Surgical pearl: large single sections in Mohs micrographic surgery

Abstract

M ohs micrographic surgery is a highly effective surgical modality for the treatment of certain cutaneous malignancies. The “fresh-tissue technique,”1 which is currently used by most Mohs micrographic surgeons, involves the saucerization excision of a neoplasm followed by the preparation of horizontal frozen sections, sequential microscopic examination of all frozen section tissue specimens, mapping of the excised tissue, and reexcision of persistent tumor until all margins are clear. Mohs micrographic surgery, which insures maximal cure rates and the conservation of normal tissue, may also be a meticulous and time-consuming procedure. In 1998, this author and Taylor2 described a method of embedding and mounting multiple, small tissue specimens on one glass slide, which can reduce the number of glass slides used to mount frozen sections and increase the rapidity with which frozen sections can be prepared and examined during Mohs micrographic surgery. This article describes the use of large single sections, another efficient, time-saving technique for preparing histologic sections for Mohs micrographic surgery. Tissue specimens excised for Mohs micrographic surgery are typically divided into 2 or more sections before being horizontally sectioned, stained, and mounted on slides for microscopic examination. The specimen is divided so that it will fit on the cryostat chuck disk and so that the lateral and deep margins can easily be flattened into a single plane. The single-section method, originally described by Randle et al,3 saves time by omitting the division of the specimen into smaller sections, thus, reducing the number of tissue specimens that must be cut by the technician and interpreted by the surgeon. According to Randle et al,3 the single-section method is most useful for small or thin specimens (eg, eyelid tumors). Conversely, large tumors or thick specimens are typically divided into 2 or more specimens so that they may fit on the cryostat chuck and to facilitate flattening the lateral and deep margins in the same horizontal plane. This article describes a method of processing large and thick specimens by the single-section method. The first step is to aggressively remove the bulk of the tumor either by curettage, scalpel excision, or shave excision. Shave excision with a bowed half razor blade is our preferred method of debulking, which produces a thin section that is easier to flatten, freeze, and cut (Fig 1). To preserve orientation, nicks are placed at 12, 3, 6, and 9 o’clock. The tumor is removed in a thin, saucerized fashion by beveling the scalpel blade 45 degrees. The excisional specimen is then placed on a glass slide and the surface is scored with partial-thickness cuts to relax the tissue so that, as it freezes, the epidermal and deep margins can be gently flattened with forceps into the same plane that is represented by the surface of the glass slide. The 12, 3, and 9 o’clock nicks are then color-coded with dyes, the map is drawn, and the specimen (that rests on the slide) is placed into the cryostat. Optimum cutting temperature (OCT) embedding compound is poured on the tissue and the cryostat chuck, and allowed to freeze to about 24°C. After placing the slide and the tissue on the chuck, the slide is separated from the specimen by warming the slide with the thumb. At this point, the undersurface of the specimen and the epidermal margin are embedded in OCT medium in the same plane in preparation for horizontal sectioning with the microtome (Fig 2). OCT medium, if allowed to adequately freeze, may extend beyond the edges of the chuck. Thus, the diameter of the cryostat chuck is not a size-limiting factor, because specimens larger than the chuck may be successfully embedded in OCT, mounted on the chuck, and horizontally sectioned. It may also be necessary to allow larger specimens to freeze for a From the University of Cincinnati. Reprints not available from the author. J Am Acad Dermatol 2003;48:506-8. Copyright © 2003 by the American Academy of Dermatology, Inc. 0190-9622/2003/$30.00 0 doi:10.1067/S1090-9622(03)0078-6