Any living tissue -- healthy or unhealthy -- cannot tolerate extreme cold. Infusing nitrogen or argon gas into the prostate gland rapidly extracts heat from the gland. As heat is drawn out, there is an instantaneous swell of ice crystals or ice balls. This results in the rupture of cell membranes. That's followed by tissue damage and, ultimately, cell death.
After the cancer cells are destroyed, white blood cells clean up the dead cells and tissue. Some studies show that during this process, the immune system strikes out and attacks cancer cells that still remain.