The base for all my calculations has been a 25mm sample. My measurements are made with precision machinist dial indicators that have a resolution in the micron range and the cryostat itself. It should be noted that my cryostat has been measured and calibrated to my specs and that I have strong opinions on the consistency of cryostat sections. The papers cited make no mention on the calculation for or the validity of there cryosections, (not all cryostats are equal and at I have noted that the thickness settings among other things is a suggestion at best, a 1 micron error on a 5 micron section over a hundred turns of the hand wheel to get a full section is considerable and unacceptable).

A Mohs surgeon is only as good as his slides.

Actas Dermo-Sifiliograficas 2012; 103(9): 759-761

Mohs at first glance may not seem economical, as it requires an investment in time, an infrastructure and trained staff.

Need a laboratory technician just one, as 2 or 3 or more taking turns would take years to learn the ins and outs of sectioning fresh tissue.

Clinical and Experimental Dermatology (2013) 38, pp589-593

The success of MMS depends not only on the expertise of the surgeon but also on factors such as tissue processing. For the latter, the role of the MMS histotechnician  (MH) is key. The skill, ability and efficiency of the MH directly affect how quickly and accurately the surgical margins may be assessed, and consequently influences the efficiency and effectiveness of MMS and thus the overall MMS experience for the patient.

Clinical and Experimental Dermatology (2012), 37, 562-573

Mohs Micrographic surgery provides the highest cure rate of all appropriate skin-cancer treatment options; however, its success relies on the preparation of high quality slides.  

 Many surgeons are aware of the lab issues that can affect patient care. It is fair to say that most will argue that high quality slides are a necessity. It is also fair to say that

 most (including the technicians themselves) are not aware of the real pitfalls that the technician faces. 

Dermatologic surgery 2013 Nov;39; 1662-1670

Taylor et al. Facing the Block and false positives

The preparation of tissue by histotechnologist is as integral to the success of Mohs surgery as the proper taking of a layer, yet a multitude of tissue processing methods are currently used


A Comparison of Four Mohs Tissue Preparation Methods Using Porcine Skin Lear et al Dermatol Surge 2010:36;1419-1425.

For the techniques in current use the above article sates.

Of the steps required to prepare tissue, embedding and freezing have the greatest variation and are considered the rate-limiting steps of Mohs tissue processing……

Tissue processing may exhibit such variation because these methods require differing levels of technical expertise and result in varied tissue quality, turn-around times and degrees of tissue facing…. 

Facing in this article refers to the often necessary trimming of a frozen tissue block  “face” before a complete section of tissue can be harvested on the slide for staining and interpretation.

The pillars of Mohs surgery are its achievement of a high cure rate and ability to spare healthy tissue. Surgeons who tolerate techniques resulting in false positives undermine the latter goal. Embedding and freezing methods that require extensive facing result in false positives, additional layers, larger surgical defects, larger and more complex repairs, and greater patient morbidity. Cases sometimes clear in a single section- by micrometers. 

Too often the lab is a black box where layers are presented on one end and slides are collected on the other. These slides can be read and scrutinized but its what didn’t make it on the slide that’s the issue. From a medical legal standpoint the pathologist or surgeon might only be responsible for what’s on the slide and Mohs lends its self well to this as it has a built in check and balance, (any positive area is removed in the next layer) but this is not tissue sparing. The lab must produce not only quality sections but also valid results. There is only one true margin and it’s not 500 microns into the block.

The two hallmarks of Mohs Micrographic surgery are tissue conservation and 100% margin control. The Lab analog to this is placing the resection margin into a single plane and aligning that plane with the microtome blade.  If you are not properly embedding and aligning your layers you are not maximizing the benefits of Mohs.

Obviously current techniques vary in there effectiveness. This is a well-known study.

Dermatologic surgery 2010 ;36: 1419-1425

Lear et al. A comparison of four Mohs tissue preparation methods.

Table 1 depth required to cut into the block to obtain a complete section

Microns, Mean ± standard Deviation

MethodSlideCryoembedderHeat sinkFloat
One piece

853 ±227




Two pieces





*Note this is for a 10mm sample.

Every Embedding aid on the market strives for perfectly parallel embedding, Every textbook, workshop and article on Mohs embedding prompts us to embed parallel to the face of the chuck.  The problem is that we assume that the chuck (the platform to which the tissue is mounted in the cryostat’s microtome) is aligned to the blade in the cryostat. The problem is inherent in the design of the blade holders and object heads that hold the chucks in place. Personal Data

Knife alignment
118 microns452 microns
Chuck alignment  83 microns226 microns
Detent477 microns769 microns
Shaft play  18 microns  37 microns
X-Y lock105 microns137 microns

Microns of tissue loss over a 25mm sample

Keep in mind that margins are cleared in 20 Microns.

The wedge of tissue loss can be calculated with simple trig

This is just an example of the chuck locking screw pushing a chuck out of plane (chuck alignment in the above chart), as we see from the chart above this error can be several hundred microns.

The tools used to help freeze the tissue plane parallel fail to do so as they are not engineered for precision. This fact is often masked in the alignment errors of the cryostat. We have become accustomed to a certain amount of alignment error and consider this acceptable, (please refer to the parallelism column in the following chart).

Please note the chart has been omitted out of respect for the devices in current use.

It can be said that improvement in is on a factor above 300% and 800% n most cases.

A graphic representation of blade misalignment and tissue loss

I agree that it is difficult to align a block to the blade but I have designed a device that does just that. Named the TM1 short for True Margin™
Why align a tissue plane to a theoretical angle and then manually try to compensate for the misalignment. Many technicians are satisfied with close enough and trim the “surplus” away.  Dermatologic surgery 2013 Nov;39; 1662-1670 Taylor et al. Facing the Block and false positives Because of the current impossibility of perfectly aligning a specimens face with a cryostats blade, even perfectly planar specimens are trimmed slightly before complete sections come into view. Inadvertent facing can also occur when a histotechnologist fail to carefully manipulate the cryostats object head and inaccurately aligns the microtome blade with the plane of the surgical margin. With a misaligned specimen, tangential sections and initially incomplete sections are discarded.   This device takes into account any alignment issues in that particular cryostat as well as the tools own errors and compensates for them as the layer freezes to the chuck. The result is a perfectly aligned tissue plane ready for sectioning. The tool is self-zeroing, easy to use, and effective. It works on both Leica and Avantik (Microm) cryostats and an adapter fits the new stem-less Leica chucks. IEC and Tisue tech are coming soon. 

Benefits of proper embedding method not only include less time wasted in trimming and aligning block faces but when combined with less false positives, smaller defects, and less complex repairs there is often time at the end of the day for an extra case.

Whether working in a hospital setting with the technician provided by the Pathology department or privately with a personal technician, (who is often paid according to skill level) the speed and validity of the slides can be inconsistent. 

It is not uncommon for patients to judge the skill of a surgeon by the scar left behind. It is also common for Mohs patients to comment that the cancer seemed so small so why is the suture line so long. A second stage can really change closure options most tumors can be cleared in one stage if the tissue is processed efficiently.

Critical areas like nasal rims, eyelids or areas that can’t afford to go deep due to nerves (sensory or motor)and important local structures really need to clear in one stage.

In a malpractice lawsuit survey, (Brown and Harvard Medical School), 

Incidence of and risk factors for medical malpractice lawsuits among Mohs surgeons.

Perlis CS et al. Dermatol Surg 2006 Jan; 32(1):79-83

it was noted that functional outcome followed by cosmetic outcome and procedural complications were the top reasons. Its should also be noted that the investigator found an increased likelihood of lawsuits with split-thickness skin grafts, a technique often used on larger repairs. 

Dermatologic surgery 2013 Nov;39; 1662-1670

Taylor et al. Facing the Block and false positives

Site ASite BDifferance

Depth to 1st section

325 microns187 microns138 microns
Ave number of stages1.531.920.39

This is trimming in microns of the block before the first section is harvested on the slide not the depth to the full section margin. By inference an increase rate of false positive by 39% with an extra 138 microns of trimming.

Taylor et al showed how an extra 138 microns equates to a 39% increase in false positives. This is just a single point there is no way to know the slope or shape of 

this function but it is safe to say that the losses due to aligning the block face to the blade are very significant and an equally significant reduction in false positives can be expected with better alignment.

The term clear margin can also be a grey zone. Surgeons vary to the size of normal margin that they consider to be safe.

Dermatologic surgery 2013 Feb;39(2): 179-86

Cartee TV, et al. How many sections are required to clear a tumor.

The number of clear sections required to declare margin negativity and terminate MMS (margin threshold) varied widely among respondents; 25% were comfortable with one clear section, whereas 19% would obtain an additional layer with eight clear sections.

..A majority of respondents would not take an additional layer with four clear sections if resecting BCC.

 I feel that skin is precious and should be cut wisely

Dermatologic surgery 2013 Nov;39; 1662-1670

Taylor et al. Facing the Block and false positives

It is the study physicians experience that patients often clear by micrometers, in practice as few as 20 microns, because a single section may determine the need for an additional layer.

I have posted a video demonstration  that make's it clear the level of both precision and accuracy that a wide tissue face can be sectioned. Perfectly flat sections perfectly aligned every time.