What happens during MOHs Micrographic Surgery

Mohs micrographic surgery is a specialised surgical method of removing specific skin cancers in cosmetically sensitive areas or on cancers that have recurred after previous treatment.  This procedure was first developed by Dr Frederic Mohs in the 1930s and is usually performed on skin cancer with unclear borders, commonly known as Basal Cell Carcinoma and Squamous Cell Carcinoma cancers.

Mohs requires a precise technique to remove layer by layer of skin so that the surgery is less invasive, removing as little healthy skin as possible, to minimise the size of the wound. This is performed under local anaesthetic and is carried out in stages during one visit and the patient waits for results between each stage. Because of the precision and speed of MOHs, the cure rate is high for both primary (new) basal cell cancers which is up to 99% and up to 95% on recurrent tumours. This compares to a cure rate of approximately 80-95% for a tumour removed by traditional surgical methods

Step-by-Step of Mohs Surgery

1. Analysis and Preparation: The surgeon will examine the visible lesion and outline it with a skin marker. The skin is numbed with a local anaesthetic injection.

2. Initial Tumour Removal: The surgeon removes a thin layer of the tumour for examination. The patient’s wound is covered up and the patient is asked to wait whilst the tissue sample is processed within the lab. Each layer can take up to an hour to be processed and examined and larger samples can take longer.

3. Mapping: A map is carefully drawn on the tissue so that the surgeon knows the exact location of the skin margins. The mapping process ensures that residual tumours seen under the microscope can be matched to the exact location on the patient further down the line.

4. Tissue processing and Examining (inside the lab): the skin sample is sent to the lab for examination of cancer cells.

  1. Tissue Dying: The exercised tissue is then divided into sections and tissue marking dyes are applied to the outer edges (peripheral and deep margins) to mark the surgical margins, helping indicate the orientation and location of the tissue.
  2. OCT Medium: An embedding matrix for cryosectioning known as OCT compound (optimal cutting temperature) it applied to embed tissue samples prior to frozen sectioning, allowing the tissue to adhere to the “chuck” which holds the sample within a Cryostat.
  3. Sectioning: The sample is then frozen in a cooling temperature of down to -35 °C + and sectioned using a Cryostat, to create thin sections that are suitable to be mounted onto a microscope slide for examination.
  4. Staining: The sample is then stained, typically using Haematoxylin and Eosin (H&E) stains to differentiate cancerous cells under the microscope.
  5. Examining under the Microscope: The slide with the sample on is placed under the microscope for precise identification of cancerous cells and their location within the tissue.
    If cancer cells are found in the examined tissue, the surgeon marks their location on the map corresponding to the patient’s skin. This step helps guide the subsequent tissue removal process.

5. Additional Tissue Removal (Stage 2): If cancerous cells are located, the surgeon will return to the patient to remove another layer of tissue specifically targeting the areas where cancer cells were identified.

6. Tissue Processing and Examining (Stage 2): This additional tissue sample is then sent to the lab to be processed and examined for cancers again.

The removal of layers of tissue and microscopic examination is continuously repeated until there are no more cancer cells are found. This means that the ‘roots’ of the skin cancer have been successfully removed and the patient is tumour-free.

7. Closure: Once the surgeon is confident that all cancer cells have been removed, the wound is closed. The method of closure depends on the size and location of the wound.


For tumours that are difficult to identify, variations of Mohs surgery may be applied that follow the basic principles of Mohs surgery but use paraffin-embedded sections instead of frozen samples allowing the use of immunohistochemical stains to help identify tumour cells. This will mean that the process is not as quick for the patient and is sometimes referred to as slow Mohs.