Chapter 7 Discussion Questions and Answers

7.1 Discuss the key principles behind the benefits of Mohs micrographic surgery.

The answer should consider the following key points:

  • The concept of complete circumferential peripheral and deep margin assessment (CCPDMA) provided by mohs procedures.
  • Mohs is extremely accurate with a high percentage cure rate of between 95-99% depending on anatomical site.
  • It involves the removal of tissue with a minimal clearance margin. Thus preserving uninvolved skin and deep soft tissues surrounding any given tumour removal
  • As a technique it lends itself to improved cosmetic effects particularly in cases of facial skin cancer tumour removal.
  • Appreciation of the difference between normal formalin fixed paraffin embedded histological assessment and tumour margin clearance compared to CCPDMA. For example the normal cut-up procedure for a skin ellipse of suspected cancer would involve inking the sample on its entire under surface and abutting to the epidermal layer all around. Sampling would then involve the transverse cutting and removal of both tips (apex) of the ellipse, to enable some comment on lateral excision margins when viewed under the microscope. One or two transverse sections from the centre (involved tumour area) will also be taken. The remainder of the tissue may then be reserved for future reference but not actually sampled.
  • Think of a medium sliced loaf of bread, in which the tips (each apex) are represented by the crust at either end, and the tumour is in one or two slices in the centre of the loaf. Normal tumour sampling would involve taking only four or five slices of this 30-pieced sliced loaf, and only the transverse face of each slice would be examined. Using the Mohs procedures and CCPDMA concept the complete circumference of the tissue is sampled (i.e. epidermal, lateral and deep tissue margins are examined in their entirety).
  • Mohs is not a procedure advocated for all cases of skin or mucosal tumour removal. The reasons for this are multifactorial; careful assessment must be made of each patient in terms of underlying clinical condition, tumour location, histological characteristics, potential risks both to the patient and staff, and also cost – Mohs is a labour-intensive process.
  • The list of primary tumour types that can be treated using the Mohs approach is quite long, the main ones being basal cell carcinoma (BCC), squamous cell carcinoma (SCC), lentigo maligna melanoma (MM) and dermatofibrosarcoma protuberans (DFSP). Other, more unusual tumours that can be treated with Mohs include recurrent aggressive mutilating keratoacanthoma, malignant fibrous histiocytoma, atypical fibroxanthoma, verrucous carcinoma, microcystic adnexal carcinoma, neuroendocrine carcinoma (Merkel cell carcinoma), extramammary Paget’s disease and leiomyosarcoma.

7.2 Discuss the key considerations required to set up a Mohs laboratory.

The answer should consider the following key points:

  • The concept of Mohs is based on the need for speed, accuracy and efficiency.
  • There should be a clear understanding of the need for

a) adequate external sunlight (windows)

b) adequate ventilation and air flow.

c) running tap water

d) good overhead lighting

e) appropriate shelving and cupboard space

f) good non-slip flooring

g) lockable doors with good general security (equipment is expensive and should be protected at all times)

h) adequate health and safety indicators and supportive signage

  • The laboratory needs to be near the clinical theatres, ideally away from direct patient areas.
  • Adjacent to the room containing the histological equipment, the laboratory ideally should have an annex with a dedicated viewing microscope for the surgical teams to assess slides.
  • Laboratory layout should adhere to good, efficient, ergonomically-orientated design. This concept, termed ‘Lean’, is based on the key premise that we focus on testing samples in the most efficient way possible in terms of cost or speed (ideally both).
  • There should be a clear sequential flow of operations with designated areas for specimen receipt, tissue embedding, microtomy and staining.
  • There should also be dedicated microscopes for quality control checking prior to slide submission to the medical teams.
  • In addition to general laboratory layout, careful consideration needs to be employed to ensure that equipment purchased is adequate for purpose. There should also be consideration of back-up equipment for use in an emergency, which should also be in close proximity and readily available.

Key pieces of equipment include:

  • A receipt bench or slotted tray
  • A computer-based logging system
  • Cut-up board and associated cut-up equipment with specimen inking stations, if required.
  • A jeweller’s eyepiece/magnifying lens.
  • Tissue embedding and orientation equipment.
  • Cryostat.
  • Cryostat chucks of various sizes
  • Staining equipment.
  • Ventilated extraction hood.
  • Staining/quality checking microscope.
  • Health and safety equipment.

7.3 What is Slow Mohs and what are the reasons for selecting this procedure over routine Mohs procedures?

The answer should consider the following key points:

  • As the term slow Mohs implies, it is Mohs done slowly! But why would you do Mohs slowly? Doesn’t it go against the whole principle of the procedure? The answer lies in the interpretive complexity of any given case. You will have realized already, having read the earlier chapters, that the tissue morphology seen in frozen sections is not as good as that in formalin-fixed, paraffin wax -embedded tissue sections.
  • Cases selection for Slow Mohs meet clearly defined criteria, as follows:

a) Some tumours selected for Mohs may present with the requirement for improved morphological detail (e.g. lentigo maligna melanoma) and also a pathologist’s input.

b) Some tumours penetrate into the fat (e.g. dermatofibrosarcoma protuberans [DFSP]). Owing to the abundance of fat and the fact that these tumours can invade deep into the subcutaneous tissue, cutting sections of fat accurately and consistently is problematic when employing frozen sections. Processing tissue with abundant fat can still be problematic, even using conventional routine paraffin processing procedures. Consideration of extended processing schedules with extra exposure to the alcohol steps of the dehydration process should be employed. Fat contains a higher water content than most other tissues.

c) Sheer complexity of tumour removal. Large tumour sites requiring extensive surgery and subsequent microscopic examination will cause delays if booked in with routine frozen section service delivery

d) Complex patient-related issues (e.g. risks associated with patients who have known infections such as HIV and HBV. Fixing and processing the tissue through to paraffin wax negates that risk.

  • Slow Mohs procedures may result in patients staying in hospital overnight, and this increases the cost of treatment significantly, especially if the process continues over several days. In some cases, however, it cannot be avoided.
  • All Slow Mohs cases should be marked ‘Urgent’ and should be embedded and cut first. Once again, the principle of displaying the entire epidermis (where appropriate) and the full inked margins applies.
  • One point of difference to note is that tissue processed and embedded into paraffin wax is often less flexible and malleable than frozen tissue. This can make tissue orientation and embedding difficult, and it often needs to be forced down in the paraffin embedding cassettes
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