Patent Application
20070149957
The inventions described below relate the field of cryosurgical systems.
Cryosurgery refers to the freezing of body tissue in order to destroy diseased tissue. Minimally invasive cryosurgical systems generally include a long, slender cryoprobe adapted for insertion into the body so that the tip resides in the diseased tissue, and source of cryogenic fluid, and the necessary tubing to conduct the cryogenic fluid into and out of the probe. These cryosurgical systems also include heating systems, so that the probes can be warmed to enhance the destructive effect of the cryoablation and to provide for quick release of the cryoprobes when ablation is complete.
Our own Visica® cryoablation system has proven effective for the treatment of lesions within the breast of female patients. The system uses Joule-Thompson cryoprobes, and uses argon gas as the cryogenic fluid. The argon gas, supplied at room temperature but very high pressure, expands and cools within the tip of the cryoprobe to generate the cooling power needed to freeze body tissue to cryogenic temperatures. The Visica® cryoablation system uses high-pressure helium flow through the cryoprobe to heat the probe. The system requires large supplies of argon gas, but is otherwise quite convenient.
Earlier cryoprobes proposed for other surgeries, such as prostrate cryosurgery, used liquid nitrogen, which has the advantage that is more readily available than argon, and the volume necessary for a given cryosurgical procedure is much smaller then argon. Cryoablation systems using liquid nitrogen, such as the Accuprobe™ cryoablation system, have been proposed and used, but these systems have been abandoned in favor of the gas Joule-Thompson systems. The literature and patent filings indicate that liquid nitrogen systems were plagued by various problems, such as vapor lock and excessive consumption of liquid nitrogen. Proposals to solve these problems, though never successfully implemented, include various schemes to prevent vapor lock and maximize efficiency of the heat exchange. See Rubinsky, et al., Cryosurgical System For Destroying Tumors By Freezing, U.S. Pat. No. 5,334,181 (Aug. 2, 1994) and Rubinsky, et al., Cryosurgical Instrument And System And Method Of Cryosurgery, U.S. Pat. No. 5,674,218 (Oct. 7, 1997), and Littrup, et al., Cryotherapy Probe and System, PCT Pub. WO 2004/064914 (Aug. 5, 2004).
To date, the problems inherent in liquid nitrogen systems have led the art to avoid them in favor of gaseous argon systems.
The devices and methods described below provide for use of liquid nitrogen in cryoablation systems while preventing the vapor lock typically associated with those systems, and minimizing the amount of liquid nitrogen used in a given procedure. The system uses cryoprobes of coaxial structure, and is supplied with cryogen from a dewar of liquid nitrogen. The system includes various enhancements to avoid heat transfer from the liquid nitrogen to the system components, and as a result permits use of very low-pressure nitrogen, and, vice-versa, the use of low pressure nitrogen permits use of the various enhancements (which could not be used in a high pressure system). The result is a system that provides sufficient cooling power to effectively ablate lesions, tumors and masses within the breast of female patients while using very little nitrogen and a compact and inexpensive system based on readily available and easy to handle liquid nitrogen.
The system includes a low-pressure liquid nitrogen supply, which requires only 0.5 to 1 bar of pressure to provide adequate cooling power for treatment of typical breast lesions. The pressure may be provided by supplying lightly pressurized air into the dewar, by heating a small portion of the nitrogen in the dewar, or with a small low pressure pump. The use of low pressure liquid nitrogen permits use of polymers for several components, such as the supply hose, the cryoprobe inlet tube, and various hose connectors which are typically made of metal, so that the system is much more efficient and uses very little liquid nitrogen. Additionally, because the liquid nitrogen is lightly pressurized, the boiling point remains low, and the liquid temperature also remains low compared with higher pressure systems.
The cryoprobe 2 comprises an inlet tube 7, a closed-ended outer tube 8, and a handle portion 9. The inlet tube comprises a small diameter tube, and the outer tube comprises a closed end tube, disposed coaxially about the inlet tube. The inlet tube is preferably a rigid tube with low thermal conductivity, such as polyetheretherketone (PEEK, which is well know for its high temperature performance), fluorinated ethylene propylene (FEP) or polytetrafluoroethylene. The cryoprobe preferably includes the flow-directing coil 10 disposed coaxially between the inlet tube and the outer tube at the distal end of the cryoprobe. The flow directing coil serves to direct flow onto the inner surface of the outer tube, thereby enhancing heat transfer from the outer tube that the cryogen fluid stream. The cryoprobe is describe in detail in our co-pending application, DeLonzor, et al., Cryoprobe For Low Pressure Systems, Attorney Docket No. 212/801 filed Dec. 23, 2005, the entirety of which is hereby incorporated by reference. The cryoprobe is supplied with cryogen through the supply hose 11, described in detail below, the control valve 5, and the dewar outlet hose 12. When used in the current system, with low-pressure liquid nitrogen, the cryoprobe having an inlet tube of 1 mm inner diameter and 1.6 mm outer diameter, and an outer tube with 2.4 mm inner diameter and 2.7 mm outer diameter works well. Probes with outer diameters of up to 4 mm and down to 1.5 mm
The cryogen source is preferable a dewar of liquid nitrogen. The dewar may comprise a composite material of low thermal conductivity, and is preferably fitted with a low pressure relief valve set to lift at about 20 to 30 psi. The dewar is lightly pressurized, in the range of about 0.5 to 1 bar over ambient pressure (about 7.25 to 14.5 psi), with air or other suitable gas, through compressor 13. Any other means of pressurizing the liquid nitrogen may be used, including use of a pump at the outlet of the dewar, heating a small portion of the liquid nitrogen or gaseous nitrogen in the dewar to boost pressure in the dewar, or heating the liquid nitrogen at the exit of the dewar.
The supply hose 11, illustrated in cross section in
In use, the cryoprobe is inserted into the body, with its distal tip within a lesion or other diseased tissue that is to be ablated, the surgeon will operate the systems through controls on the control system 6. The dewar is pressurized to about 0.5 to 1 bar (about 7.25 to 14.5 psi). The control valve is operated to provide flow to the cryoprobe at about 0.5 to 2 grams per second to effect cryoablation of the lesion. The flow of cryogen is continued as necessary to freeze the lesion to cryogenic temperatures. Preferably the operation of the system is controlled automatically via the control system, though it may be implemented manually by a surgeon, including manual operation pressurizing means of the dewar and manual operation of the control valve. When used to treat lesions in the breast, the system may be operated according to the parameters described in our U.S. Pat. No. 6,789,545.
The systems described above may be employed with various liquid cryogens, though liquid nitrogen is favored for is universal availability and ease of use. Also, though system has been developed for use in treatment of breast disease, it may be employed to treat lesions elsewhere in the body. Thus, while the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. Other embodiments and configurations may be devised without departing from the spirit of the inventions and the scope of the appended claims.
