by Gina B. Rainwater, M.D.

Traditional “bread loaf” sectioning of surgical excision specimens allows examination of <0.1% of the surgical margin. On frozen sections, it is challenging and sometimes impossible to distinguish residual MIS from the background actinic damage containing reactive melanocytes.

Modified Mohs surgery, or “Slow Mohs”, using rush permanent sections achieves 100% margin control.

Margin-controlled staged surgery can maximize the cure rate and minimize post-operative wound defect by sparing normal tissue. The slow Mohs technique involves en face mounting and examination of margins combined with traditional vertical sectioning of the “debulk” specimen (Figure 1).

Upon arrival at ProPath, formalin-fixed slow Mohs sections are examined and oriented for embedding by a dermatopathologist. Experienced histotechnicians then cut the paraffin-embedded tissue into 4µm thick sections in a direction parallel to the margin, maintaining orientation to the “true” margin. Multiple sections are cut from the block. If a full face (en face margin) is not achieved, more sections are generated from the block. The central portion of the specimen (debulk) is processed using a bread-loafing technique for diagnostic purposes (confirm diagnosis, assess the depth of tumor, evaluate for perineural invasion) and to assess the deep margin. Total processing time is approx. 7-8 hours. Results are often communicated by late morning, but, in the event deeper sections are required, a specimen may require more time to be fully evaluated. We recommend scheduling the patient for surgery for the first appointment in the afternoon as almost all results are communicated by 1 pm.

To facilitate this process and the timely turn-around of slow Mohs cases, it is important to include an initial pathology report and a Mohs map with your specimen. Including each en face margin in a separate cassette ensures that the tissue is properly fixed and processed.

ProPath dermatopathologists are committed to timely and efficient communication with the clinician to ensure the best care for our surgical patients.