetic is then infiltrated into the tumor. Prior to taking the Mohs layer, many surgeons will debulk the cancer with curette or scalpel to better delineate its dimensions. A saucer-shaped piece of tissue is then excised from the cancerous area by beveling the surgical blade at about a 45-degree angle (in contrast to traditional surgical techniques, where the incision is made perpendicular to the skin).
The specimen is then sectioned into manageable pieces, if needed, and these slices are color coded with ink. Using an anatomic cartoon representation of the site, a map is drawn to precisely label the area from which the tumor is taken. The tissue is then submitted to the Mohs histotechnician, who prepares frozen tissue sections under the supervision of the Mohs surgeon. These sections are obtained by cutting the undersurface in a horizontal plane so that the depth and most peripheral skin edge will be available for microscopic examination. Theoretically, 100% of the margin is examined, in contrast to routine histologic processes. If tumor is identified, the map is used as a guide to obtain additional tissue layers only from the areas of positivity until a tumor-free plane is reached. Between stages of Mohs surgery, the wound is covered with a temporary dressing, and the patients wait in a comfortable area. Various steps of the procedure are illustrated in Fig. 53-1 using an apple model.
Davis DA, Pellowski DM, Hanke WC: Preparation of frozen sections, Dermatol Surg 30:1479–1485, 2004.
8. What histologic stains are used in Mohs surgery?
The majority of Mohs surgeons stain their fresh frozen tissue with hematoxylin and eosin (H&E), the most common stain used in paraffin fixed sections. Some Mohs surgeons use toluidine blue, a stain that is particularly useful for highlighting basal cell carcinoma, but this has become less common. The most common immunohistochemical stain used by Mohs surgeons is a Melan-A (MART-1) stain used to highlight melanocytic cells in lentigo maligna and other melanocytic tumors. Other immunohistochemical stains that can be used for more rare tumors include CD-34 for dermatofibrosarcoma protuberans and cytokeratin-7 for squamous cell carcinoma. However, the majority of Mohs surgeons use only H&E staining in their day to day practice.
9. How are the defects created in Mohs surgery repaired?
The precision of Mohs surgery allows for maximum preservation of normal tissue. However, the surgical defect created during Mohs surgery must be repaired in most cases. Some tumors can be allowed to heal in by themselves (second intention healing), but this should be limited to smaller, shallow defects on concave surfaces. The majority of Mohs defects will need to be reconstructed, using elliptical (primary) closures, flaps, or skin grafts (Fig. 53-2). Mohs surgeons, especially those who have completed fellowships, are well trained and experienced in aesthetic reconstruction.
Key Points: Mohs Surgery
2. Mohs micrographic surgery allows maximum preservation of uninvolved tissue and, consequently, results in smaller defects compared to conventional tumor excision protocols.
3. Mohs micrographic surgery is indicated for basal cell and squamous cell carcinomas arising on the face and other sites where the maximal preservation of normal skin and cosmesis are important goals.
10. Who performs Mohs surgery?
The American College of Mohs Micrographic Surgery currently recognizes about 50 training centers in the United States where qualified applicants receive comprehensive training in Mohs micrographic surgery. The period of training is 1 to 2 years, during which time the dermatologist acquires extensive experience with all aspects of the technique. Once training is completed, the physician becomes eligible for membership in the College.
Many dermatologists know the basic techniques and employ them in their practices. However, when patients require more extensive surgery, they usually are referred to a member of the College.
11. What are the advantages and disadvantages of Mohs surgery?
Mohs surgery allows for the targeted removal of tumor, while sparing the greatest amount of normal tissue. The technique allows for the visualization of 100% of the margin of the specimen, while traditional sectioning allows for the examination of <0.1% of the margin. Mohs surgery has the highest cure rate of any technique to treat skin cancer. It also spares the patient the risks of general surgery and allows the removal of tumor and reconstruction to occur in a single day. One disadvantage of the procedure is that it can be time consuming, and the long days can be hard for patients. Additionally, Mohs surgery can be more expensive than other modalities for treating skin cancer.
