A Novel Method of Processing Single Sections Too Large to Fit on One Glass Slide in Mohs Micrographic Surgery

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Mohs surgeons frequently divide large excisional specimens into 2 or more pieces before frozen section processing so that the tissue may fit on the cryostat chuck and to facilitate flattening of peripheral and deep margins into the same horizontal plane. Delays can be expected as each piece of tissue has to be separately processed and microscopically examined. The single-section method is a useful way to quickly process tissue that avoids the subdivision of excisional specimens into smaller pieces, which can save time by reducing the number of tissue specimens that must be cut by the histotechnician and interpreted by the surgeon.1–3 The limiting factor of the size of specimen that can be processed by the single section method is the diameter and length of the glass slide (typically 2.5 × 7.5 cm) on which the specimen is mounted. The authors describe a method whereby a single section that is larger than one glass slide can be successfully processed without subdivision of the tissue specimen into smaller pieces. This method saves time by avoiding having to divide large specimens into 2 or more pieces which have to be processed separately.
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.
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