American Mohs: A Critical Review

American Mohs: A Critical Review
Article Type: Review
Authors: Brett M. Will1, MS; David J. Elpern2, MD; Roy Grekin3, MD; Douglas W. Johnson4, MD

Affiliations: 1Medical Student, Georgetown University School of Medicine, Washington, DC; 2Practicing Clinical Dermatologist, Williamstown, Massachusetts; 3Department of Dermatology, Clinical Professor, University of California San Francisco School of Medicine, San Francisco, California; 4Department of Dermatology, The Queen’s Medical Center, Honolulu, HI

Keywords: Basal cell carcinoma, Cost effectiveness, Economics, History, Mohs, Squamous cell carcinoma

Funding Sources: None
Conflict of Interest Disclosure: None declared

Capsule Summary

  • Mohs Micrographic Surgery (MMS) is a valuable tool in the treatment of skin cancer whose indications and implementation have changed significantly since its inception
  • Improved understanding of MMS practices and costs to healthcare may better clarify the role it should play in the American healthcare system


Over 80 years ago, a medical student conceived of a novel approach to remove difficult-to-treat nonmelanoma skin cancers.  The procedure, called Mohs Micrographic Surgery (MMS), has been refined over the ensuing years and now large numbers of practitioners provide the service.  As the indications have continued to evolve and enlarge, the appropriate use of MMS needs to be addressed.  We look at the history of MMS since its inception and present questions that clinical dermatologists are asking. Most importantly, is MMS overused and if it is, should precautions be taken to temper its overuse?


A medical school research project completed in 1936 spawned the evolution of an effective and frequently used procedure for management of difficult and recurrent skin cancers. Dr. Frederick Mohs had observed a lay practitioner in West Texas managing skin cancers in cowboys and oilrig workers with “black paste” (Roy Grekin, M.D., personal conversation, 2019). He took this concept to the University of Wisconsin Medical School as a Brittingham research assistant where they were studying chemical reactions with cancer cells in rodents and primates. Mohs used a zinc chloride paste made of 40gms of Stibinite (80 mesh seive), 10gms Sanguaria Canadensis and 34.5ccs of zinc chloride saturated solution. The preparation was applied as a paste and when examining the treated tissue histologically, it was noted that it fixed the tissue without drastically altering the histology.1,2 After his internship in 1936, Mohs returned to Wisconsin and used the technique, called Chemosurgery for political reasons, to manage difficult-to-treat skin cancers. It was not accepted by mainstream medicine and was met with criticism and rejection by surgical specialties in particular.2

In 1956 Mohs published the first edition of his book, “Chemosurgery in Cancer, Gangrene and Infections.”3 He developed a kit and along with the textbook, the technique was shared with other physicians including Ted Tromovitch, George Vavruska, Perry Robbins, Harry Szuejewski and a handful of others in the early 1960s. His clinic developed a reputation for being able to handle large difficult tumors.2 By the 1960’s, multiple articles were published documenting chemosurgery’s efficacy in treating recurrent or otherwise difficult-to-treat skin cancers.4-6

The American College of Chemosurgery was formed in 1967 in Munich with the first formal meeting being held at the 1967 annual American Academy of Dermatology (AAD) conference in Chicago, Illinois. The next year, Dr. Alfred Kopf started the first training program at New York University Medical Center. A six-month fellowship required 150 cases completed. In 1983 fellowships were extended to one year and 250 cases and in the 1990’s, 500 cases. In 2020, it will increase to 650 cases. By 2004, the AGCME began accrediting fellowships and it was noted that 90% of all skin cancers were being managed by dermatologists. The procedure’s name had now evolved to Mohs Micrographic Surgery (MMS).2,7

By the 1950’s Dr Mohs had used local anesthesia and fresh tissue excision on the delicate skin of eyelid tumors. By 1970, Tromovitch and Stegman presented several cases with this technique at the annual College meeting. It was met with initial resistance but the technique became widely adopted because it was less painful, multiple procedures could be completed in a single day, and it left a fresh tissue bed for immediate reconstruction. In the early days, wounds had to heal by second intention. At that time, plastic surgeons frequently did the reconstructions as Mohs surgeons were not trained in extensive repairs. Now repairs are routinely done by most Mohs surgeons.2,8

Early Mohs Surgery

In his initial 1941 paper introducing MMS, Frederick Mohs established the indications for his new procedure. These were largely based on tumor location with special considerations for cosmetically sensitive areas and mucosal lesions. Mohs argued that MMS may also be preferable for tumors that arise along the embryonic fusion plates of the nose, lips, and cheeks, where tumors may invade deeply. Preservation of function and normal anatomy were relevant concerns in areas such as the hands, eyelid, parotid gland, anus, and genitalia. He also notes that distant metastases or local extension into vital structures may be contraindications for the use of MMS. Of note, there is no mention of histologic variants in Mohs’ initial discussion of indications for the technique. Additionally, 95% of the tumors treated in his initial paper were either basal cell carcinomas (BCC) or squamous cell carcinomas (SCC).1

By the 1980’s MMS was recognized by physicians as an effective treatment for recurrent and treatment-resistant BCC and SCC skin cancers. In a 1983 article on MMS, indications for its use had now expanded to include tumors with perineural invasion, aggressive histologic subtypes such as sclerotic, morpheaform, or keratotic BCCs, size > 2cm, or concerning clinical qualities such as multicentricity, recurrence, incomplete excision, or ill-defined borders. Furthermore, the article provides an illustration of the “H zone”, or high-risk locations on the face where MMS may be indicated (Figure 1).8

Figure 1. High-Risk (H-zone) areas of the face.

MMS Guidelines

The National Comprehensive Cancer Network (NCCN) developed guidelines for the appropriate treatment of BCCs in 2000. They describe three different regions, area H, area M, and area L, with high, medium, and low risks of recurrence. Treatment recommendations include 4mm margins for complete excision of low risk lesions and MMS or >4mm margins in high risk tumors. NCCN guidelines also recognize clinical and histopathologic factors in determining a lesion to be high or low risk (Figure 2). Of note, any lesion <6mm in diameter, regardless of location, is considered to have a low risk of recurrence.9

Figure 2. NCCN Risk factors for BCC Recurrence

As the use of MMS continued to increase, there developed a need for standardized criteria for its appropriate use.10,11 In 2012, appropriate use criteria (AUC) to standardize and limit the use of MMS were developed. This was a collaboration between the American College of Mohs Surgery (ACMS), American Society for Mohs Surgery (ASMS), American Academy of Dermatology, and American Society for Dermatologic Surgery Association leadership in order to standardize the indications for MMS and identify appropriate use of the technique.12

A 17-member panel of Mohs and non-Mohs academic dermatologists was presented with 270 scenarios where MMS might be considered. The panelists considered H, M, and L areas along with predisposing factors for non-melanoma skin cancer (NMSC). Additionally, the histologic features considered aggressive for BCCs and SCCs were greatly expanded upon as compared to MMS in the 1980’s (Figure 3).8,12 Each of the scenarios was given a score from 1-9. Scores from 1-3 mean MMS would be inappropriate for that indication, 4-6 means it is uncertain whether MMS would be appropriate for that indication, and 7-9 means MMS is appropriate for that indication.12

Figure 3. Aggressive features fir Mohs AUC

Adherence to Guidelines and Appropriate Use

A study examining the Centers for Medicare and Medicaid Services (CMS) databases from 1996-2008 calculated absolute changes over that time period. The study found MMS utilization increased by 248% and charges for MMS increased from 28% of all skin cancer treatment costs in 1996 to 64% in 2008. These changes correlated with a 137% increase in the total cost of skin cancer treatments. The authors note this increase may be due to a shift towards dermatologists and away from general or plastic surgeons, whose high facility-based treatment costs were largely not captured by the dataset and analysis.13 A study evaluating treatment costs using the 2008 Resource-Based Relative Value Scale found that MMS is more expensive than electrodessication and curettage or topical chemotherapy. Comparative analyses of NMSC treatment modalities have estimated MMS is between 25-100% more costly than simple excision, although this difference lessens with larger lesions. On the other hand, MMS is, on average, cheaper than excision with frozen section or radiotherapy.14,15

An analysis of the Medicare Provider and Utilization Data set for MMS procedures in 2013 suggested possible overuse of the procedure. Where 8.6% of Medicare Part B-funded MMS procedures in 2009 were performed on the trunk, arms, or extremities, 12.3% of procedures were performed on these L area locations in 2013. The top quintile of providers in MMS service volume performed the greatest percentage of procedures in these areas, also accounting for over half of all reimbursements for Medicare Part B MMS codes in 2013. In other words, the study demonstrated that the leading providers in both service volume and Medicare Part B reimbursement also performed the greatest percentage of procedures in low-risk areas.11 A similar 2019 Australian study also reported a correlation between service volume and percentage of MMS procedures performed on low-complexity areas.16

Differences in MMS utilization also exist between the United States and other countries. The United States has 3.7 times as many Mohs surgeons per capita as Australia and 15.8 times as many as Canada (Table 1). A 2011 paper comparing MMS utilization in Ottawa, Canada and Houston hospitals found a similar trend. Where the Houston area has 24 Mohs surgeons serving approximately 8.1 million people (1 Mohs surgeon/337,500 people), The Ottawa Hospital in Ottawa, Ontario has a single Mohs surgeon serving a population of approximately 2.1 million people. In this study, Ottawa patients exhibited a preoperative tumor area three times that of their Houston counterparts. They also demonstrated, on average, one extra MMS stage, larger post-surgical defects, and significantly longer wait times between scheduling and treatment.17

Table 1

The United States also exhibits a greater case volume per provider as compared to Mohs surgeons in the United Kingdom. Only the top 20.3% of British Mohs surgeons performed more than 200 cases per year in 2011, while, in contrast, the average American Mohs surgeon performed 317.9 MMS procedures in 2013.18

These differences in MMS utilization between countries are interesting when comparing rates of NMSC. Australia has the highest incidence of BCC in the world at 884 cases/100,000 person-years, ten times that of the United Kingdom. The incidence of BCC in the United States, on the other hand, varies by latitude from 170-935.9 cases/100,000 person-years. Similar trends were observed in SCC incidence rates, with Australia demonstrating higher incidences of SCC than both the United States and United Kingdom.19

Identifying and providing feedback to MMS outliers has been shown to decrease both mean number of stages per procedure and overall Medicare spending. In a 2019 article on a transparency intervention in 2329 surgeons to prevent MMS overuse, outliers and inliers were given feedback on their performance as compared to peers (Figure 4). The study observed that outliers who received feedback demonstrated a 12.6% decrease in mean stages per case as compared to a 9.0% decrease in outliers who did not receive feedback. This statistically significant difference between the two groups resulted in an estimated savings of over $11,000,000 to Medicare.20

Figure 4. Mohs surgeons’ performance feedback


The role of MMS in the management of skin cancer has continued to evolve over the eight decades following its inception. While the technique was originally intended primarily for BCCs or SCCs in high-risk areas, regardless of histologic subtype, guidelines for its application have since been expanded to include risk stratification by area, lesion size, twenty different histologic features, and concerning host or clinical features.1,9,12 Accordingly, MMS utilization has continued to increase in recent years.11

As MMS has become more widely utilized, so has it increased its share of Medicare expenditures.13 Mohs surgeons are reimbursed significantly more than their non-Mohs dermatologist counterparts. In 2013, the top quintile of Mohs surgeons in service volume received over $350,000 more in Medicare Part B reimbursements for MMS procedures than non-Mohs dermatologists did for all other procedures combined.11 It is also notable that nationwide analysis of Medicare Part B reimbursements demonstrated MMS use in low-risk regions, such as the trunk and extremities, increased between 2009 and 2013, even after the implementation of the Mohs AUC in 2012. A statistically significant correlation was identified between percentage of MMS procedures performed on the trunk and extremities, and overall caseload. The Mohs providers performing the most cases also have a greater tendency to operate on areas not typically indicated for MMS.11,12 It is clear that a subset of Mohs practitioners are inappropriately selecting cases and overutilizing the procedure.

American MMS utilization exhibits differences in contrast to that of other countries. In comparison to Australia and Canada, the United States demonstrates a greater number of Mohs surgeons per capita (Table 1: Comparison of MMS utilization in Australia, the United States, and Canada. *In 2013 there were 513 dermatologists and other practitioners billing for Mohs codes. We do not know how many there are in 2020 but used 513 as a conservative estimate (Stephania Brooks, email communication, June 2020).23,24,32-36). Additionally, American Mohs surgeons perform more cases per year than do their British counterparts.11,18 However, it is difficult to derive conclusions from these observations alone. Differences between skin cancer prevalences, health care systems, cultural views on surgery/healthcare, access to resources, and population ages are just a few factors that may change the role MMS plays in different communities. These observations raise important questions such as “Why are there more Mohs surgeons per capita in the United States than other developed countries?” and “Does a greater number of MMS procedures equate to more favorable patient-centered outcomes?” Large-scale comparisons of CMS or other datasets to foreign equivalents would help clarify the role MMS should play in the healthcare system of a developed nation.

The United States has at least two (and possibly three) classes of Mohs surgeons. One is the ACMS, comprised of fellowship-trained practitioners who have had a one to two-year rigorous fellowship training.21 As of June 2020, there are 1,627 ACMS members in the U.S (Stephania Brooks, email communication, June 2020). A second pathway for micrographic surgeons who have taken shorter, often informal training is provided by the ASMS, established in 1990.22 There are 753 ASMS members as of June 2020.23 Additionally, there are dermatologists and other practitioners who bill Medicare for Mohs codes. In 2013, there were 513 of such practitioners. Fellowship trained Mohs surgeons practice mostly in urban areas and those with less, or possibly no, training tend to operate in rural settings.24  It should be investigated which groups contribute to the overutilization and in which states they reside.

MMS overutilization can impact patient care and patient safety. While MMS is a relatively safe procedure when performed by a trained practitioner, significant complications may occur, particularly in patient populations with existing risk factors.25-27 In patients with significant comorbidities, advanced age, or low life expectancy, non-surgical management may be preferred over MMS after considering post-surgical complications, patient preferences, and efficacy of non-surgical management.28

Although development of the Mohs AUC provided much-needed standardization, further research into nationwide guideline adherence may help identify potential areas of improvement in MMS appropriate use. The issue of appropriate use in the field of MMS is one that has garnered increasing attention in recent years, especially by institutions such as the U.S. Department of Justice and CMS.29,30 The efficiency and cost-effectiveness of identifying outliers and providing feedback is reassuring that improved awareness of Mohs AUC, targeted interventions, and provider education can be useful in reinforcing MMS standards nationwide.20

We believe that there may be a need to set more rigorous guidelines to rein in the subset of MMS outliers. The overuse and abuse of MMS by a subset of micrographic surgeons is a fact known to most clinical dermatologists and it paints a poor picture of the profession.31 This could lead to unwarranted restrictions on a necessary and useful procedure. We also hope to avoid the poor patient outcomes observed in some geographic areas with a lack of access to MMS and encourage appropriate action to protect providers’ abilities to care for their patients.17

Conclusion MMS is a valuable tool in the treatment of NMSC and some melanomas that has changed significantly in its technique, indications, and widespread implementation over the past eight decades. Improved understanding of its applications and impact on healthcare leads to an appreciation of the need for more rigorous standardization to assure appropriate use and prevent overuse. Costs to healthcare and, ultimately, patient care will benefit from increasing


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