A Novel Method of Processing Single Sections Too Large to Fit on One Glass Slide in Mohs Micrographic Surgery
The following case illustrates this technique. Mohs excision of a dermatofibrosarcoma protuberans produced a tissue specimen measuring 3.5 cm in diameter. The tissue is inked for orientation corresponding to the map, placed on a glass slide, transferred into the cryostat, embedded with optimum cutting temperature (OCT) compound, and allowed to freeze. The slide with the tissue specimen is flipped over onto the cryostat chuck and leveled with a heat extractor. Once the tissue has adhered to the chuck, the slide is separated from the specimen by warming the slide. The chuck is then placed onto the specimen head in preparation for horizontal sectioning with the microtome (Figure 1). Once the OCT has adequately frozen, the tissue can be cut with the microtome. The tissue specimen in this example, however, is 3.5 cm in diameter and is too large to be mounted on one glass slide, which is 2.5 cm in diameter. To circumvent this problem, the technician mounts the specimen on 2 separate slides held together (Figure 2). Thus, approximately half of the tissue specimen is mounted on one glass slide, and the other half of the tissue specimen is mounted on the adjacent glass slide. Each slide is then stained, cover slipped, and ready for microscopic examination (Figure 3).
Some authors have advocated the use of large chucks and large glass slides to process large specimens.4 Additional costs are associated with acquiring these items. Our technique does not necessitate the purchase of equipment that is not typically needed in the standard Mohs laboratory.
If the specimen is mounted and frozen directly on the cryostat chuck, there is a risk of creating false positive or negative margins if sectioned tissue edges “roll” onto or away from the slide before cutting. This risk is minimized by freezing the specimen on a glass slide before mounting it on the cryostat chuck in frozen section medium. Mounting the specimen on a glass slide will place the peripheral and deep margins in the same plane, thus ensuring that each piece of tissue crosses the cryostat blade at the same depth, which reduces the risk of a false positive reading.