Mohs surgery

Last updated date: 06-Nov-2023

Originally Written in English

Mohs surgery


Mohs micrographic surgery is a precise, tissue-sparing method of skin cancer excision named after the surgeon who invented it, Frederick Mohs. It is a surgical procedure that has a high cure rate for a range of skin malignancies, including basal cell carcinomas (BCC) and squamous cell carcinomas (SCC) (SCC). The fundamental benefit of Mohs surgery is that it allows for exact microscopic control of the whole tumor border while preserving as much healthy tissue as possible. 


What is Moh surgery?

Moh surgery Definition

Mohs micrographic surgery is a precise, tissue-sparing method of skin cancer excision named after the surgeon who invented it, Frederick Mohs. It is a surgical procedure that has a high cure rate for a range of skin malignancies, including basal cell carcinomas (BCC) and squamous cell carcinomas (SCC) (SCC). The fundamental benefit of Mohs surgery is that it allows for exact microscopic control of the whole tumor border while preserving as much healthy tissue as possible.

Clinical evidence to date indicates that, when compared to conventional surgical excision, MMS resulted in a significantly higher cure rate for the treatment of recurrent NMSC, and that it may have a role in the treatment of melanoma in situ and some other unusual skin cancers such as Merkel cell carcinoma and dermatofibrosarcoma protuberans. 


Skin cancer Mohs surgery

Skin cancer Mohs surgery

Dr. Mohs invented this procedure in the 1930s. Because the approach involves the administration of a chemical fixative to the in-situ tumor, the treatment was first dubbed "chemosurgery." The tumor was removed and microscopically inspected after 24 hours of in-situ fixing. The procedure was performed several times until the tumor was fully eliminated.

Mohs conducted the final few layers of the fixed-tissue procedure for a basal cell carcinoma (BCC) of the eyelid without the zinc chloride fixative in 1953 to speed up the process. Mohs used this fresh-tissue approach for all eyelid carcinomas because the tangential frozen slices he acquired functioned so effectively. He reported a 5-year cure rate of 100% utilizing the fresh-tissue approach to remove eyelid carcinomas in 1969. 

Over the next several decades, Mohs surgery transitioned away from zinc chloride fixation and toward processing fresh tissue that was frozen and sectioned in a cryostat microtome. When compared to the previous chemosurgery procedure, this technique had various advantages, including shorter processing times (15 to 30 minutes), lower patient pain, and increased tissue conservation.

Mohs surgery is recommended for skin malignancies that have a high risk of recurrence and when tissue preservation is critical. A narrow margin of tissue is removed circumferentially around and deep to the clinical margins of a skin tumor. To simplify tissue processing, the specimen is routinely removed with a 45-degree bevel.

The tissue is then swiftly frozen and sectioned in a cryostat microtome, allowing for speedy tissue processing (about 15 to 30 minutes). By sectioning the tissue horizontally, nearly all of the tissue margin (peripheral and deep margins) may be studied under the microscope. The procedure is repeated until the tumor's histopathological margins are negative.

Mohs micrographic surgery is particularly beneficial in regions of functional and cosmetic relevance, such as the head and neck area, anogenital area, hands, and feet, due to its tissue-sparing qualities. 


Skin cancer

Skin cancer

Skin cancer is characterized by the abnormal development of skin cells, which is mainly induced by exposure to UV radiation. Basal cell carcinoma and squamous cell carcinoma are the two most frequent kinds of skin cancer (usually grouped under non-melanoma skin cancers - NMSC). Melanoma, the least frequent but most lethal form of skin cancer, claimed the lives of 1,250 Canadians in 2017.

Merkel cell carcinoma, dermatofibrosarcoma protuberans, atypical fibroxanthoma, and sebaceous carcinoma are all less prevalent kinds of skin cancer. Skin malignancies can be invasive (invading the basement membrane) or in situ (limited to the epidermis), and tumor features such as size, location, and pathology affect the probability of deep tumor invasion and recurrence following therapy.

Non-melanoma skin cancer is typically treated with surgical removal of the tumor, whereas melanoma may be treated with surgery, radiation therapy, chemotherapy, and immunotherapy. Small skin cancer lesions may be treated with simple excision, electrodesiccation and curettage, or cryosurgery; larger or recurring lesions may be treated with conventional wide excision of the tumor or Mohs surgery.

Mohs surgery, also known as Mohs micrographic surgery (MMS), is a surgical process in which small layers of the tumor are gradually removed and evaluated until only cancer-free tissue remains. It may be completed in a single outpatient clinic visit. MMS's greater accuracy can help lessen scarring and the chance of requiring further treatment or surgery.



Mohs surgery Indications

Mohs surgery is recommended for skin malignancies that have a high risk of recurrence and when tissue preservation is critical. The Mohs Appropriate Use Criteria (AUC) criteria were created to help doctors determine whether a particular tumor would be adequately handled by Mohs surgery. 

Mohs surgery is particularly suitable for areas of the body in the "H" area:

  • Central face, eyelids/canthi, eyebrows, nose, lips, chin, ear, and periauricular area
  • Genitalia
  • Hands, feet, ankles, and nail units
  • Nipples/areola


Higher-risk patient characteristics include:

  • Immunocompromised
  • Genetic syndromes 
  • Prior radiated skin
  • Patient with history of high-risk tumors
  • Tumor characteristics include:
  • Positive margin on recent excision


Aggressive features that are high risk for recurrence of BCC:

  • Aggressive histologic subtype: morpheaform, infiltrating, micronodular
  • Perineural involvement
  • Metatypical/keratotic


Aggressive features of SCC:

  • Poorly or undifferentiated (characterized by a high degree of nuclear polymorphism, high mitotic rate, or low degree of keratinization)
  • Perineural/perivascular
  • Spindle cell
  • Breslow depth 2 mm or greater
  • Clark level IV or greater


Other rare cutaneous cancers with aggressive traits or in extremely aesthetically sensitive places, such as, but not limited to, the following, are candidates for Mohs surgery:

  • Verrucous carcinoma
  • Keratoacanthomas
  • Extramammary Paget disease
  • Microcystic adnexal carcinoma
  • Dermatofibrosarcoma protuberans
  • Sebaceous carcinoma
  • Atypical fibroxanthoma
  • Malignant melanoma

While the Mohs AUC can be useful in identifying whether a certain lesion is appropriately addressed with Mohs surgery, it does not rule out the possibility of using other treatment modalities to treat the same lesion (e.g. curettage, electrodesiccation & curettage, or excision).


Advantages of Mohs surgery

Efficient, cost-effective treatment

  • Single-visit outpatient surgery
  • Local anesthesia
  • Lab work done on-site

Precise results

  • Physician examines 100% of tumor margins
  • Spares healthy tissue
  • Leaves the smallest scar possible

The highest cure rate

  • Up to 99% for a skin cancer that has not been treated before
  • Up to 94% for a skin cancer that has recurred after previous treatment


Disadvantages and limitations of Mohs surgery

Disadvantages of Mohs surgery include the following:

  • If the situation is complicated or complex, the operation may become boring and lengthy for the patient.
  • If a large or difficult tumor cannot be removed in one day, prompt reconstruction following full excision may be impossible.
  • The process necessitates the use of a professionally qualified dermatologist as well as supporting workers.
  • Multiple injections of local anesthesia might be painful for the patient.


Limitations of Mohs surgery may include the following:

  • Recurrence may occur due to noncontiguous tumors and/or unconnected foci in tumors.
  • Adjunctive therapy may be required to achieve complete recovery.
  • The tumor's size may be too large to be surgically removed.


Traditional tumor excision methods

Traditional tumor excision methods

Traditional tumor excision procedures entail the removal of the clinically visible tumor as well as an extra margin of normal-appearing tissue. The extra margin is excised since malignant expansions are tiny and cannot be clinically seen or palpated.

Following tumor removal, traditional microscopic processing is utilized to evaluate the specimen's margins for remaining tumor. The bread-loaf approach and the quadrant method can be used to create representative vertical slices at 2- to 4-mm intervals throughout the specimen and in each of four quadrants. When a tissue sample is shown to be tumor-free under a microscope, the whole margin from whence the sample was acquired is presumed to be tumor-free as well.

However, because less than 1% of the interface between the material and the patient is evaluated histologically, a tumor that is considered to have clean margins using these techniques may really contain fingerlike expansions in the inspected intervals. The inability of these approaches to consistently identify residual tumor explains the high rate of local recurrence despite the discovery of tumor-free margins


Preoperative evaluation

Issues to consider and evaluate prior to Mohs surgery include the following:

  • General health and history of the patient (including medicines, allergies, past surgeries/hospitalization, potential aggravating diseases such as diabetes, cardiovascular/pulmonary impairment, history of prolonged bleeding, or proclivity for keloid/scar development)
  • Examine the physical appearance and histology characteristics of the skin tumor
  • Discussion with the patient about the Mohs surgical process, alternative treatment choices, probable procedure problems, and post-operation wound care and reconstructive possibilities.
  • Discussion with the patient about quitting drinking and/or smoking during the perioperative period
  • Discussion with the patient concerning diet, drugs, and clothing for the day of operation, as well as postoperative concerns.
  • Consultations with many specialists (e.g., dermatology, pathology, cutaneous/oncologic surgery, reconstructive surgery)



Mohs micrographic surgery necessitates equipment for both the operating room and the lab, where tissue is processed and inspected microscopically. To provide optimal visualization and access to the tumor, the operating room requires good lighting and an adjustable table.

A scalpel, delicate forceps, scissors, gauze, and an electrosurgical instrument for coagulation comprise the surgical equipment. An enlarged tray containing needle holders, scissors, fine forceps, skin hooks, and a scalpel can be used for reconstruction.

The Mohs histology laboratory is comprised of microtomes that freeze tissue before cutting very thin slices to mount on glass slides. After that, the slides are either put in an automatic stainer or stained by hand. To reduce exposure to the chemicals used in the staining process, this method may need the use of a vent cover.

The completed slides are then analyzed under light microscopy by the Mohs surgeon to evaluate if the tumor is still present in the tissue. Many Mohs labs additionally contain immunohistochemical stainers and chemicals to allow for tissue immunohistochemistry. The technique necessitates the presence of the surgeon and at least one helper in the operating room.



The operation is performed in phases, all in one visit, with the patient waiting in between. Following the removal of a layer of tissue, the surgeon analyzes it under a microscope at an on-site laboratory. If any cancer cells remain, the surgeon knows exactly where they are and removes another layer of tissue while saving as much good tissue as possible. This procedure is repeated by the doctor until no cancer cells remain.

The technique of Mohs surgery is as follows:

  1. Prior to injecting a local anesthetic, the tumor is first delineated. Any visible tumor is excised or "debulked" after anesthesia with a curette, flexible blade, or knife.
  2. Prior to removal, the tissue layer is meticulously orientated by making small superficial etch marks with a scalpel around the tissue layer and matching in-situ skin 
  3. After that, a narrow margin of tissue is excised circumferentially and deep to the debulked tumor defect. This "layer" of tissue is removed with a beveled angle of around 45 degrees, allowing for easier tissue processing.
  4. Once removed, the tissue layer is frequently split into halves or quadrants and colored dyes are applied to aid in the exact mapping of the tumor. The tissue is subsequently pushed flat such that the epidermal edge and deep margin share the same tissue plane. This flattening process is aided by the "beveled" edge acquired tissue loss.
  5. The tissue is then sliced and processed in a horizontal orientation, allowing nearly 100 percent of the peripheral and deep margins to be studied under the microscope on the same tissue segment. In contrast, standard vertical, or "breadloaf," tissue processing evaluates just a tiny area of the tumor edge.
  6. If residual tumor is observed under the microscope, the Mohs map is indicated, and the appropriate in-situ tissue is precisely excised from the patient in the area where tumor was located. This procedure is continued until the tumor is histologically negative, achieving full tumor excision while preserving as much healthy tissue as possible.
  7. After the tumor is removed, the defect is closed using a number of procedures, including primary closure, flaps, grafts, and second intention healing. A recent analysis of Mohs stages per case for experienced Mohs surgeons revealed a median of roughly 1.7 stages to clear per tumor. Obviously, in more intricate scenarios, the figure can be substantially larger.


Hematoxylin and eosin (H&E) and toluidine blue are the most often utilized tissue stains during Mohs surgery. While H&E is used frequently by the majority of Mohs surgeons, a substantial minority prefer toluidine blue for processing basal cell carcinoma because mucopolysaccharides and hyaluronic acid, which are linked with BCC, stain metachromatically with a magenta hue.

To be optimally successful, the Mohs technique requires continuous tumor development (no "skip" zones). Fortunately, this feature is present in the majority of skin malignancies.


Clinical Significance

Mohs surgery has had a high degree of clinical success.

  • Mohs surgery has good 5-year cure rates for non-melanoma skin malignancies (NMSC), especially basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) (SCC). Primary BCC (99 percent), recurrent BCC (94.4 percent), primary SCC (92-99 percent), and recurrent SCC (94.4 percent) are examples of 5-year cure rates (90 percent ).
  • Other less frequent cancers that can be treated with Mohs surgery include dermatofibrosarcoma protuberans, microcystic adnexal carcinoma, extramammary Paget disease, Merkel cell carcinoma, and sebaceous carcinoma. Mohs micrographic surgery has lately showed tremendous promise in treating various types of malignant melanoma, particularly lentigo maligna, lentigo maligna melanoma, and thin melanomas, thanks to the availability of accurate immunohistochemistry stains.


Mohs surgery recovery

The type of correction utilized by the surgeon influences postoperative care after Mohs surgery. There are several ways of bandaging and washing that are suitable. This section describes the method used by the main author.

Immediate postoperative period

Cleanse the lesion with regular saline solution before applying topical petrolatum ointment to wounds allowed to heal via granulation (secondary aim). Instruct patients to repeat the procedure 1-2 times every day.

Cleanse the surgical site with saline and apply topical petrolatum ointment beneath a pressure dressing for defects repaired with linear closure or flaps. Patients should be instructed to leave the bandage alone for 24 to 48 hours. Following that, teach patients to change their dressings on a daily basis using the same technique.

For wounds healed with skin grafts, use a petrolatum gauze dressing and a topical petrolatum ointment directly to the graft (eg, Xeroform). A few layers of sterile gauze should be placed on top of the petrolatum gauze to create bulk for a proper pressure dressing.

Apply a liquid dressing adhesive (e.g., Mastisol) to the skin a few centimeters away from the wound, then fix the bulky dressing using paper tape. Instruct patients to leave the dressing alone until the following week.

After undergoing more elaborate treatments, such as flaps and grafts, especially for bigger wounds, patients are frequently given oral antibiotics.


After wound healing

Fluorouracil cream, imiquimod cream, or photodynamic treatment may be indicated after the surgical site has healed fully for individuals with significant photodamage and actinic keratoses near surgical regions. Other superficially ablative procedures can also be employed for skin resurfacing 



Even under perfect settings, there is always a danger of complications while performing any surgical surgery. Complications from Mohs surgery are rare and typically mild.

The most common postoperative complications from Mohs surgery are, as with other dermatologic surgical procedures, bleeding, hematoma formation, nerve damage, seroma formation, wound dehiscence, flap necrosis, graft failure, infection, contact dermatitis due to antibiotic ointments or dressing materials, excessive granulation formation, keloid and/or hypertrophic scar formation, hyperpigmentation and/or hypopigmentation, and tumor recurrence (although the likelihood of tumor recurrence is much less with Mohs surgery than with other more routine therapeutic modalities).


Postoperative bleeding is uncommon in granulating wounds, particularly when patients ignore the wounds. Postoperative bleeding is more common with repairs, particularly big flaps.

Reduce bleeding risks by obtaining an adequate preoperative patient medical history and having a solid preoperative plan in place (e.g., avoidance of nonsteroidal anti-inflammatory drugs [NSAIDs] and acetylsalicylic acid [aspirin] when medically safe; monitoring and maintaining an international normalized ratio [INR] of 3 for warfarin patients.

In patients who require ongoing anticoagulation during Mohs surgery, it is critical to establish good hemostasis and use a pressure bandage. Recent studies have revealed that continuing to use warfarin and clopidogrel increases bleeding issues following Mohs surgery, despite the fact that none of the subjects had severe long-term difficulties. However, aspirin, NSAIDs, and vitamin E did not cause significant elevations in postoperative hemorrhage.

Nerve damage

Because tiny sensory fibers are destroyed during tumor removal, sensory nerve loss is common during Mohs surgery. Such deficiencies are generally temporary due to nerve fiber regrowth.

By utilizing correct understanding of human anatomy, you can avoid motor nerve injury. Spend extra time studying the anatomy in high-risk locations where motor neurons pass superficially.


Infections following Mohs surgery are uncommon when correct washing and surgical methods are used. Oral antibiotics are generally prescribed for individuals with lesions in a surgical site region with a high infection risk. Consider prescribing fluoroquinolones to treat pseudomonal infections in wounds involving cartilaginous tissues.


Is Mohs right for me?

Mohs surgery is the gold standard for treating numerous basal cell and squamous cell carcinomas (BCCs and SCCs), including those in aesthetically and functionally important locations such as the eyes, nose, lips, ears, scalp, fingers, toes, and genitals. Mohs surgery is also advised for BCCs or SCCs that are big, aggressive, or fast developing, have unclear margins, or have recurred after earlier treatment. Mohs surgery is also being used effectively by some surgeons in specific cases of melanoma.


Mohs surgery on nose

Mohs surgery to remove skin cancer on the tip of the nose is a typical reason for nasal reconstruction. The face plastic surgeon will go through surgical alternatives for reconstructing your nose with you, such as a local flap, a skin transplant, or a staged operation like a forehead flap.



Mohs surgery is a surgical method used to treat different skin malignancies that uses tangentially sliced frozen-section histology to provide perfect microscopic control of the margins. Mohs surgery has become the treatment of choice for the majority of head and neck skin malignancies, as well as recurring or histologically aggressive lesions.

Mohs surgery is suggested as a first-line treatment option for high-risk primary or recurring basal cell carcinoma. MMS may be regarded as one of the choices for high-risk primary or recurring squamous cell carcinoma, especially when tissue preservation or margin controls are difficult, or when the tumor is at a vital anatomical region. MMS may be appropriate for digital and penile tumors, as well as recurring or incompletely excised lesions in squamous cell carcinoma in situ (Bowden's disease).

Mohs surgery may also be explored for melanoma in situ (lentigo maligna) and Merkel cell carcinoma, particularly if the tumor is in a sensitive place and there is a risk of functional impairment from a too-radical excision.