| For adequate
destruction of tumor tissue, the entire volume of a lesion must be subjected
to cytotoxic temperatures. Hence, effective heating throughout the target
volume (i.e., the tumor and 5- to 10-mm thickness of normal tissue) is required.
Thus, an essential objective of ablative therapy is achievement and maintenance
of a 50°-100°C temperature throughout the entire target volume for at least
4-6 minutes. However, the relatively slow thermal conduction from the electrode
surface through the tissues increases the duration of application to 10-30
minutes. Tissues cannot be heated to greater than 100°-110°C without vaporizing,
and this process produces significant gas that both serves as an insulator
and retards the ability to effectively establish a radio-frequency field.
The vaporization or carbonization around the electrode tip can also retard
heat conduction within the tissue, often in an asymmetric fashion. This
process coupled with the rapid decrease in heating at a distance from the
electrode essentially limits the extent of induced coagulation to no greater
than 1.6 cm in diameter. Many investigators have explored and several corporations
have manufactured new radio-frequency ablation devices based on technologic
advances that increase heating efficacy. To accomplish this increase, the
radio-frequency output of all commercially available generators has been
increased to more than 150 watts, which may potentially increase the intensity
of the radio-frequency current deposited at the tissue. Expandable electrodes
permit the deposition of this energy over a larger volume. In addition,
this design decreases the distance between the tissue and the electrode,
thereby ensuring more uniform heating that relies less on heat conduction
over a large distance. Alternate strategies to increase the energy deposited
from a single radio-frequency electrode have also been developed. The internally
cooled electrode design minimizes carbonization and gas formation around
the needle tip by eliminating excess heat near the electrode. The removal
of this heat by a "heat-sink" effect of flowing fluid in the electrode permits
increased energy deposition and deeper tissue heating. Preferential cooling
of the tissues near the electrode allows heating of tissues farther from
the electrode when high radio-frequency current is being applied. A combined
approach that involves use of a multiprobe cluster of internally cooled
electrodes with pulsing has also been described with the claim of even greater
coagulation than that achieved by any of the individual methods alone. Viewed
in total, these technologic developments can be used to create an ablation
lesion with a maximum diameter of 5.0 cm. We are using a new commercially
available device using a single probe system with continuous infusion of
saline without exceeding a 110°C maximum temperature threshold (60W, BerchtoldŽ/
Germany). The continuous infusion of saline at the tip of the needle allows
to increase heat and electrical conductivity. The BerchtoldŽ radio frequency
device relies on an electrical measurement of tissue impedance to determine
that tissue boiling is taking place. The impedance rises can be detected
by the generator, which can then reduce the current output and increase
the saline flow. Injection of NaCl solution during RF ablation can increase
energy deposition, tissue heating, and induced coagulation. Radio-frequency
ablation is limited, however. With currently available devices, the largest
focus of necrosis that can be induced with a single application is approximately
4-5 cm in greatest diameter. Thus, the diameter of suitable lesions must
be less than 3-4 cm. Other limitations are the proximity of the tumors to
large vessels, which may prevent adequate heating, as well as proximity
to central bile ducts, which predisposes the patient to a risk of biliary
complications. Finally, treatment of superficially located tumors carries
a risk of injury to adjacent organs. |
Radio-frequency
device with injection of continuous saline
