Mohs Histographic Surgery

Increased awareness of skin cancer has resulted in increased numbers of patients visiting their dermatologists.  When warranted, the dermatologist may perform a biopsy of any suspicious lesion and submit it to a pathology laboratory for diagnosis by a pathologist.  If the diagnosis is a cancer, the lesion will have to be excised, since the cure for skin cancer is surgical removal of the tumor.  For lesions located on the face, head, neck or other cosmetically sensitive areas, the dermatologist may elect to employ the method of Mohs histographic surgery to effect the complete surgical removal of the tumor.  This procedure is usually reserved for basal cell carcinoma and squamous cell carcinoma.  Malignant melanoma is usually treated with standard elliptical excisions.


Dr. Frederic Mohs performed basic cancer research from 1929 to 1934 while attending medical school at the University of Wisconsin in Madison, Wisconsin.  He determined that a zinc chloride solution could be used to treat skin cancer cells by causing necrosis and that the histology of the tissue was well preserved.

The procedure was originally referred to as chemosurgery since chemical fixation preceded the surgery.  However, around 1953, Dr. Mohs began to eliminate the zinc chloride fixation, especially for tumors located near the eye, and by 1970 the procedure involved use of frozen sections only.  The technique was then referred to as the Mohs procedure.  Since then, many modifications have been added, including increased detailed mapping afforded by colored inks, such that currently the procedure is called Mohs histographic surgery.


As in any procedure where there is danger of infection from blood borne pathogens, it is extremely important that technicians in the micrographic laboratory area utilize universal precautions for infection control.  This includes, but is not limited to wearing gloves, safety glasses and a lab coat/gown when handling tissues and chemicals.  In addition technicians must employ proper waste disposal techniques.  Any contaminated materials must be disposed of as Biohazard Waste, and waste chemicals must be properly stored and disposed of in accordance with state and federal regulations.

The laboratory area must be in close proximity to the operating theater as the procedure is performed while the patient is waiting under local anesthetic throughout.  The area must contain a cryostat, grossing board with sharp blades and/or scalpels, a writing area for mapping, different color marking inks and applicator sticks.  A staining set up for hematoxylin and eosin (H&E) staining and coverslipping must also be available, with sufficient ventilation for safety.

The patient is prepped and under local anesthetic throughout the procedure. The surgeon makes the first excision of the lesion, removing the grossly visible tumor that has been previously biopsied and proven cancerous.  The inferior/superior orientation is made in the specimen with a notch made by the surgeon.  This notch designates “12:00”.  The tissue is then transported to the laboratory area on a sterile moist gauze and is designated Stage 1 (S1).  The term “stage” refers to each time a patient is prepped and the surgeon removes tissue for processing.  The term “layer” refers to each time the surgeon removes a level of tissue from the patient.  The number of the layer changes each time a deeper layer of tissue is removed.

Once received in the laboratory, the specimen is described, measured and drawn on the report form, noting any notches present.  Then the specimen is inked: one color from 12:00 to 6:00; and a different color from 6:00 to 12:00.  If the specimen is large, it can then be bisected from 12:00 to 6:00.  The specimen is then frozen in OCT compound on the cryostat chuck with the dermis (underlying part of the skin) facing up, such that it will be sectioned first.  Once the specimen is completely frozen throughout, cryostat sections are made and picked up on appropriately labeled glass slides.  It is imperative to obtain the very first sections on the slide, as they represent the true surgical margin.  The first slide is designated as Stage 1-Layer 1 (S1L1).  The slides are then stained with H&E, coverslipped and brought to the pathologist.  This orientation of the specimen, with the underlying dermis cut first, allows for assessment of the presence of cancer cells throughout the entire surgical margin.

The pathologist looks at the slides and determines if the tumor has been completely excised.  If this is the case, then the surgeon is notified and the wound is repaired.  However, if tumor cells are present at the inked margin of the specimen, the pathologist indicates the positive area(s) on the diagram.  The surgeon can then go back to the patient and remove this remaining tumor.  The exact location is determined by referring to the detailed map.   This specimen is brought to the laboratory area and designated Stage 2.  The diagramming and inking are repeated and the specimen is frozen for cryostat H&E sections, now designated as Stage 2-Layer 1 (S2L1).  This procedure is repeated as many times as is necessary to remove all the tumor cells.   If only one or two stages are performed, the excision area on the patient may be repaired by the surgeon in the operating theater.  For more extensive repair of multiple stage excisions, the patient may have to be transported to hospital for skin grafting repair.

The above article is based on excerpts from CM Chapman and IB Dimenstein.  Dermatopathology Laboratory Techniques.  Copyright 2015.  In press.  Please contact the author at to reserve a first print run copy of this valuable reference.


  1. Theory and Practice of Histological Techniques. Chapter 10.  JD Bancroft, A Stevens ed.  Churchill Livingstone, NY.  Fourth edition. 1996
  2. Theory and Practice of Histotechnology.  Chapter 9.   DC Sheehan, BB Hrapchak.  CV Mosby Company, St. Louis. First edition. 1980.
  1. Luna L.   Manual of Histologic Staining Methods of the Armed Forces Institute of Pathology.  3rd ed., McGraw-Hill, New York, 1968, page 10.
  2. Chapman CM. Dermatopathology: A Guide for the Histologist.  Copyright 2003.
  3. Chapman CM and Dimenstein IB.  Dermatopathology Laboratory Techniques.  Copyright 2015.  In press.