Fixation on Histology

Tissue processing is an essential and first step in anatomic pathology laboratory techniques. It is designed to remove all extractable water from the tissue, replacing it with a support medium that provides sufficient rigidity to enable tissue sectioning without damage or distortion.

There are two types of tissue processing machines in the histology world. They are a carousel-type processor (traditional tissue transfer) and a microwave tissue processing machine. The former was the first automated tissue processor used in the histology laboratory that transports tissues in cassettes through a series of steps known as fixation, dehydration, clearing, and impregnation by using chemicals of formalin, alcohol, xylene, and paraffin wax, respectively. It requires 12 hours to complete the whole processing, mostly histology laboratory professional process overnight, to receive the completed, processed tissue the next morning. The latter, microwave processing, in principle uses previously fixed tissue that is dehydrated rapidly using microwave energy to heat the reagent alcohol to just below its boiling point. Isopropanol further dehydrates the tissue and prepares it for paraffin infiltration. The residual isopropanol is effectively “boiled out” using microwave energy to heat. The heat uses for the faster penetration of the chemicals through the tissues resulting in rapid processing. The tissue should be thoroughly fixed before being placed in the microwave for tissue processing. It uses only three reagents: ethanol, isopropanol, and paraffin wax, and it takes a  maximum of two and half hours to complete the whole processing. When we look at the differences of these two tissue processing machines, the differences are mainly: in the chemical they use, the quality of microscopic tissue, the time taken to the whole process, and safety.

As mentioned above, the microwave uses only three types of chemicals: ethanol, isopropanol, and paraffin wax, whereas a carousel-type processor uses formalin, ethanol, xylene, and paraffin wax. Overall, the quality of microscopic tissues from traditional and microwave processing methods was highly similar. It was not possible to distinguish between the two techniques by studying the tissue sections. The time taken to complete the whole tissue processing by microwave and traditional processing is two and half hours and 12 hours, respectively. This shows the microwave method is better in shortening turnaround time (TAT) to complete the diagnosis. Turnaround time has been an important issue for many years. It has become increasingly important in this age of managed care and commitments to reducing healthcare services costs.

As far as safety is concerned, it is essential to mention that microwave tissue processing uses high temperatures for a rapid chemical reaction to penetrate the tissue for rapid processing. The histology laboratory professional must be careful when removing the cassettes in the beaker from the microwave to avoid burns and be far from the beaker when opening the cover lid as high vapor comes from the heated alcohol. Avoiding xylene by microwave tissue processing machine can be an advantage over the traditional one.

References:

1. L Carson, C. Cappellano, Histotechnology, A self-Instructional text, America Society for Clinical Pathology press, 4^th ed. Page 48, 2016 
2. L. Ralph Rohr, Lester J. Layfield, D. Wallin, D. Hardy, A Comparison of Routine and Rapid Microwave Tissue Processing in a Surgical Pathology Laboratory, America Society for Clinical Pathologists, Am J Clin Pathol,  2001;115:703-708
3. Experience With an Automated Microwave-Assisted Rapid Tissue Processing Method, America Society for Clinical Pathologists, Am J Clin Pathol, 2004;121:528-536
4. D. Bancroft, M. Gamble, Theory and Practice of Histological Techniques, Churchill Livingstone, 6^th edition, page 84, 2008