Sociedade Brasileira de Dermatolodia Surgical & Cosmetic Dermatology


ISSN-e 1984-8773

Volume 3 Number 3

Back to summary


Continuing medical education

Mohs micrographic surgery

Cirurgia micrográfica de Mohs

Nilton de Ávila Reis1, Luciana Cirillo Maluf Azevedo1, Hamilton Ometto Stolf1, Keyvan Nouri1, Arash Kimyai-Asadi1, Leonard Harry Goldberg1

Received on: 20/05/2011
Approved on: 10/8/2011

This study was carried out at the author’s
private practice - São Paulo (SP), Brazil.

Conflicts of interests: None
Financial support: None



Mohs Micrographic Surgery is regarded as a very useful technique for the excision of difficult to han- dle skin cancers. The procedure is divided into clearly defined steps: tumor evaluation and marking, tumor exeresis, tissue preparation and mapping, histologic processing and analysis, and closing of the surgical wound. The histologic analysis of all surgical margins leads to higher cure rates and tissue conservation, which make the procedure safer and more reliable.



Mohs Micrographic Surgery (MMS) is considered to be the most reliable conservative method for treating cutaneous malignancies. It is a surgical procedure carried out in progres- sive stages; sections are made observing meticulously mapped margins until the tumor is completely removed. In addition to boasting higher cure rates than other treatment types, MMS helps preserve the maximum amount of healthy tissue 1 .

The MMS technique has continuously evolved since it was first described and is currently the treatment of choice for tumors located in critical areas, sites that have previously undergone radiotherapy, large or recurring tumors, and tumors with aggressive histological characteristics In 1941, Frédéric Mohs described a new surgical technique for the phased removal of skin cancers through in situ fixation of cutaneous tissue 2 . After fixing the tumor, Mohs excised the cancer and cut the excised piece into tangential sections that included as much of the epidermis as the underlying tissue. These cuts were car- ried out with enough depth to allow microscopic analysis of the margins.

Several types of procedures, such as curettage and elec- trodissection (CE), cryotherapy, photodynamic therapy, radio- therapy, conventional surgery and MMS, have been used to treat skin cancers. Cryotherapy, CE and radiotherapy are destructive procedures based on visual and clinical assessments of the extent of the tumor; there is only a limited ability to check the free margins when using these methods. With 3-6 mm safety mar- gins, conventional exeresis is used in the majority of skin can- cers 3 . Although conventional exeresis is systematically followed by histological analysis of the surgical margins 4 , these assess- ments are limited compared to those of MMS, in which all peripheral and deep margins are analyzed (Figure 1).

Although conventional treatments yield high cure rates for small and well-delimited skin cancers in general, MMS yields higher cure rates for both primary and recurrent tumors 5,6 . The 5-year recurrence rates for primary and recurrent basal cell car- cinomas treated conventionally are 10% and 17%, respectively. When treated with MMS, those rates fall to 1% and 6% 6 .

Besides presenting considerably higher cure rates than con- ventional surgery, MMS allows greater preservation of healthy tissue around the lesion. In order to obtain cure rates similar to those achieved with MMS, conventional surgery frequently needs much more extensive margins. For instance, the cure rate for conventional excisions of basal cell carcinomas with less than 10 mm with 3-mm margins is 85%. A 4-mm margin would increase the cure rate to 95%7. For large sclerosing or recurrent basal cell carcinomas, sections with 13-15 mm margins are nec- essary in order to obtain a 99% cure rate 8 .

A biopsy to confirm a skin cancer diagnosis is usually car- ried out before MMS is recommended. The indications for MMS are well established, especially for non-melanoma skin cancers (Table 1). The role of MMS in the treatment of other tumors such as melanoma and Merkel Cell Carcinomas is more controversial; its indication depends on the dermatologist''''''''s pref- erence and comfort.


The MMS technique includes the examination and mark- ing of the area to be excised, the removal and mapping of the surgical piece, and the histological processing and microscopic examination (Figure 2). The procedure is repeated until there are no traces of tumor in the surgical margins. Once the tumor is completely removed, the surgical wound is closed.

When examining the patient, the clinical margins of the tumor are initially delimited using a surgical marking pen. The tumor''''''''s location is then confirmed by the patient with the help of a mirror. Once confirmed, local anesthesia, usually with lido- caine and epinephrine, is applied. Curettage can be used before the excision to remove any excess tumorous tissue and check for any subclinical expansion of the tumor 9,10 . Nonetheless, curettage should not be considered as a necessary step, since it does not always benefit the procedure.

A study analyzing the effectiveness of curettage before MMS in non-melanoma skin cancers concluded that, although it is useful for removing excess friable tumorous mass before MMS, it does not completely delimit the extent of the tumor 11 . In addition, pre-operative curettage might not reduce the num- ber of MMS phases. When treating a patient with MMS, one should take into account the fact that 24% of non-melanoma skin cancers are completely removed when histologically exa - mined 12 . For those tumors, aggressive curettage can cause more damage to the surrounding tissue without necessarily increasing the accuracy of the procedure.

Once the clinical delimitation of the tumor is defined, with or without using curettage, the surgical marking is carried out 2 mm outside the previous marking to indicate the scalpel''''''''s point of incision.

In classic MMS, the scalpel blade forms a 45º angle with the skin during the excision of the tumor''''''''s margins. This allows the epidermis, dermis and deep tissues to be cut in a straight line by the cryostat, allowing the examination of a single plane 13 . The excision of peripheral margins at 90º is a variant of the original technique. In that variant, the surgical piece is divided into peripheral and deep margins, with a separate analysis of each 14 (Figure 2).

The processing of the excised tissue efore the microscopic examination includes marking it with dye, leveling, freezing, cutting and staining. Once the tumor is excised, the removed tissue is divided into fragments to allow its inclusion underneath a coverslip.. The fragments must be mapped and color-coded with tissue dye to produce a map of the tissue in order to ori- ent the surgeon while he or she analyzes the tissue under the microscope. A study carried out in the USA 15 showed that most Mohs surgeons use hand-drawn maps of excised tissue. This is a straightforward, cheap and fast method, which allows the surgeon greater flexibility to illustrate the size and format of the tumorous tissue and the surgical defect. However, hand drawings are not as accurate when analyzing the skin in recur- rent cases. Digital pictures allow more precise representations of the excised tissue and surgical defect, and provide better dimen- sions and information for the follow-up. Digital pictures are currently used by a minority of Mohs surgeons (less than 2%), a number that will certainly rise with the increasing use of digi- tal pictures and electronic medical records.

The excised tissue must be level on the microscope slide in order to cut the combined epidermis and dermis in a single plane. Although in most cases the tissue is leveled spontaneous- ly or by using light mechanical pressure, it is sometimes neces- sary to relax the tissue by making cuts on its surface – a partic- ularly useful technique for thick tissue.

The tissue is then sectioned, and the slides are prepared for histological analysis. Hematoxylin-eosin is the most commonly used stain in MMS, and can be used for all cutaneous neoplasias. Toluidine blue is a particularly useful alternative stain for basal cell carcinomas, since it clearly demarcates the islands of tumor- ous cells by staining the surrounding mucopolysaccharides pink.

Dermis and epidermis sections are usually 5-6 mm thick, while those of adipose tissue normally measure 15-25 µm 16 . Exceptionally thick sections are difficult to analyze and can lead to misinterpretation. Conversely, details in the cells are more clearly defined in thinner sections.

Oblique sections of adnexal structures can be mistaken with basal cell carcinomas. However, serial cuts help distinguish the two.

Laboratory technicians play a key role in the process and should always position the tissue so that the correct section is made in the epidermis'''''''' surface. Mohs surgeons must know how to flatten, freeze, cut and stain the tissue in order to communi- cate efficiently with laboratory technicians in case the quality of damaged tissues needs to be discussed.

One of the biggest advantages of MMS for the dermato- logic surgeon is that it maximizes the preservation of normal tis- sue. The surgeon must analyze the microscope slides to deter- mine whether the margins are affected. If the tumor has been completely excised, the surgical defect can be reconstructed. If tumorous cells are present in the tissue sample, their correspon- ding location is marked on the map. If the lateral margin is com- promised, an additional 1-2 mm of tissue is removed. If the tumor is present in the deep margin, an incision along the base of the surgical wound is made in order to remove a thin slice of its bottom. These steps are repeated until the margins are found to be tumor free; reconstruction follows.

MMS does not require prophylactic antibiotic therapy afterwards. This type of treatment is restricted to specific cases and is not linked to the MMS surgical technique 17 .


The MMS technique was developed to allow the complete histological control of the margins of excised cutaneous tumors. In spite of significant variations in the techniques used by dif- ferent Mohs surgeons, these techniques share several common points, including: (1) the complete histological control of both peripheral and deep margins, (2) tissue preservation due to the use of narrow margins, (3) clinical-pathological correlation and surgical/histopathologic assessments that are carried out by the same physician. Since its introduction, techniques used in MMS have evolved continuously. In spite of some technical differ- ences, the accuracy and meticulousness applied in each step of the procedure lead to consistently high, replicable cure rates. When carried out by properly trained professionals, MMS is a safe and reliable method that is increasingly used in dermatolo- gist physicians'''''''' daily practice.


1 . Rapini RP. On the definition of Mohs surgery and how it determines appropriate surgical margins. Arch Dermatol. 1992; 128(5): 673-8.

2 . Mohs FE. Chemosurgery: a microscopically controlled method of cancer excision. Arch Surg. 1941; 42: 279-95.

3 . Berezovsky AB, Rosenberg L, Cagniano E, Silberstein E. The role of frozen section histological analysis in the treatment of head and neck skin basal and squamous cell carcinomas. Isr Med Assoc J. 2008; 10(5): 344-5.

4 . Rapini RP. Comparison of methods for checking surgical margins. J Am Acad Dermatol. 1990; 23(2 pt 1): 288-94.

5 . Rowe DE, Carroll RJ, Day CL. Mohs surgery is the treatment of choice for recurrent (previously treated) basal cell carcinoma. J Dermatol Surg Oncol. 1989; 15(4): 424-31.

6 . Karampoiki V, Flores FJ, Altinoz H, et al. Screening Evaluation System - Europe (SESy_Europe) met skin cancer screening. Cent Eur J Public Health 2007; 15(2): 71-3.

7 . Wolf DJ, Zitelli JK. Surgical margins for basal cell carcinoma. Arch Dermatol. 1987; 123(3): 340-4.

8 . Breuninger H, Dietz K. Prediction of subclinical tumor infiltration in basal cell carcinoma. J Dermatol Surg Oncol. 1991; 17(7): 574-8.

9 . Glen MB, George LW, John WG. Mohs micrographic surgery. Am Fam Phys. 2005; 72(5): 845-8.

10 . Ratner D, Bagiella E. The efficacy of curettage in delineating margins of basal cell carcinoma before Mohs micrographic surgery. Dermatol Surg. 2003; 29(9):899-903.

11 . Jih MH, Friedman PM, Goldberg LH, Asadi AK. Curettage prior to Mohs micrographic surgery for previously-biopsied nonmelanoma skin cancer: What are we curetting? A retrospective, prospective and com parative study. Dermatol Surg 2005; 31(1): 10-5.

12 . Swetter SM, Boldrick JC, Pierre P, Wong P, Egbert BM. Effects of biopsy induced wound healing on residual basal cell and squamous cell carci nomas: rate of tumor regression in excisional specimens. J Cutan Pathol. 2003; 30(2): 139-46.

13 . Cottel WI, Bailin PL, Albom MJ, Bernstein G, Braun M 3rd, Hanke CW, et al. Essentials of Mohs micrographic surgery. J Dermatol Surg Oncol. 1988; 14(1): 11-3.

14 . Arnon O, Rapini RP, Mamelak AJ, Goldberg LH. Mohs micrographic surgery: current techniques. Isr Med Assoc J. 2010;12(7):431-5.

15 . Silapunt S, Peterson SR, Alcalay J, Goldberg HL. Mohs tissue mapping and processing: a survey study. Dermatol Surg 2003; 29(11): 1109-12.

16 . Snow SN, Madjar DD Jr. Mohs surgery in the management of cutaneous malignancies. Clin Dermatol. 2001; 19(3): 339-47.

17 . Reis NA, Timoner FR, Machado Filho CAS. Profilaxia em cirurgia derma- tológica. Surg Cosmet Dermatol. 2010:2 (1): 47-53.

Facebook Twitter Linkedin

© 2021 Sociedade Brasileira de Dermatologia - Todos os direitos reservados

GN1 - Sistemas e Publicações