Mohs Surgery - Differences Between Histology of Transverse Sections and Vertical Sections

Differences Between Histology of Transverse Sections and Vertical Sections

Sometimes, for confirmation purposes, a second opinion will be asked of a pathologist to review pathology slides from Mohs cases. Traditional histology of skin tissue uses vertical sectioning – with the subcutaneous tissue at the bottom and the epidermis at the top. Mohs surgery uses tangential or horizontal sectioning, which can confuse the pathologists trained in the traditional method.

First, one has to determine the method of chromacoding or color coding. The orientation of the Mohs map must be able to distinguish between medial, lateral, superior, and inferior.

Next, one has to determine if the surgeon followed the convention of mounting only 2 sections per case; as preferred by some authors; or did he/she perform serial sectioning through the block as preferred by other authors. If serial sectioning is performed, the distance between sections should be confirmed. Some surgeons utilize 100 micrometres between each section, and some utilize 200 micrometres between the first two sections, and 100 micrometres between subsequent sections (10 crank of tissue set at 6 to 10 micrometre is roughly equal to 100 micrometres if one allows for physical compression due to the blade).

Next, one determines if the entire epidermal border is present. Ideally 100% of the epithelial border should be present. Convention requires at least 95% of the epidermis to be present. However, some surgeons will make an exception for some missing epithelium at the apices of an elliptical excision around the specimen. Ideally, oval sections should be performed. However, for practical purposes on some lesions, a surgeon might cut the Mohs section to approximate the final closure defect. The apexes are often 1 cm or more from the tumor, so clear margins at the apices can be ignored. This is not the ideal by convention, but is appropriate on a case by case basis.

Next, one determines if the surgical margin is clear. With serial sectioning, one has to recreate the surgical specimen in a 3 dimensional way. The first section that touches the blade begins the 3 dimensional reconstruction. By using the 3-D reconstruction of the specimen, one can say that all the epithelial margin is present as one progresses from deep to superficial. If only 2 sections are present, ideally, both the sections should be clear. If the deeper section is positive, one has to ascertain the distance between the sections. Convention often calls for a clear margin of at least 200 micrometres. For ambiguous structures that resemble both adnexal structure and carcinoma, following the serial sections will allow for one to identify the structure as benign or malignant. With the 2 slides method, this might be impossible to perform, as no 3-D reconstruction is possible with only 2 sections.

Carcinoma appearance under Mohs micrographic sectioning can be difficult. Tangential cut of squamous cell can mimic squamous cell carcinoma (but without the atypia). Sections through the buds of hair follicles can resemble isolated islands of basal cell cancers, often even with retraction artifact. Serial section analysis is best for Mohs surgery.