Mohs Micrographic Surgery
Mohs micrographic surgery, also known as Mohs surgery, is a surgical technique for skin cancer removal. It was conceptualized in the early 1930s by Dr. Frederic E. Mohs, who observed that cancer tissues treated with 20% zinc chloride solution maintained well-preserved microscopic morphology. This discovery set the foundation for a technique of removing cancer under microscopic control, the basis of Mohs surgery. The procedure was modified over the following decades; of note was the introduction of the fresh-tissue technique in 1953, which avoided zinc chloride fixation and worked particularly well on eyelid tumors.
Indications for Mohs surgery vary, but may include:
- Lesions that are large, recurrent, aggressive
- Lesions with ill-defined borders
- Lesions involving functionally-sensitive (i.e. fingers) areas
- Lesions involving cosmetically-sensitive (i.e. eyelids, lips, and nose) areas
- Lesions situated in regions with a high likelihood of recurrence
- Lesions with perineural invasion
- Lesions situated in embryonic fusion planes
- Lesions with extension to bone and cartilage
- Lesions situated in irradiated skin
- Lesions in immunocompromised individuals.
In the context of periocular skin cancer, Mohs surgery can be especially advantageous. The periocular region is highly susceptible to skin cancer due its direct exposure to ultraviolet radiation. In this context, preservation of maximal tissue using a multi-disciplinary approach combining Mohs and oculoplastic surgery may be particularly beneficial to enable surgical clearance of tumor while addressing functional concerns, such as complete eyelid closure, and achievement of a cosmetically acceptable outcome.
Mohs surgery is typically performed under local anesthesia, which is infiltrated into the tumor area and surrounding skin. The first excision commonly involves the removal of the clinically visible lesion with a small margin of normal tissue. The tissue is taken to the laboratory, where a histotechnician flattens the tissue so the superficial and deep margin lie in the same plane; in this way, Mohs surgery offers circumferential histologic margin evaluation. If skin cancer cells are detected, the surgeon will remove an additional portion of tissue. This process is iterated until no skin cancer cells are detected under the microscope. In total, the procedure usually takes a few hours.
After the skin cancer is removed, several avenues may be taken to manage the wound depending on its nature. It may be able to heal by secondary intent, or it may require primary closure or additional reconstructive techniques, such as grafts or flaps. Often, the wound closure may be performed on the same day as the Mohs surgery.
The goal of Mohs surgery is to remove all histopathologic evidence of malignancy while conserving as much healthy skin as possible. The technique is particularly effective as it allows the practitioner to intraoperatively ensure microscopic evidence of tumor-free margins.
In addition to serving as an attestation of complete tumor excision, Mohs surgery has demonstrated advantages in sparing tissue and decreasing risk of recurrence in prior studies. One randomized trial reported a smaller median area of surgical defects in patients with basal cell carcinoma managed with Mohs surgery compared with standard surgery. Another randomized trial documented a lower number of post-treatment recurrences in patients with high-risk facial basal cell carcinoma who were treated with Mohs surgery compared to those treated with traditional surgical excision.
The limitations associated with Mohs surgery primarily concern tissue quality and location as well as time. Successful microscopic analysis of the excised tissue requires high-quality frozen sections, which may be compromised if the specimen is too thin or too thick, frozen inadequately, or contains adipose tissue. Multifocal tumors – those located in more than one region – present an additional challenge, as the Mohs surgeon may successfully detect a clear margin around one tumor portion when others still exist elsewhere. Furthermore, Mohs surgery, particularly when it involves several rounds of excisions, may be time-consuming.
In the periocular area, there exist several additional limitations in the use of Mohs surgery, particularly with regards to lesion extension and tumor type. Extension of lesions to orbital fat may impede margin control as adipose tissue is known to compromise frozen sections and thereby hinders effective margin control. Sebaceous cell carcinomas, due to their multicentricity and pagetoid spread, may also hinder accurate detection of negative margins. Finally, in the context of periocular melanoma, frozen section processing may compromise the microscopic morphology of the epidermis. This may be resolved using Slow-Mohs, which involves multi-day processing of permanent sections instead of frozen sections; an alternative approach utilizes MART-1 immunohistochemical staining to maximize detection of atypical melanocytes.
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