HOW TO CUT: Fat

Fatty tissue is, without question, the arch nemesis of the Mohs tech.  There you are, cutting beautiful sections of non-fatty tissue when that spot of fat hits the blade and BOOM!  
It melts away, leaving a gaping hole in your section and a smudge of yellow mush on your blade.  

The problem is simple: Fat does not freeze well.  It takes a great deal more time and LN2/Freeze-It to reach the optimal cutting temperature.

Unfortunately, the temp at which fat hardens is often too cold for obtaining good sections of the non-fatty tissues that may also be present in the same sample.  If this is the case, you can use your A slide to get several cuts with full epidermis and then work on the fat for the B slide.

Step 1 - A sharp blade and a clean stage.

If your blade is getting dull, this is the perfect time to put in a new one.  The knife stage should also be free of any frost, OCT, streaks of tissue, and clumps of melted fat.  Use a piece of dry gauze to wipe residue up and away from the blade.  

Step 2 - The fat must be EXTRA COLD.

Once the tissue is lined up to the blade and you've cut through the safety layer, spray the yellowish unfrozen fat with short bursts of LN2 or Freeze-It until it turns white.  The picture on top shows fat that's a little on the warm side. (Not the best example, since I'd already frozen it and made slides for the doctor by this point)  On the bottom, the frozen fat appears more white.  The tissue may require several rounds of spraying with the LN2 before it's finally ready to cut, so patience is a must.  It's better to take an extra minute with this step than to attempt to cut too soon and waste tissue. 

Freeze artifact
with sub-epidermal splitting

CAUTION:  Excessive freezing of the block may cause freeze artifacts such as sub-epidermal splitting or the appearance of "bubbles" due to the expansion of water in the tissue or blood.  As mentioned above, you can always make the A slide with sections of non-fatty tissue at the regular cutting temperature and the B slide with fatty sections at the colder temperature.

Step 3 - Cut THICK and FAST.

Once the fat is frozen white (and STAYS white for a few seconds) discard the first one or two wafers. Double ratchet so that your sections are ~6-8µm thick and then slowly cut through the OCT until you've cut into the edge of the specimen.  Make sure that the tissue won't roll up, then grab the edge under your brush and coordinate the speed of your right hand with the guiding motion of the brush in your left.  Do not press the brush and tissue against the stage or it will stick and you won't be able to cut anything until you clean it.  If you manage to crank the wheel in a clean, swift jerk, you'll be able to get reasonable sections of some fairly fatty tissues. 

Some rolling on the bottom edge. Oops.

Maximizing Blade Usage - GG

It is possible to use THREE sections of the blade instead of just two. When you slide the blade in, just make sure that it's parallel to the edge of the knife.  The blunt edge of the blade should fit snugly into the groove.  Use your screwdriver to double check the tightness of the top screws, and you'll be good to go!

Disclaimer: I've tested this with three different knives without any problems. I have not, however, tested it with the smaller black-top knives. Just to be safe, take a few practice cuts from the safety layer or from a tissue block that you no longer need before cutting actual slides for the doctor.

A side note: If you make really wide OCT mounts (wider than the metal disc), you probably won't get good results with this method.  All that excess OCT will dull the adjacent section of the blade and you'll only get two sharp areas of the blade before you have to change it out.

Position #1: Insert the blade just far enough to cut the block.
Position #2: Roughly in the middle. You won't be able to see either of the 2 elliptical holes.
Position #3: Move the blade all the way over until just the tail end of the blade is cutting the block.

Safety Update: In Position #3, notice how there's quite a bit of exposed blade on the left-hand side. You run the risk of cutting yourself, since your left hand is the one that's working inside the machine.  I've found that it's safer to take the blade out and flip it over so that the blade looks like Position #1 but with the circular hole on the right side.

HOW TO PREP: Dog Ears

Dog ears (aka Burow’s triangles or cones) are triangles of tissue that are removed from the sides of a circular or elliptical wound before doing a closure.  Their removal allows the skin to heal flat without puckering at the ends, but also lengthens the incision and resultant scar. 

If the Mohs surgeon is fairly certain that the next level of tissue will be clear of cancer, they may decide to kill two birds with one stone by excising the dog ears and having them processed as a level.  

You will need to show the epidermis from the two sides of the cone (marked purple in the picture) as well as the surgical margin that makes up the base of the piece.  Bisect through the apex of the triangle to get the epi around the tip to lay down easily.  From here, continue processing it like a regular Mohs level.