Surgery – Instruments – Electrical application
Reexamination Certificate
2001-11-13
2004-02-03
Gibson, Roy D. (Department: 3739)
Surgery
Instruments
Electrical application
C606S049000, C606S027000, C607S101000
Reexamination Certificate
active
06685700
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the use of radio frequency energy for heating and necrosing solid tissue. More particularly, the present invention relates to a control method and system for delivering radio frequency current to the tissue through an electrode or electrode array disposed within the tissue.
The delivery of radio frequency energy to target regions within solid tissue is known for a variety of purposes. Of particular interest to the present invention, radio frequency energy may be delivered to diseased regions in target tissue for the purpose of tissue heating and/or necrosis, referred to generally as hyperthermia. For example, the liver is a common depository for metastases of many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. Electrosurgical probes for deploying single and multiple electrodes have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. See, for example, the references cited in the Description of the Background Art hereinafter.
A primary goal in such hyperthermic treatments, particularly those intended for tumor treatment, is the complete, thorough, and uniform heating of the target tissue mass. Uniform heating of the tissue mass, however, can be difficult to achieve, particularly in highly vascularized tissues where variability in local blood flow can have a significant effect on the heating characteristics of the tissue. For example, creation of a lesion having a selected volume in some highly perfused tissue locations may require twice as much power as an identically-sized lesion in less highly perfused locations. While a variety of approaches for achieving such complete, thorough, and uniform heating of tissue have been proposed, most such approaches are somewhat complex and require the use of electrodes which are capable of measuring temperature, impedance, or the like. See, for example, the radio frequency power supply described in published PCT Application WO 93/08757. In general, many approaches for achieving uniform tissue heating have relied on slow, gradual heating of the tissue in order to avoid the formation of charred or otherwise necrosed, high radio frequency impedance regions within the target tissue mass. Such approaches, however, are complex, can result in an undesirable prolongation of the treatment, and are not always successful,
For these reasons, it would be desirable to provide improved treatment methods, systems, and apparatus which allow for effective and efficient delivery of a radio frequency energy to solid tissue masses using electrodes. In particular, it would be desirable to provide such methods, systems, and apparatus which are useful with many or all tissue-penetrating electrode systems which are now available or which might become available in the future. The methods, systems, and apparatus should be simple to implement and use, and should preferably reduce the complexity, cost, and treatment time required to achieve complete heating and/or necrosis of the target tissue mass. The methods, systems, and apparatus should preferably require no information or feedback from the tissue region being treated, other than information which can be acquired from the power delivery characteristics which can be monitored in the radio frequency power delivery system itself. In particular, the methods, systems, and apparatus should be able to operate solely by monitoring the power and/or current delivery characteristics of the radio frequency energy into an electrode system present in the target tissue. At least some of these objective will be met by the present invention as claimed hereinafter.
2. Description of the Background Art
The heating of solid tissue with radio frequency current using the preferred electrode structures of the present invention is described in WO 96/29946 and co-pending Applications Ser. Nos. 08/410,344; 08/559,072; 08/766,154; 08/764,085; and 08/858,414, filed on May 19, 1997, the full disclosures of which are incorporated herein by reference.
WO 97/06739; WO 97/06740; WO 97/06855; and WO 97/06857 describe RF treatment electrodes and note that power delivery can “impede out” if levels are raised too high.
Assignee of the present application has developed a radio frequency power supply (Model RF-2000, Radio Therapeutics Corporation, Mountain View, Calif.) which provides power levels up to 100 W and is intended for the coagulation (ablation) of soft tissue. The power supply is controlled by a programmable microprocessor which is capable of continuously monitoring power delivered to an electrode system.
Patents and published applications describing radio frequency tissue ablation using electrodes having various configurations include U.S. Pat. Nos. 5,662,680; 5,599,346; 5,599,345; 5,562,703; 5,536,267; 5,489,161; 5,472,441; and 5,458,597; and published International Applications WO 97/06857; WO 97/06855; WO 97/06740; WO 97/06739; WO 96/04860; and WO 95/13113.
A radio frequency power supply having impedance monitoring capability is described in WO 93/08757.
Other radio frequency power apparatus and methods are described in U.S. Pat. Nos. 5,556,396; 5,514,129; 5,496,312; 5,437,664; and 5,370,645; and WO 95/20360, WO 95/09577, and WO 95/20360.
BRIEF SUMMARY OF THE INVENTION
The present invention provides improved methods, systems, and apparatus for delivering radio frequency energy to electrodes disposed in tissue for inducing hyperthermia and other purposes. It has been found that the delivery of radio frequency power to electrode(s) disposed in tissue can, if the power is delivered for a sufficient time and/or at a sufficient power delivery level or flux, result in an abrupt increase in the electrical impedance between the electrode(s) and tissue. While such an abrupt increase in impedance is undesirable since it results in an immediate fall-off of energy delivery (for a voltage limited radio frequency power source), the present invention relies on the occurrence of the abrupt reduction in power delivery (which may be observed as a reduction in current delivery to the electrodes) to provide information about the heat capacity and heat delivery characteristics of the local target tissue region. The present invention uses such information to control subsequent delivery of energy to the target tissue region using the same electrode(s).
The present invention still further depends, in least in part, on the observation that the abrupt rise in the electrode-tissue interface impedance diminishes very rapidly when the power delivery is stopped, typically disappearing within several seconds. Delivery of the radio frequency power can be resumed after the impedance has diminished, typically to impedance levels substantially equal to those observed prior to the abrupt increase. Based on these observations, it is possible to determine improved or optimized radio frequency power delivery levels and protocols based on the power levels and/or time periods required to induce the abrupt impedance increases and associated power declines in specific target locations. In particular, the protocols rely on appropriate adjustments to the power levels which are resumed after the tissue impedance diminishes.
It is presently believed that the abrupt increase in electrode-tissue interface impedance results from the formation of a thin gaseous layer over the electrode surface, apparently resulting from vaporization of water within the tissue as the temperature approaches the local boiling point. Surprisingly, the thin gaseous layer appears to spread from an initial nucleation site to cover most or all of an electrode surface in a very short time period, typically less than 30 seconds, resulting in an increase in electrode-tissue interface impedance which is very large when compared to the total system impedance prior to formation of the thin gaseous layer. In the exemplary systems described herein after, typical system impedance prior to formation of the thin gaseous layer will be in the range from
Behl Robert S.
Kermit Eben L.
Lyons Peter F.
Nichols Colin J.
Bingham & McCutchen LLP
Gibson Roy D.
RadioTherapeutics Corporation
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