Cryoablation is a minimally invasive procedure that uses extreme cold to freeze and destroy cancerous cells. It is a safe and effective treatment for a wide variety of benign and malignant tumors, including breast, lung, kidney, liver, bone and many other lesions. Yet how does cryoablation work to actually destroy the tumor by “freezing”?
What happens during cryoablation?
Under ultrasound or CT (computed tomography) imaging a cryoprobe is guided into the tumor. A freezing agent, such as liquid nitrogen (used by the ProSense cryoablation system) or argon, is circulated within the cryoprobe once it has been inserted, reaching temperatures as low as − 190 °C to create an ice ball around the targeted tissue.
CT scans verify that the ice ball covers the entire tumor as well as a margin of healthy tissue to ensure the area is completely treated. Once this has been accomplished, the cryoprobe is warmed by the cryoablation system (thawed) and safely retracted from the tumor and the patient’s body.
Most procedures involve a freeze-thaw-freeze cycle where the tumor is frozen, thawed and frozen for a set time for optimal tumor destruction. The time to ensure this process varies depending on the size, location, and type of tumor.
Cryoablation: Mechanism of Action
This freezing action on the tumor causes cellular damage, death, and necrosis of tissues both directly and indirectly. So to understand how does cryoablation work we must look at these 5 key mechanisms of action:
- Osmotic injury
- Mechanical injury
- Vascular injury
- Apoptotic cell death
- Immunogenic response
The success of these processes to destroy the tumor is influenced by four factors during cryoablation[1]:
- Cooling rate – reaching low freezing temperatures results in better and direct cold-induced cellular injury
- Target temperature – different types of cellular injury occur at different temperatures with the lowest target temperatures closest to the cryoprobe
- Time at target temperature – increasing the time at target temperature will increase the likelihood that cells in the outer portion of the ice ball will have sufficient time and temperature for lethal intracellular ice to form
- Thawing rate – slow thawing can increase the chance of cell injury and repeated freeze-thaw cycles lead to a higher degree of tumor ablation
How does cryoablation work?
Cryoablation destroys cancer through five mechanisms of action at the cellular level.
- OSMOTIC INJURY:
Osmotic injury is permanent and takes place at the coldest parts of the iceball closest to the cryoprobe tip at temperatures reaching -40°C.
- Freezing
In the extracellular space (the space between cells), rapid and sustained freezing of the tumor causes the formation of ice crystals, which results in dehydration of the extracellular space.
Electrolytes and proteins are now concentrated in the extracellular space, creating an osmotic gradient. This forces water to rush out from the cells’ interiors (which haven’t yet frozen) to the extracellular space to restore the water balance. Cells shrink, crinkle, or dehydrate when the water is forced out of them, causing cracks in the membrane (solution-effect injury).
- Thawing
Ice crystals in the extracellular space melt during the subsequent thaw cycle, causing water to flow back into the dehydrated cells. This causes them to swell and eventually rupture because the wall has been weakened. Dehydrated cells shrink, their salt concentration impairs enzyme function, and their membranes are directly destabilized due to the concentration of salts. According to research, a long thaw cycle has the same killing effect as a long cold freeze cycle.
Intracellular ice crystal formation, important for tumor destruction, is most prominent during rapid freezing and during the thawing phase when temperatures reach -20 to -25°C.
- MECHANICAL INJURY:
Mechanical injury takes place at temperatures as low as -20°C a little further out from the cryoprobe tip center. Mechanical damage to the cell membrane leads to permanent dysfunction of the cell.
Sustained freezing during a freeze cycle causes the formation of larger ice crystals to form inside cells. This causes direct damage to intracellular organelles such as nuclei containing DNA and mitochondria that create energy. This damages the intracellular skeletons that maintain cell shape and induces pore formation in the plasma membranes, resulting in permanent dysfunction of the cellular transport systems and leakage of cellular components.
Each subsequent freezing cycle increases the efficiency of freezing, leading to a progressively larger cryoablation zone. At -20 to -25°C, when rapid freezing and thawing occur, intracellular ice crystals are most prominent.
- VASCULAR INJURY:
In order for cancer cells to survive, they need blood vessels to deliver oxygen, supply nutrients and eliminate waste. Cancer cells are killed in a similar way during cryoablation when small and medium-sized vessels are destroyed in and around the tumor.
During freezing, at −20 to −10 °C, intracellular ice crystal formation damages cells lining the blood vessels. This causes vasoconstriction and blood clot formation in the blood vessels feeding the tumor, depriving the tumor of oxygen and nutrients.
During thawing, blood flow restores around the cryoablation zone releasing chemicals (free radicals). These re-injure the blood vessel lining and cause further blood clot formation (blood platelet aggregation and thrombi formation). This leads to accidental cell death by blood/oxygen restriction.
Near the outer edge of the iceball, where temperatures may be sublethal, vascular injury is one of the major causes of tissue necrosis.
- APOPTOTIC CELL DEATH OR PROGRAMMED CELL DEATH:
The outer portion of the ice ball does not reach such low, direct tumor killing temperatures (-6 to -10 °C) as the center portion. However, it is still possible to activate enzymes within cancer cells that can destroy intracellular proteins and DNA at “warmer” sub-lethal temperatures, such as -6 to -10 °C. Known as apoptosis or programmed cell death, tumor cells essentially commit suicide 8-12 hours after the freezing injury.
Unlike apoptosis which causes permanent cell death, hypothermic stress leads to reversible injury. This occurs at the ice ball’s warmer (< 32°C) outer edges where cell membranes and organelles become less fluid and ion pumps lose their transport capabilities. This is an entirely transient and reversible effect.
- IMMUNOGENIC RESPONSE:
Cryoablation may have an additional role beyond tumor destruction, where a patient’s own immune system is harnessed to fight cancer. A patient’s systemic antitumor immune response is activated by abnormal tumor cell proteins:
1) T-cells bind to abnormal tumor proteins and target them for attack by antibodies (and possible immunity) derived from granulocytes, monocytes, and macrophages.
2) T-cells are directly stimulated by antigen-presenting cells such as dendritic cells and macrophages to attack abnormal tumor proteins.
Another mechanism of action for immunogenic injury occurs by stopping the release of “checkpoint” proteins. These suppress the immune response so the body can now recognize and attack the remaining cancer cells.
Immune checkpoint inhibition involves cancer cells releasing proteins called “checkpoints” that suppress the immune system. Through cryoablation, cancer cells are prevented from releasing immune-suppressive “checkpoint” proteins. As a result, the immune system is able to recognize and attack the remaining cancer cells. Investigating cryoablation with immunotherapy is a new and attractive therapeutic approach to prevent or cure metastasis.
The success of cellular injury from cryoablation
The extent and success of cellular injury or ablation of the tumor by cryoablation can be influenced by four factors: cooling rate, target temperature, time at the target temperature, and thawing rate.
As described above, different mechanisms of cell injury occur at different temperatures and have different effects.
The choice of cryoablation system for a procedure is therefore highly important in providing rapid, accurate, and consistent cooling rates and temperatures.
A faster rate of cooling results in a higher intracellular water content before freezing, which will maximize the intracellular ice crystal formation adjacent to the probe tip during osmotic and mechanical injury. Furthermore, increasing the time at the target temperature will increase the likelihood that cells in the peripheral portion of the ice ball will have sufficient time and temperature for lethal intracellular ice to form.
Finally, the rate of thawing also plays a role. Rapid thawing can increase the chance of cell survival by limiting the size of intracellular ice crystals. This idea is supported by studies[1] that show a greater degree of cell death with passive thawing than with active thawing and that repeated freeze-thaw cycles lead to a higher degree of tumor destruction.
Summary
To summarize how cryoablation works; Cryoablation is a safe and effective minimally invasive treatment that destroys tumors by freezing. It works by causing cellular damage, death, and necrosis of tissues through direct mechanisms, from cold-induced injury to cells, and indirect mechanisms, which affect the cellular microenvironment and impair cell viability leading to tumor death.
To learn more about how cryoablation works visit this notable website developed by Dr. Dennis Holmes, an internationally recognized breast surgeon and cancer researcher who uses cryoablation.
References
[1] Erinjeri, JP & Clark, TWI. Cryoablation: Mechanism of Action and Devices. J Vasc Interv Radiol. 2010 Aug; 21(8 Suppl): S187–S191.