Mohs Surgery: Fixed Tissue Technique

Summary

•The fixed tissue technique is rarely used today.

•The fixed tissue technique was particularly useful for deeply invasive cancers and highly vascularized tumors.

•The fixed tissue technique did not permit immediate repairs

•The fixed tissue technique often required the surgery being extended over multiple days.

•The in vivo fixation process was quite painful.

P.G. Lang (*)

Trident Dermatology, Charleston, SC, USA e-mail: pearonlang@bellsouth.net

M. Braun III

Braun Dermatology, George Washington University,

Washington, DC, USA

7.1Mohs Surgery-The Early Days

When Dr. Mohs initially began to develop his technique for removing skin cancer, he had concerns that transecting the cancer might result in the introduction of tumor into the lymphatics and blood vessels, thus leading to metastases. There was also concern that viable tumor cells could be implanted into the adjacent tissues resulting in a recurrence (Figs. 7.1 and 7.2). Working with the Department of Pharmacology at the University of Wisconsin, he developed a black paste, which contained zinc chloride, sanguinaria canadensis, and stibnite (Figs. 7.3 and 7.4). The application of this paste allowed the tissue to become “fixed” in vivo. This fixed tissue had the consistency of cardboard. Because of the fixation process, this tissue did not bleed when cut, and there was no sensation of pain; thus, there was no need for local anesthesia as long as one stayed within the confines of the fixed tissue. Using 20% merbromin, lines could be drawn on the floor of the wound as tissue was removed, resulting in

Fig. 7.3 Formula for the fixative

Fig. 7.4 Bottle of “the paste”

Therefore, immediate repairs were not possible. The sloughing of residual fixed tissue could also result in a full-thickness defect of a nose or ear. Also, if a large vessel had only been partially fixed, hemorrhage could occur when the slough occurred. Although the process of fixation could occur within several hours, it normally required at least 12 h. Thus, usually only one stage of Mohs surgery was done in a day. Thus, for tumors with extensive subclinical spread, the surgery could extend over many days. Because the zinc chloride fixative was routinely used in the early days of Mohs surgery, the procedure became known as chemosurgery. There were several disadvantages to this name. First of all, patients often confused the procedure with chemotherapy, with all of its side effects, and they often were afraid of undergoing surgery. Secondly, the term conjured up the idea that the procedure might be a form of quackery or witchcraft.

Typically, on the first day of surgery, the zinc chloride fixative was applied to the obvious tumor under occlusion (Fig. 7.6). Dichloroacetic acid was often applied prior to the fixative to increase its penetration. On the second day, the tumor was debulked, and the fixative was applied to the wound bed and adjacent skin. Dichloroacetic acid was applied to the adjacent epidermis prior to applying the fixative to increase its penetration. On the third and subsequent days, tissue was removed and microscopically examined for the presence of tumor. Zinc chloride was applied to any areas containing tumor, along with an occlusive dressing. The process was continued until there was no longer any residual tumor. If a repair was anticipated, this had to be delayed until the residual fixed tissue had separated; however, the majority of wounds were allowed to heal by second intent.

How deep the fixation process extended depended on a number of factors, including: (1) the thickness of the layer of paste applied, (2) the length of time the dressing was left in place (the paste was usually “used up,” i.e., inactive after 18 h), (3) the vascularity of the tissue, and (4) the amount of edema that occurred. For most cases, a 1–3-mm-thick layer of paste was applied. Avascular tissue was very susceptible to fixation, and one needed to be careful if, for example, one was working in an area of prior irradiation. Over fixation could result in a full-thickness defect of an ear or nose or even the skull. Achieving a “good” fix around the eye was sometimes difficult because of the accumulation of fluid, which diluted out the paste and interfered with fixation. Intact skin was somewhat impervious to the penetration of the fixative; thus, it was necessary to pretreat with dichloroacetic acid prior to applying the fixative.

An occlusive dressing was necessary to prevent the unwanted spread of the paste and to promote its penetration. The sequence of application of the various constituents was as follows: (1) first, the dichloroacetic acid was applied to the intact epidermis surrounding the wound bed; (2) next, the fixative was applied to the wound bed and adjacent epidermis; (3) then, the area was ringed with Orabase, which helped prevent the spread of the paste (Fig. 7.6); (4) then, several layers of Vaseline-coated orthopedic cotton were applied. The piece of cotton applied first had petroleum on its outer surface, but not on its inner surface. Over this was placed gauze and tape. The next day, the patient presented for surgery. The dressing and paste were

Fig. 7.6 Orabase and fixative have been applied to the lesion

removed, and surgery was initiated. The fixed tissue was very white in appearance and had the consistency of cardboard. As long as the excision was confined to the fixed tissue, there was no bleeding or pain. As the tissue was removed, 20% merbromin was used to mark on the floor of the wound, thus denoting the source of the specimens. A corresponding map was drawn. After removal, the tissue was submitted for microscopic examination utilizing frozen sections. Any areas positive for residual tumor were noted on the map of the wound. Zinc chloride fixative was applied to those

areas positive for residual tumor, along with an occlusive dressing. The patient was instructed to return the following day for additional surgery, if so indicated. This sequence of events continued until the patient was tumor-free.

In the early days of Mohs surgery, most histotechnicians used a cryotome in conjunction with carbon dioxide gas and a water bath. Unfortunately, the sections generated were quite thick, which made the microscopic interpretation of these sections more difficult (Fig. 7.5). The advent of the cryostat revolutionized Mohs surgery in that ultra-thin sections could be generated quickly. These sections were, microscopically, much easier to interpret.

Although the “fixed tissue” technique had many disadvantages and thus was eventually replaced by the “fresh tissue” technique, there were situations in which it was quite helpful. For example, if one had exposed calvarium, the paste could be applied to the exposed bone. Up to 12 weeks later, if this had been done expertly, a few taps with a hammer and osteotome would result in the separation of a thin layer of bone, revealing healthy granulation tissue, which could serve as the foundation for healing by second intent or the bed for a split-thickness graft. Unfortunately, this technique was not effective if the bone had previously been irradiated. The use of the fixative also facilitated the removal of very vascular tumors and deeply invasive tumors. Deeply invasive tumors could probably be removed more safely and accurately and with less pain than with the fresh tissue technique.