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Mohs Surgery - The Procedure

The Mohs Micrographic Surgery procedure relies on a highly specialized and contoled sequence of surgical tumor resection and pathological investigation. Using the presently named 'Fresh Tissue Technique', Mohs surgeons mark the area of clinically recognizable tumor (seen with the naked eye) and freeze the area to be removed with local anesthetic. The tissue is surgically removed, divided and marked with reference points on the patient.

Tissue specimens are then labeled with vital dyes that allow the surgeon to reference the tissue seen on microscopic slides directly back to the patient. Technicians at surgical suite produce frozen section slides of the removed tissue that is microscopically analyzed by the Mohs surgeon. If any tumor remains in the resection tissue, the surgeon knows that tumor remains in the patient. The patient is returned to the operating room for another 'level' (removal of another thin segment of tissue). The patient's resection site is referenced to a 'map' (produced at the initial resection) that allows the surgeon to precisely remove the remaining tumor roots.


  • The surgeon identifies the visible tumor
  • The tumor and surrounding skin are anesthetized
  • The visible tumor is removed
  • A thin layer of additional tissue is removed around the tumor site
  • A diagrammatic map is created to guide the surgeon to precise locations if any tumor roots remain in the thin layer
  • The thin layer is processed for microscopic analysis
  • The surgeon examines the entire layer with a microscope identifying any remaining cancer cells

If any of the microscopic sections contain tumor:

  • The map guides the surgeon to the precise location where tumor roots remain
  • An additional layer of tissue is removed in the location of the roots
  • Processing of this tissue is now conducted (as before)
  • The surgeon examines the new layer with a microscope identifying any remaining cancer cells

The tumor resection follows the same step-by-step process until no tumor cells remain in the microscopic sections and the patient is declared tumor free. At this point, a discussion ensues regarding options to reconstruct the tumor site.

Advantages & Disadvantages



  • Highest cure rate
  • Maximal preservation of tissue
  • Procedure can be done on patients unsuitable for general anesthesia (if done under local anesthesia)
  • Less psychological morbidity (don't have to wait days for results)
  • More cost effective

  • Requires Mohs unit and very specially trained personnel (only a limited number of practicing Mohs Surgeons in Canada)
  • Long tedious day in some cases


Tumor Factor

Technical Factors

  • Tumors prone to skip lesions may be prone to recurrence
  • Significantly lower cure rates are seen in patients at high risk for metastatic disease
  • Extensive tumors
    • Surgery may be mutilating
    • Patient may refuse
  • Difficult at times to distinguish reactive tissue change and malignant change
  • Inflammatory foci in the tissue preparation may obscure tumor cells
  • Incomplete or inadequate Histologic specimens may lead to recurrence
  • Frozen section quality may affect the accuracy of the analysis