microwave processing

From:Debbie Pepperal

Subject: Microwaves and cooking

Several recent communications in Histonet concerning the use of Microwaves
have prompted this reply.

I would make the following suggestions learned from our long and extensive
experience with microwave applications in histotechnology.

We have successfully used a Milestone microwave histoprocessor in our
routine and research laboratories for the past 18 months.  Like most new
instruments, it took our laboratory staff a while to be familiar with the
instrument to produce the consistent results which we now enjoy.  We also
introduced an extra step of ten minutes in isopropyl alcohol following the
exposure to JFC solution and this has ensured consistency in all our
processing runs, irrespective of the size of blocks or the nature of the
tissue. It is also important that the tissue is well fixed before
processing through the microwave assisted procedure.

The comments regarding "cooking" of tissue may have resulted from the
unfortunate use of the term in the book by Doctors Boon and Kok.  While
these authors argued that the effects of microwaves are due to heat (hence
their use of the term "cooking"), there is also evidence that molecular
kinetics resulting from the very rapid 1800 oscillation at 2.45 million
cycles/sec in the electromagnetic fields may also contribute.  The rapidly
moving molecules may collide and hasten chemical reaction. The heat
generated is thus a reflection of the molecular movement or kinetics.
Undoubtedly, heat may also have a role to play but the actual temperatures
generated in the tissue are not known and measurement is only made of the
solution in which the tissue is immersed. Thus, the temperature attained
(optimal around 70 degrees C) by itself is probably insufficient to fix
tissue but it is the kinetics or a combination of kinetics and heat that
achieves this effect. The temperature to which the tissue is subjected to
is critical and tissue susceptibility varies. 72 degrees C is the maximum
tolerated temperature beyond which most human tissues will show the effects
of cooking.

Among the many ways of achieving fixation is irradiating in normal saline,
which is by no means considered to be a fixative.   We have previously
shown that irradiation of tissue to a temperature of 70#161#C in normal saline
results in very satisfactory fixation. We do not believe that this brief
exposure to heat can produce the desired effect and kinetics must have an
important role. Certainly, irradiation in recognised fixatives such as
formalin or alcohol will hasten penetration into the tissue and accelerate
the fixation process.  When done in the correct manner with due heed to the
temperature attained, not only is fixation greatly accelerated but
cytomorphology and antigenicity are excellent as we have shown often enough
- with none of the undesirable artefacts of "cooking".

There are many useful and extensive applications of microwaves in
histotechnology but like any other laboratory modality, especially newly
introduced ones, it should be employed with care and strictly to
established protocol.

I know that there are many laboratories throughout the world that currently
employ microwaves in one way or another in routine and research procedures
with a great deal of success.  These laboratories have reaped the many
benefits of this innovative introduction to the histopathology laboratory.

Anthony S-Y Leong, MD

Professor Anthony S-Y Leong, M.D.
Medical Director
Hunter Area Pathology Services
Head, Discipline of Anatomical Pathology, University of Newcastle,
Locked Bag 1
Hunter Region Mail Centre
Newcastle 2310, Australia
TEL: 612 4921 3042; FAX: 612 4921 4440 OR 612 4921 4794 or 
612 4921 4507
email: aleong@mail.newcastle.edu.au OR        aleong@hunter.health.nsw.gov.au 

<< Previous Message | Next Message >>