Cryosurgery

Cancer

Cancer is a disease of multicellular organisms in which the normal regulatory systems that keep each cell type behaving in the manner that best serves the organism as a whole break down. These systems tell each cell what it should do, what genes it should transcribe, what it should attach to, whether it should divide; everything that defines the cell as belonging to a certain tissue. This regulation is essential if each cell is to contribute to the function of the organism. If a particular cell loses its ability to have cell division regulated, for example, then it may start to divide constitutively. If this cell is part of the gut lining, then this probably won't be a problem, since the excess cells will simply be shed into the gut. If this cell is in the kidney, though, then a lump of these cells might form. The lump will not grow to such a degree that it starts to overun the organism, since each cell requires a minimal amount of nutrients and oxygen to continue growing, thus a blood supply must be maintained throughout the lump as well. This type of lump is called a benign tumor and is usually easy to remove surgically.

There are more regulatory mechanisms in place besides just the control of cell division. A malignant tumor can result if some of these other regulatory systems break down as well. All mammalian cells are capable of moving over a substrate that they can attach to. One regulatory system keeps cells of a certain type in the place where they belong. Loss of this system can allow cells to move through basement membranes and invade the vasculature, where they can travel through the body and reverse the process at some remote location. An invasive cell that does not respond to growth regulation can form a metastase, or secondary tumor. This sort of malignant growth is usually fatal to a multicellular organism, so there have been many approaches used to remove or kill malignant cells before metastasis occurs.

One approach that has been tried, and is now being found to be particularly effective for certain cancers, is cryosurgery. The tumor is frozen in order to kill the cells, leaving the detritus for the body to clean up on its own. The crucial aspect of any treatment of a cancerous tumor is that all the cancer cells must be killed. Since these cells have escaped growth regulation, they also are no longer responsive to programmed cell death--the suicidal mechanism by which cells limit their lifetime in multicellular organisms. The consequence of leaving any cancerous cells alive after treatment is that a new tumor will form and metastatic disease becomes more likely.

Cryosurgery was introduced in the early 60's after the mechanisms of freezing injury started to be studied by cryobiologists. The procedure is analogous to the cryopreservation of organized tissues, but the many problems that make cryopreservation difficult are virtues, when the goal is to kill the tissue. The major impediments, however, are the difficulty in killing all of the cells without also damaging the normal tissue that surrounds the tumor. The injury that results from a cryosurgical procedure is complex and is still not well understood. In 1964, Cooper published some of his experimental results and stated that all of the tumor must be taken below -20C for the procedure to be effective. Unfortunately, the variables that can be manipulated during a procedure are numerous, and the relative effects of changing these variables is still not firmly established. The idea of a critical isotherm of -20C remains as dogma in the field, but with renewed activity and improvements in technology, cryosurgical treatment is likely to rapidly become a precise intervention, with optimization of the protocol for individual cases.

Cryoinjury

The injury that results from cryosurgery is complex, but it is currently thought that there are two primary mechanisms of damage. The first is direct injury to the cells from the freeze-thaw cycle and the second is the injury that results from damage caused to the biological structure-- primarily the vasculature--of the tumor.

Factors Affecting Cryosurgical Injury

There are several parameters that can be controlled (to varying degrees) during the freeze-thaw cycle of a cryosurgical procedure. Due to the complexity of the injury response and its delayed nature, the relative contributions of each parameter is not yet well understood. In addition, the nature of the procedure makes it difficult to separate out the effects of each parameter since they are all interelated to some extent. Experiments are just starting to unravel the mystery but a full understanding will have to await more data.

Optimization of Clinical Cryosurgery

There are many aspects of the cryosurgical operation that are subject to variation. The type of cryoprobe that is used, its size, geometry, and material construction are all important to how it performs, as are the cryogen used and whether heating and cooling can be done.


Fig. 10.1.1 Typical Cryoprobes.

The size and temperature of the iceball; the time that the iceball is held at a certain temperature--indeed, the entire thermal history of the iceball; the number of freeze-thaw cycles (with the possibility of different thermal protocols on different cycles); adjuvant therapy, where chemicals (cryosensitizers) are added to increase the effectiveness of the procedure; these are all parameters that can be modified right now to tailor cryosurgery to specific ends. In addition, the improvements in monitoring what happens within the iceball by ultrasound, thermometry, infra-red thermography, CT scanning, and magnetic resonance imaging will all serve to improve the control available to the cryosurgeon.

Clinical cryosurgery is now in a position to be implemented as a treatment of optimization, tailored to the particulars of a case, rather than implemented as a standard procedure. In addition to examining the particulars of each case, such as tumor geometry, nearby blood vessels, nearby sensitive tissues, tumor type, etc.; the procedure as a whole is a problem in optimizing several competing goals. The procedure must achieve the complete destruction of malignant cells, although not necessarily immediately after thawing. Peripheral injury, especially to critical tissues, must be minimized. The vascular injury should be maximized and the direct cell injury should also be maximized up to the cryoshock limit, if an excessive cell death is the cause of cryoshock. Given the effectiveness of the procedure for certain types of tumors already (also given the right cryosurgeons), it is a given that with more research into this rapidly growing field, cryosurgery will become one of the dominant clinical procedures in surgical oncology.

Tissue Cryo-Engineering

The biological processes of growth, repair and remodeling depend on the interaction between the host and the graft tissue. It is the living cells in these tissues that orchestrate this recovery process; responding to their micro-environment (the milieu of chemicals that surround and interact with each cell) and a more integrated, long-range communication system that encompasses the whole organism (mediated by hormones and the nervous system). These systems have not evolved for coping with surgical interventions, thus the response following reconstruction is often inappropriate and leads to a poor outcome. By modulating the biology of the tissue that is put into the host, it is possible to direct the repair activities toward a better recovery.

The process of freezing cells can lead to two types of injury, one that occurs during slow-cooling and another that occurs when cooling is more rapid. Although the mechanisms by which the injuries occur are different, they both target the plasma membrane of the cell as the primary site of injury. Without an intact membrane, the cell dies in a very short time following thawing. The fact of these two types of injury, though, is what allows tissue cryo-engineering to be performed. Because one type of injury occurs when you cool slowly, and a different injury occurs when you cool quickly, there is an optimal cooling rate that results in the best recovery. Each cell type has a different optimal cooling rate, sometimes dramatically so, thereby allowing the recovery within a tissue to be tailored to a particular cell type. The growth of ice through organized tissues has also been found to be governed by complex interactions. This phenomenon allows another means to engineer the freezing process for differential recovery of cells based on location within the tissue. Thus, by the process of freezing and thawing, the recovery as well as injury of the cells within a tissue can be controlled to meet pre-specified criteria. The biological response following transplantation of such an engineered tissue can be exploited to enhance the recovery following joint reconstruction; a benefit that will have a widespread impact on orthopaedics and biomedical engineering.

In a recently published experiment, a cryoprobe was used to cause a discrete cryoinjury in cardiac tissue, and then transplanting myoblasts into the cryo-scar to regenerate new heart tissue. The interesting point was that they considered the cryo-scar a good place to transplant cells because of all the growth factors released by the fibrotic process. The scope for tissue cryo-engineering in vitro (for allogeneic transplants) and in vivo (like the heart regeneration experiment) is clearly increasing rapidly. Hopefully this new growth will complement the recent interest in cryosurgery to help us understand the freezing and thawing process better.


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Document last updated Feb. 16, 1999.
Copyright © 1999, Ken Muldrew.