Controller for array of miniature radiation sources

[0002] This application claims benefit of U.S. Provisional Application Ser. No. 60/351,809, entitled “Array of Miniature Radiation Sources” and filed on Jan. 25, 2002.

[0003] Not Applicable

[0004] Not Applicable

[0005] The present invention relates to a controller for an array of miniature therapeutic radiation sources.

[0006] In the field of medicine, radiation may be used for diagnostic, therapeutic and palliative purposes. Therapeutic use of radiation such as x-rays and γ-rays typically involves using these rays to eradicate malignant cells. Conventional radiation treatment systems used for medical treatment, such as linear accelerators that produce high-energy x-rays, utilize a remote radiation source external to the targeted tissue. A beam of radiation is directed at the target area, for example a malignant tumor inside the body of a patient. The x-rays penetrate the patient's body tissue and deliver radiation to the cancer cells, usually seated deep inside the body. This type of treatment is referred to as teletherapy because the radiation source is located at some distance from the target. This treatment suffers from the disadvantage that tissue disposed between the radiation source and the target is exposed to radiation. To reach the cancer cells, the x-rays from an external radiation source must usually penetrate through normal surrounding tissues. Non-cancerous tissues and organs are thus also damaged by the penetrating x-ray radiation.

[0007] Brachytherapy, on the other hand, is a form of treatment in which the source of radiation is located close to, or in some cases within, the area receiving treatment. Brachytherapy, a word derived from the ancient Greek word for close (“brachy”), offers a significant advantage over teletherapy, because the radiation is applied primarily to treat only a predefined tissue volume, without significantly affecting the tissue adjacent to the treated volume. The term brachytherapy is commonly used to describe the use of “seeds,” i.e. encapsulated radioactive isotopes, which can be placed directly within or adjacent to the target tissue that is being treated. Handling and disposal of such radioisotopes, however, may impose considerable hazards to both the handling personnel and the environment. Also, introduction of the radioisotopes requires invasive procedures which have potential side-effects, such as the possibility of infection. Moreover, there is no ability to provide selective control of time dosage or radiation intensity.

[0008] The term “x-ray brachytherapy” is defined for purposes of this application as x-ray radiation treatment in which the x-ray source is located close to or within the area receiving treatment. An x-ray brachytherapy system, which utilizes a miniaturized low power radiation source that can be inserted into, and activated from within, a patient's body, is disclosed in U.S. Pat. No. 5,153,900 issued to Nomikos et al., U.S. Pat. No. 5,369,679 to Sliski et al., U.S. Pat. No. 5,422,926 to Smith et al., and U.S. Pat. No. 5,428,658 to Oettinger et al., all owned by the assignee of the present application, all of which are hereby incorporated by reference.

[0009] The x-ray brachytherapy system disclosed in the above-referenced patents includes a miniaturized, insertable probe capable of producing low power x-ray radiation while positioned within or in proximity to a predetermined region to be irradiated. In this way, x-ray radiation need not pass through the patient's skin, bone, or other tissue prior to reaching the target tissue. The probe may be fully or partially implanted into, or surface-mounted onto a desired area within a treatment region of a patient. X-rays are emitted from a nominal, or effective “point” source located within or adjacent to the desired region to be irradiated, so that substantially only the desired region is irradiated, while irradiation of other regions are minimized. X-ray brachytherapy offers the advantages of brachytherapy, while avoiding the use and handling of radioisotopes. Also, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.

[0010] X-ray brachytherapy treatment generally involves positioning the insertable probe into or adjacent to the tumor or the site where the tumor or a portion of the tumor was removed to treat the tissue adjacent the site with a local boost of radiation. X-ray probes of the type generally disclosed in U.S. Pat. No. 5,153,900 include a capsule, and a hollow, tubular probe or catheter extending from the capsule along an axis, and having an x-ray emitting target element at its distal end. The probe may enclose an electron source, such as a thermionic cathode. In one form of a thermionic cathode, a filament is resistively heated with a current. This in turn heats the cathode so that electrons are generated by thermionic emission.

[0011] In another form of an x-ray brachytherapy device, as disclosed in U.S. Pat. No. 5,428,658, an x-ray probe may include a flexible probe, such as a flexible fiber optic cable enclosed within a metallic sheath. The x-ray probe may also include a substantially rigid, evacuated capsule that is coupled to a distal end of the flexible probe. The capsule encloses an optically activated electron source, such as a photocathode, and an x-ray emissive target element. In a photocathode configuration, a photoemissive substance is irradiated by a LED or a laser source, causing the generation of free electrons. Typically, a flexible fiber optic cable couples light from a laser source or a LED to the photocathode.

[0012] U.S. Pat. No. 6,480,568 entitled “Optically Driven Therapeutic Radiation Source,” issued on Nov. 12, 2002 to Mark Dinsmore, and hereby incorporated by reference in its entirety)(hereinafter the “'568 patent”) discloses an optically driven (for example, laser driven) therapeutic radiation source using a reduced-power, increased efficiency electron source, which generates electrons with minimal heat loss. The '568 patent discloses the use of laser energy to heat an electron emissive surface of a thermionic emitter, instead of using an electric current to ohmically heat an electron emissive surface of a thermionic emitter. With the optically driven thermionic emitter, electrons can be produced in a quantity sufficient to produce the electron current necessary for generating therapeutic radiation at the target, while significantly reducing the requisite power requirements. U.S. patent application Ser. No. 10/005,290 and hereby incorporated by reference)(hereinafter the “290” application) discloses a therapeutic radiation source having an in situ radiation detector, which permits real-time monitoring of the therapeutic radiation that has been generated and delivered.

[0013] Even though the above-discussed miniature radiation sources can generate x-rays local to the target tissue, it is difficult to provide a uniform, or other desired, dose of radiation to an irregularly shaped target tissue, using these radiation sources. These miniature radiation sources generally act as point sources of therapeutic radiation. The intensity of the radiation from a point source decreases uniformly with approximately the square of the distance (R) from the source (i.e., 1/R²). Since body cavities, or the beds of resected tumors, are not generally spherically symmetrical, a point source within a body cavity or central to the resected tumor bed will not deliver a uniform dose of radiation to the tissue lining of the cavity or bed. Likewise, a point source at the center of a non-spherical tumor will not deliver radiation with an isodose contour matching the peripheral surface of the tumor.

[0014] The organs or body cavities being treated during radiation therapy usually have arbitrary and irregular shapes and geometries. The areas of a patient's body requiring treatment may be characterized by twists and bends. In some cases, the geometry of the target region may not be fixed, as in the bladder for example, which has a flexible inner wall without a well-defined shape. Also, some treatment procedures may require delivery of localized radiation to portions of the human body that are not easily accessible. Cancerous tumors are usually shaped irregularly, and are distributed randomly across a given anatomical region.

[0015] A single point source of therapeutic radiation, even when inserted into and activated within a patient's body, cannot deliver a uniform dose of radiation to a desired area within an irregularly shaped body cavity or organ, nor can it deliver more complex radiation dose patterns that may be desirable or required for some cases. Similarly, a single point source at the center of a non-spherical tumor will not deliver radiation with an isodose contour matching the peripheral surface of the tumor, as discussed earlier.

[0016] U.S. Pat. No. 6,556,651 (entitled “Array of Miniature Radiation Sources,” issued on Apr. 29, 2003 to Euan Thomson and Mark Dinsmore, and hereby incorporated by reference in its entirety)(hereinafter the '651 patent), discloses a system for delivering therapeutic radiation that includes a plurality of point-like sources that are arranged over the desired treatment region as a one- or multi-dimensional array. A plurality of point radiation sources permits a more effective and versatile delivery of radiation over irregularly shaped treatment regions, as compared to a single point source of therapeutic radiation.

[0017] In order for a surgeon or other radiotherapy to use such an array of sources to build up complex, multi-dimensional dose patterns that he believes appropriate for the conditions of his patient, a controller is needed that can manipulate and selectively drive the array. There is a need for a controller that can control the intensity and duration of the therapeutic radiation emitted from each individual therapeutic radiation source in the array. The physician or other operator of the array of sources must be able to independently vary, for each therapeutic radiation source, parameters such as the turn-on time of the laser source, turn-on time of the high voltage source, the magnitude of the accelerating voltage provided to the electrons, and the magnitude of the electron beam current formed by the accelerated electrons.

[0018] The present invention is directed to a controller for an array of therapeutic radiation sources, for example x-ray sources. Each therapeutic radiation source includes an electron source and an associated target element. The controller allows each therapeutic radiation source to be selectively operated, so as to generate therapeutic radiation at selected time intervals and at selected intensities. The controller includes intensity control circuitry for independently controlling the intensity of the therapeutic radiation generated by each therapeutic radiation source. The controller also includes duration control circuitry for independently controlling the duration of the therapeutic radiation generated by each radiation source. The controller may also include means for controlling the position of each radiation source within the treatment region, for example a mechanical introducer for inserting the therapeutic radiation sources into a treatment region and withdrawing the radiation sources from the treatment region.

[0019] In one embodiment, each therapeutic radiation source is coupled to an associated fiber optical cable, so that the array of therapeutic radiation sources includes a plurality of fiber optical cables and a corresponding plurality of therapeutic radiation sources. Alternatively, a single fiber optical cable can be used, having one originating end and a plurality of terminating ends. Light is generated by an optical source and transmitted through a fiber optical cable, and impinges upon the electron source, causing emission of electrons. An accelerating voltage is provided between each electron source and each associated target element. The target element emits therapeutic radiation in response to incident accelerated electrons from the electron source. The generated electrons may form an electron beam along a beam path.

[0020] The intensity control circuitry may include programmable means for user-controlled adjustment of the amplitude of the accelerating voltage, or the magnitude of the current formed by the electron beam. The duration control circuitry may include means for selectively activating a high voltage power supply that provides the accelerating voltage between each electron source and its associated target element. The duration control circuitry may also include means for selectively activating the optical source.

[0021] The mechanical assembly may be an introducer for controllably effecting a reversible displacement of a linear or planar array of therapeutic radiation sources, into and out of a desired treatment region. Alternatively, the mechanical assembly may be an introducer that allows independent movement of each therapeutic radiation source with respect to each other.

[0022] The controller may regulate an in situ radiation detecting system that may be provided for monitoring in real time an amount of the therapeutic radiation emitted by each therapeutic radiation source. The controller may also regulate an image-guided surgery system that can generate in real time a visual image representing a cumulative dose of radiation delivered by each therapeutic radiation source to desired locations within the treatment region.

[0023] The present invention features a method for treating an anatomical region in a patient. The method includes positioning an array of x-ray sources near the anatomical region. For example, the array of x-ray sources may be inserted into a body passageway, such as a blood vessel, and the array may be guided through the body passageway so that each x-ray source is positioned at a desired location with respect to the anatomical region. The method includes selectively activating one or more of the x-ray sources in the array, so as to irradiate the anatomical region according to a desired and predetermined irradiation profile.

[0024] FIG. 1(a) is a schematic illustration of one embodiment of a controller for an array of therapeutic radiation sources, constructed according to the present invention.

[0025] FIG. 1(b) is a schematic illustration of one embodiment of a controller for an array of therapeutic radiation sources, constructed according to the present invention, which includes a plurality of fiber optical cables and a corresponding plurality of therapeutic radiation sources.

[0026] FIG. 2(a) illustrates an x-ray spectrum for a tungsten target that includes a continuous bremsstrahlung spectrum as well as the characteristic spectral lines for tungsten.

[0027] FIG. 2(b) illustrates the continuous bremsstrahlung x-ray spectrum emitted from a tungsten target for two different values of incident electron energy.

[0028] FIG. 3 illustrates one exemplary embodiment of the intensity control circuitry, in accord with the present invention.

[0029] FIG. 4 is a schematic illustration of one embodiment of a controller for an array of therapeutic radiation sources in accordance with the present invention, in which the controller regulates an in situ radiation detecting system and an image-guided surgery system for the array of therapeutic radiation sources.

[0030] In overview, the present invention provides a controller for a therapeutic radiation system, formed of a plurality of miniaturized radiation sources that are strung together into an array. The controller enables the operator of the array to control the intensity of the therapeutic radiation to which a given area within the target tissue is exposed. The controller is also operable to control the length of time a given area within the target tissue is exposed to therapeutic radiation. The controller may also be operable to control the location of the radiation source within the target area that is receiving therapeutic radiation. For example, the controller may include one or more mechanical introducer assemblies for inserting the sources within, and withdrawing the sources from, the desired treatment region.

[0031] In a preferred embodiment of the invention, each therapeutic radiation source in the array is an electron-beam activated x-ray source which operates at relatively low voltages, i.e. approximately in the range of about 10 kV to about 90 kV, and relatively small electron beam currents, i.e. approximately in the range of about 1 nA to about 1 mA. At those operating voltages and currents, the x-ray output is relatively low, so that each therapeutic radiation source can be made small enough to be adapted for implantation in medical applications. In an embodiment in which the array of radiation sources is used for intravascular therapy, the x-ray source is preferably operated in the range of approximately 30 kV to about 40 kV.

[0032] In view of the low level x-ray output, adequate tissue penetration and cumulative dosage may be attained by locating the x-ray source adjacent to or within the region to be irradiated. Each miniature x-ray source may selectively apply a relatively low dose rate of x-rays to any part of a tumor, at any desired intensity level dictated by the clinical conditions of the region being treated. The x-rays may be applied for any desired time interval, either continually or periodically.

[0033] FIG. 1(a) is a schematic illustration of one embodiment of a controller 140 for a system 100 for delivering therapeutic radiation, constructed according to the present invention. The system 100 includes a plurality of therapeutic radiation sources 20₁, 20₂, . . . , 20_i, . . . , 20_n. In the illustrated embodiment, the plurality of therapeutic radiation sources are shown as being arranged in a one-dimensional array, for simplicity. However, multi-dimensional arrays (i.e. planar, two-dimensional arrays, or higher-order arrays) are also within the scope of the present invention, as discussed in the '651 application. In the illustrated embodiment, the therapeutic radiation sources 20_nare x-ray sources, however, other types of therapeutic radiation sources are also within the scope of this invention.

[0034] In the illustrated embodiment, each x-ray source 20_icomprises an electron source 50₁, and an associated target element 60_i. Preferably, the electron source 50_iis a cathode. In particular, the cathode 50_imay be a resistively heated thermionic cathode, or a laser-heated thermionic cathode which emits electrons when heated to a sufficient temperature. Alternatively, the cathode 50₁may be a photocathode, which emits electrons by the photoelectric effect in response to an incident light beam. Alternatively, the cathode 50_imay be a cold cathode.

[0035] The system 100 includes means for providing an accelerating electric field which acts to accelerate electrons emitted from the electron source 50_itoward the target element 60_i. The accelerating voltage may be provided by a high voltage power supply 80. In the illustrated embodiment, a single high voltage conductive cable 90 provides a connection to each electron source 50_ifrom the high voltage power supply 80. The single high voltage conductive cable 90 is tapped at different locations along the array, so as to connect the cathode 50_iin each x-ray source 20_ito the high voltage power supply 80. The outer flexible tube 14 couples a ground return from each target element 60_ito the high voltage power supply 80, thereby establishing a high voltage field between each cathode 50_iand each associated target element 60_i. An insulator (not shown) provides insulation between the high voltage conductive cable 80 and the grounded flexible tube 14.

[0036] The controller 140 allows a surgeon or other radiation therapy professional to selectively operate each individual therapeutic radiation source 20₁, so as to generate therapeutic radiation at selected time intervals and at selected intensities. The controller 140 includes intensity control circuitry 142 for independently controlling the intensity of the emitted x-rays from each individual therapeutic radiation source 20_i, and duration control circuitry 144 for independently controlling the duration of the emission of the x-rays from each individual therapeutic radiation source 20_i. In the illustrated embodiment, the controller 140 also includes a mechanical introducer 95 for appropriately positioning the array of radiation sources within a body passageway, and for guiding the array through the passageway.

[0037] FIG. 1(b) is a schematic illustration of another embodiment of the invention, in which the controller 140 is adapted for use with an array that includes a plurality of fiber optic cables 110₁, . . . , 110_i, . . . , 110_n, and a corresponding plurality of therapeutic radiation sources 20₁, . . . , 20_i, . . . , 20_n. Each fiber optical cable 110_ihas an originating end 11_i^aand a terminating end 110_i^b. Each x-ray source 20_iis coupled to the terminating end 110_i^bof an associated fiber optical cable 110_i. A laser source 30 generates a laser beam directed to the originating end 110_i^aof each fiber optical cable 110_i. Each fiber optical cable 110_itransmits optical radiation that is incident on the originating end 110_i^aof the fiber optical cable 110_ito the terminating end 110_i^bthereof. The electron source 50_iis responsive to incident laser light from the terminating end 110_i^bof the associated fiber optical cable 110_ito emit electrons.

[0038] In the embodiment illustrated in FIG. 1(b), the electron source 50_imay be a laser-heated thermionic cathode having an electron emissive surface. Electrons are emitted from the electron emissive surface of the thermionic cathode 50_iinto the surrounding vacuum, when the surface is heated to a sufficient temperature. The thermionic cathode 50_imay be formed of a metallic material, including tungsten, thoriated tungsten, other tungsten alloys, and tantalum. The thermionic cathode 50_imay also be an oxide coated cathode.

[0039] In one form of the invention (not shown), a single optical fiber may be used to transmit light from the laser source 30 to the plurality of electron sources. Using fiber splicing techniques known in the art, an optical fiber having a single originating end and a plurality of terminating ends can be constructed to be used in the present invention.

[0040] Referring to the elements in FIGS. 1(a) and 1(b) in more detail, the target element 60_iincludes an x-ray emissive element, for example a high atomic number element such as gold (Au) or tungsten (W), that emits x-rays in response to incident accelerated electrons from the electron beam. In the illustrated embodiment, the electrons emitted from the cathode 50_igenerate an electron beam 70_ialong a beam path. The associated target element 60_iis positioned in the electron beam path.

[0041] Each x-ray source 20_imay include a shell or capsule 64_iwhich encloses the cathode 50_iand the target element 60_i. According to one embodiment, the capsule 64_imay be rigid in nature, and generally cylindrical in shape. In this embodiment, the cylindrical capsule 64_ienclosing the x-ray source 20_ican be considered to provide a substantially rigid housing for the cathode 50_ias well as for the target element 60_i. The capsule 64₁defines a substantially evacuated interior region along a reference axis, between the cathode 50₁at an input end of the capsule 64₁and an x-ray transmissive window at an output end of the capsule. The material forming the capsule 64_imust therefore be amenable to vacuum-tight connections. The inner surface of the capsule 64_iis lined with an electrical insulator, while the external surface of the capsule 64_iis electrically conductive.

[0042] The target element 60_iis preferably spaced apart from and opposite the electron emissive surface of the cathode 50_i. In one embodiment, the target element 60_iis a small beryllium (Be) substrate, coated on the side exposed to the incident electron beam with a thin film or layer of a high-Z material, such as tungsten (W), uranium (U) or gold (Au). As the atomic number of the x-ray emissive material increases, the peak output in the spectral distribution curve of the emitted x-rays, and the characteristic spectral lines of the x-rays, shift to higher energies. The efficiency of x-ray generation is highly dependent on the acceleration voltage provided by the high voltage power supply 80, although independent of the electron beam current. By way of example, when the electrons are accelerated to 30 keV-, a 2.2 micron thick tungsten layer absorbs substantially all of the incident electrons, while transmitting approximately 95% of any 30 keV-, 88% of any 20 keV-, and 83% of any 10 keV x-rays generated in that layer. In this embodiment, the beryllium substrate 60_iis 0.5 mm thick. With this configuration, 95% of the x-rays generated in directions normal to and toward the target element 60_i, and having passed through the tungsten layer, are then transmitted through the beryllium substrate. The x-rays are then directed outward through the x-ray transmissive window of the housing onto a desired region-to-be-treated.

[0043] The controller 140 allows each individual x-ray source 20_ito be individually controllable. The intensity control circuitry 142 allows user-controlled adjustment of the intensity of the x-ray radiation, and the duration control circuitry 144 allows user-controlled adjustment of the duration of the x-ray radiation. The intensity of the x-rays emitted from a particular x-ray source 20₁may be controlled by adjusting either 1) the magnitude of the accelerating voltage provided between the cathode 50_iand the target 60_iin the x-ray source 20₁; or 2) the intensity of the electron beam current formed by the electrons emitted from the cathode 50_iin the x-ray source 20_i.

[0044] The intensity of the x-rays emitted from an x-ray source 20_iis directly proportional to the intensity of the electron beam current in the x-ray source 20_i, because x-rays are produced when incident electrons from the cathode 50_i, interacting with the target nuclei, are decelerated and eventually brought to rest. An electron having some initial kinetic energy (provided by the accelerating voltage between the cathode 50₁and the target element 60_i) is decelerated during an encounter with a heavy target nucleus, because of the Coulomb interaction between the electron and the nucleus. The energy lost by the electron appears as x-ray photons, in the form of bremsstrahlung (“braking”) radiation. The energies of these x-ray photons correspond to the energy lost by the colliding electron. Increasing the electron beam current therefore results in a directly proportionate increase in x-ray emission. An electron in the incident beam typically loses different amounts of energy in many such encounters with the target nucleus, before the electron is finally brought to rest. The x-rays produced by numerous electrons making a large number of different encounters therefore produce many discrete photons having continuously varying wavelengths, i.e. a continuous bremsstrahlung spectrum is produced.

[0045] FIG. 2(a) illustrates an x-ray spectrum for a tungsten target that includes the continuous bremsstrahlung spectrum. FIG. 2(a) also includes the characteristic spectral lines for tungsten, in addition to the bremsstrahlung spectrum. These discrete spectral lines are characteristic of the transitions between bound electron energy levels of tungsten, as allowed by the selection rules. When an incident electron passes near an electron in an inner subshell of a target atom, the electron sometimes imparts enough energy to the inner subshell electron so as to remove it from its very negative energy level and eject it from the target atom. The highly excited target atom emits a set of high frequency photons, which produce the characteristic x-ray line spectrum of the atom forming the target material, in this case tungsten.

[0046] FIG. 2(b) illustrates the continuous bremsstrahlung x-ray spectrum emitted from a tungsten target for four different values of incident electron energy. While increasing the electron beam current results in a directly proportional increase in x-ray emission at all spectral ranges, a change in the acceleration voltage between the cathode 50₁and the target 60_iresults in a total x-ray output variation proportional to the square of the voltage, since the kinetic energies of the electrons are proportional to the square of the accelerating voltage. A corresponding shift in the peak x-ray photon energy is also observed, as illustrated in FIG. 2(b).

[0047] In the present invention, the intensity control circuitry 142 permits a user to vary the intensity of the x rays emitted by each x-ray source 20_ias desired, by adjusting the accelerating voltage provided to each source 20_i, and/or the electron beam current generated in each source 20_i. The duration control circuitry 144 permits a user to regulate the time intervals during which the accelerating voltage and the laser radiation is provided to each x-ray source 20_i.

[0048] FIG. 3 illustrates in more detail one possible embodiment of the intensity control circuitry 142 for controlling the magnitude of the accelerating voltage, in accord with the present invention. As illustrated in FIG. 3, the intensity control circuitry 142 includes a switching converter and regulator 200, and a high voltage transformer 210. The power supply 80 includes a high voltage multiplier 220. The transformer 210 is coupled to a control voltage terminal 220a,which also serves as a high voltage multiplier input terminal. The voltage multiplier 220 and establishes an output voltage at a high voltage terminal 220a.In one embodiment, the voltage multiplier 220 may include a set of series connected diodes and sets of series coupled capacitors (not shown).

[0049] A voltage sensor 230 senses the voltage level at the high voltage terminal 220a,and a voltage feedback signal representing the sensed voltage level is sent on line 225 to the regulator 200. The switching converter and regulator 200 establishes a high voltage amplitude feedback loop, in response to a detected difference between the voltage feedback signal on line 225, and an applied high voltage control signal on line 226. The high voltage control signal may be selectively controlled to establish a desired variation of the amplitude of the potential at the high voltage terminal 220a.The output voltage at the high voltage terminal 220amay be controlled by varying the density of the constant energy pulses from the power transformer 210. The output voltage can be electrically compared to the control signal from line 226, and, if the output voltage is too low, the frequency of the energy pulses can be increased. The high voltage terminal 220ais connected to the high voltage conducting cable 90, which provides the regulated output voltage to each cathode 50_i.

[0050] The intensity of the electron beam current generated by a laser-heated thermionic cathode or a photocathode depends, inter alia, on the intensity of the laser beam that impinges upon the cathode 50_i. The intensity control circuitry 142 may thus include a current sensor 240 coupled to each x-ray source 20_i, and a laser driver 250 for the laser source 30. The current sensor 240 measures the electron beam current in each x-ray source 20_i. The laser driver 250 regulates the intensity of the laser beam generated by the laser source 30, so as to vary the intensity of the electron beam current generated by the cathode 50₁.

[0051] The intensity control circuitry 142 may include a difference amplifier 260 which establishes a current feedback loop by driving the laser driver in response to a detected difference between a current feedback signal from the current sensor 240 on line 245, and an applied emission control signal from the difference amplifier on line 265. The emission control signal may be selectively controlled to establish a desired variation in the x-ray tube cathode current.

[0052] In one embodiment of the invention, the laser source 30 may be a pulsed laser source, capable of generating sequential pulses of laser light. In this embodiment, the intensity control circuitry 142 may include means for increasing or decreasing the frequency of the pulses of laser light.

[0053] The duration control circuitry 144 controls the duration of the emission of the x-rays. The duration control circuitry 144 may include switching means 146 coupled to the high voltage power supply 80. The switching means 146 may selectively turn on and off the voltage provided by the power supply to any desired one of the plurality of x-ray sources, thereby controlling the time interval during which the accelerating voltage is provided to each x-ray source 20_i. The duration control circuitry 144 may also include means (not shown) for selectively activating the laser source 30, i.e. means for selectively turning on or off the laser source 30, thereby effectively turning on and off one or more of the plurality of laser-driven x-ray sources 20_i. In one embodiment of the invention, the duration control circuitry 144 may include an optical switching network (not shown) operable to selectively activate and de-activate each of the plurality of x-ray sources 20_iby selectively connecting and disconnecting each x-ray source 20_ifrom the laser source 30.

[0054] In one embodiment of the invention, the controller 140 includes a mechanical introducer assembly 95 that is operable to selectively insert and withdraw each x-ray source 20₁to and from the treatment region. Preferably, the introducer 95 controllably effects reversible displacements of the array of therapeutic radiation sources along the passageway, so as to insert the array into a treatment region, and withdraw the array from the treatment region. For example, the introducer 95 may insert a linear array of sources into a blood vessel in a patient's vascular system. The introducer 95 may include a guide member for guiding and navigating the array of x-ray sources through the blood vessel or other body passageway, and onto a desired treatment region.

[0055] Alternatively, the introducer 95 may allow each individual therapeutic radiation source to be independently inserted into, and withdrawn from, the treatment region. In this embodiment, the introducer 95 may allow selective and independent movement of the individual sources relative to each other, along the axes that define the one- or multi-dimensional array of radiation sources. Alternatively, the introducer 95 may allow a planar array of sources to be guided onto a desired treatment region, by mechanically manipulating one linear row or column of sources at a time, in a multi-row or multi-column array of radiation sources.

[0056] Because the x-ray sources 20_iare individually controllable, a radiation dose distribution can be built up so as to match any desired pattern. For example, each of the plurality of x-ray sources 20_imay be selectively switched on for different lengths of time, in a manner deemed appropriate by the surgeon or other radiotherapy professional, and as dictated by the conditions of the patient. Alternatively, the intensity of each x-ray source 20_imay be adjusted, so that for example the acceleration voltage and beam current are increased for an x-ray source 20_ithat is being used to destroy a tumor that is relatively large in size, while the acceleration voltage and beam current are decreased for an x-ray source 20_ithat is being used to destroy a tumor that is relatively small in size. In some cases, for reasons such as the topology of the treatment regions, it may not be possible for the introducer 95 to position the x-ray sources 20_iat an exact, desired location. In these cases, the intensity of an x-ray source 20_imay be adjusted according to need. For example, if an x-ray source 20_iis located further from a tumor than would be desired, due to a deficiency in the implant process, the accelerating voltage, the electron beam current, and the duration of x-ray emission, can all be increased, in order to build a higher dosage for the treatment region.

[0057] In operation, an array of therapeutic radiation sources, preferably x-ray sources, are positioned near a desired treatment region within a patient, using a mechanical introducer. For example, the array may be inserted into a body passageway (such as a blood vessel), and guided through the passageway, until each x-ray source is positioned at a desired location with respect to the treatment region. Each of the plurality of x-ray sources are selectively activated, so as to irradiate the treatment region. Using the controller featured in the present invention, the intensity and duration of the therapeutic radiation generated by each source in an array of sources can be controlled by the user, so as to achieve a desired, predetermined irradiation profile.

[0058] FIG. 4 is a schematic illustration of another embodiment of a controller 140 for an array of x-ray sources 20₁in accordance with the present invention, in which the controller 140 also regulates an in situ radiation detecting system for real time monitoring of the radiation dose distribution. In this embodiment, the controller 140 also regulates an image-guided surgical system that uses images of the treatment region to guide the position of each of the plurality of x-ray sources 20₁, and permits the surgeon to visualize during the surgical procedure the cumulative dose of radiation delivered by each source 20_i. A method and apparatus for image-guided radiotherapy is disclosed in U.S. patent application Ser. No. 09/656,878, and is hereby incorporated by reference. The image-guided surgical system includes an imaging system 350 which generates a visual image representing the cumulative dose of radiation delivered by each source 20_ito a desired point within the treatment region. The visual image is displayed on the display unit 360, to guide the surgeon during his delivery of therapeutic radiation.

[0059] The in situ radiation detection system includes a scintillator 68_idisposed along a path of the x-rays emitted by each x-ray source 20_i. The scintillator 68₁is adapted to generate flashes of scintillator light, in response to the x-rays that are incident thereon. The intensity of the scintillator light is proportional to the quantity of the incident x-ray photons, as well as to their energies. In the illustrated embodiment, the scintillator 68_iis affixed to the terminating end of the associated fiber optic cable 10_i. The scintillator 68_iis thus arranged so that the x-rays incident on the scintillator 68_iare predominantly those x-rays that are emitted from the target element 60₁in a backward direction, with respect to the path of the incident electron beam. It should be noted, however, that any configuration that places the scintillator 68_ialong any path of the emitted x-rays is within the scope of this invention.

[0060] A photodetector 210 is placed in optical communication with each scintillator 68_i. The photodetector 210 converts scintillator light into a signal proportional to the intensity of the scintillator light, and hence indicative of the amount of x-rays emitted by the associated target element 60_iand incident on the scintillator 68₁. The photodetector 210 may be a photomultiplier 210, by way of example. In one form, the photomultiplier may include a plurality of anodes. The radiation detection system may also include an optical system 220 for selectively directing light so that only light from the scintillator 68_iis incident on the photodetector 210, and all other types of optical radiation, such as ambient visible light and light from the laser source 30, are filtered out. In one embodiment, the optical system 220 may include a dichroic beam splitter (not shown) and a filter (not shown) disposed between the laser source 30 and each fiber optic cable 10_i. The photodetector 210 is preferably coupled to a feedback circuit 230, which feeds the indicative signal from the photodetector 210 back to the controller 140. The controller 140 is responsive to the feedback signals from the photodetector 210, to calculate in real time a cumulative dose of the x-rays detected by the scintillator 68_i. The calculated cumulative dose information may be displayed on a display unit 360, such as a monitor screen. This permits the surgeon or other radiotherapy professional to monitor, in real time, the cumulative dose of the delivered x-rays.

[0061] In one form of the invention in which a single optical fiber (having an originating end and a plurality of terminating ends, as mentioned earlier) is used, the controller 140 may include means (not shown) for gating on and off the incident optical radiation from the laser source 30. The intensity of the scintillator light may be measured during the off-times.

[0062] In sum, by stringing together a plurality of individually controllable therapeutic radiation sources, and by controlling parameters such as the turn-on time and position of each individual therapeutic radiation source, as well as the duration and intensity of the therapeutic radiation emitted by each source, any desired dose and spatial distribution of the output radiation can be attained. An arbitrarily complex radiation density distribution may be attained over a desired treatment region.

[0063] While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.