Abstract:
A method of performing microbeam radiosurgery on a patient whereby target tissue within a patient is irradiated with high energy electromagnetic radiation from an inverse Compton scattering radiation source via microbeam envelopes.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to methods for performing radiosurgery on a patient, and in particular, to methods for performing radiosurgery using microbeam radiation. 
         [0003]    2. Description of the Related Art 
         [0004]    For over a century, high energy radiation (e.g., X- and y-radiation) has been used to destroy cancerous tumors located deep within the bodies of patients. This form of cancer therapy, known as radiotherapy, is one of the three major methods for treating cancer, surgery and chemotherapy being the remaining two. Radiotherapy is widely used. Indeed, nearly 60% of all cancer patients receive radiotherapy as an element of their overall treatment protocols. 
         [0005]    Recently, radiation has also been used to treat non-cancerous tissues which are otherwise diseased or compromised. A particularly exciting emerging medical protocol utilizes radiation to either destroy or modulate the function of brain tissue associated with psychiatric or neurological disorders. Such treatments hold the promise of curing problems such as depression, chronic pain, and obesity. 
         [0006]    The use of radiation to treat all forms of disease and biological dysfunction is known as radiosurgery. 
         [0007]    Conventional radiosurgery employs three methods to generate high energy radiation. In a first method, the physical phenomenon of radioactivity is used. In a second method, the physical phenomenon of bremsstrahlung (i.e., “braking radiation,” arising from decelerating charged particles) is used. In a third method, the physical phenomenon of oscillating charged particles is used. 
         [0008]    Conventional radiosurgery systems also generate three types of radiation spatial patterns with which to expose tissue. In a first case, the spatial pattern is uniform, and is described as a broad or non-segmented beam. In a second case, the spatial pattern is comprised of a two dimensional array of substantially mutually parallel circular or rectangular beams, and is described as a grid or segmented beam. In a third case, the spatial pattern is comprised of a linear array of substantially mutually parallel rectangular beams, and is described as a segmented beam. If the diameter of the circular beams, or the width of the rectangular beams, is less than 1 mm, such beams are described as microbeams. 
         [0009]    Referring to  FIG. 1 , in the physical process of radioactivity for the nuclide  60 Co, a neutron of the  60 Co nucleus emits a β −  particle  10  (a.k.a., an electron), leaving behind an also radioactive  60 Ni nuclide. The activate  60 Ni nucleus in turn emits two high energy γ-ray photons 12 and 14 at 1.17 and 1.33 MeV, respectively, yielding a stable  60 Ni nuclide. 
         [0010]    Referring to  FIG. 2 , a conventional radiosurgery system uses the radioactive nuclide  60 Co. The  60 Co material  20  is placed in the hollow portion of an otherwise solid Pb sphere  22 . A patient is irradiated with photons  12 ,  14  when a slide mechanism  24  brings the  60 Co material  20  into position over a channel  26  within the Pb sphere  22  which is aligned with the patient (not shown). A collimator  28  between the  60 Co material  20  and the patient shapes the radiation field to provide either a broad or segmented beam. 
         [0011]    Referring to  FIG. 3 , in the physical process of bremsstrahlung, a high energy electron  30  inelastically scatters off the nucleus  32  of a target atom, such as W. In the collision with the target nucleus  32 , the electron  30  decelerates and loses energy. Some of the energy lost by the electron  30  emerges from the collision as a high energy X-ray photon  34 . 
         [0012]    Referring to  FIG. 4 , a conventional radiosurgery system which employs bremsstrahlung uses a linear accelerator  40  to provide a beam of high energy electrons  42  which is directed at a W target  44 . High energy X-ray photons  34  emerge from the W target  44 . A collimator  28  between the W target  44  and the patient (not shown) shapes the radiation field to provide either a broad or segmented beam. 
         [0013]    Referring to  FIG. 5 , the generation of radiation via the oscillation of a charged particle is shown. An electron  50  is made to oscillate between two points A and B in space. As a result of this oscillation, a photon  52  emerges. 
         [0014]    Referring to  FIGS. 6A-6B , in a conventional radiosurgery system using the oscillation of charged particles to produce high energy radiation, a linear accelerator  40  ( FIG. 6A ) provides a beam of high energy electrons  42  which is injected into a synchrotron  60 . The output of the synchrotron  60  is, in turn, injected into a storage ring  62 . Located along a portion of the storage ring  62  circumference is a device known as a wiggler  64 . The wiggler  64  ( FIG. 6B ) includes a series of magnets  66  providing an oscillating magnetic field pattern. As the electrons  50  move through the wiggler  64 , the electrons  50  oscillate in a plane perpendicular to the plane of the oscillating magnetic field. The oscillating electrons  50 , in turn, produce high energy radiation  52  ( FIG. 6A ) which is directed at a patient (not shown). A collimator  28  yields either a broad or segmented beam. 
         [0015]    Referring to  FIGS. 7A-7C , the three types of radiation spatial patterns typically used by conventional radiosurgery systems are depicted: a broad, non-segmented beam ( FIG. 7A ), a grid segmented pattern with a two dimensional array of substantially mutually parallel circular beams ( FIG. 7B ), and a segmented pattern with a linear array of substantially mutually parallel rectangular beams ( FIG. 7C ). If the individual beam dimensions  70 ,  71 ,  72  are less than 1 mm, the associated beam is considered a microbeam. 
         [0016]    A major difficulty presented by these conventional radiosurgery systems is that the radiation which destroys diseased tissue also destroys normal healthy tissue. For most conventional radiosurgery systems, this problem is dealt with by exposing the diseased tissue from several angles, thereby maximizing the dose to the diseased tissue while minimizing the dose to neighboring normal tissue. Even so, the maximum dose which can be deposited in the diseased tissue, which determines the effectiveness of the radiation in destroying the diseased tissue, is limited by the susceptibility of the neighboring normal tissue to damage. 
         [0017]    As indicated in Slatkin et al., U.S. Pat. No. 5,339,347 (the disclosure of which is incorporated herein by reference), experiments show that microbeam radiation patterns essentially resolve the problem of damage to normal tissue. Although the normal cells in the direct path of the microbeams are destroyed, the region of destroyed cells is so narrow that the healthy cells on either side are capable of healing the damaged region of tissue. Furthermore, as shown in Dilmanian et al., U.S. Pat. No. 7,194,063 (the disclosure of which is incorporated herein by reference), there exist microbeam targeting strategies which assure the destruction of diseased tissue while sparing the functionality of neighboring normal tissue. 
         [0018]    One problem that can disannul the effectiveness of microbeam radiosurgery, however, is tissue movement during irradiation. Such movement may arise from patient breathing, or the pulsing of blood through the tissue. Movement of the tissue effectively broadens the regions irradiated by the microbeams. As the irradiated regions become wide, the healing capability of surrounding tissue is compromised. To avoid this problem, the microbeam radiation is preferably delivered extremely quickly so that the range of tissue motion during the irradiation is sufficiently small. Thus, the radiation source providing the microbeams preferably has a high dose rate. 
         [0019]    Of the conventional radiosurgery systems described herein ( FIGS. 2 ,  4  and  6 A- 6 B), only the synchrotron source utilizing oscillating charged particles ( FIGS. 6A-6B ) has the ability to provide a sufficiently high dose rate to assure the effectiveness of microbeam radiosurgery. At the current state of the art, a synchrotron source has a maximum dose rate of nearly 2×10 4  Gy/s, while a linear accelerator utilizing bremsstrahlung ( FIG. 4 ) has a maximum dose rate of 4×10 −1  Gy/s, and a  60 Co source utilizing radioactivity ( FIG. 2 ) has a maximum dose rate of 7×10 −2  Gy/s. These dose rates must be compared against the dose rate required to successfully treat the most challenging problem presented to microbeam radiosurgery, that of a moving lung tumor. A minimum dose rate of 7×10 3  Gy/s is required to ablate a lung tumor using microbeam radiation. 
         [0020]    Unfortunately, a synchrotron is a very large and expensive device. The synchrotron source which has been used for most microbeam radiosurgery experiments to date is the European Synchrotron Radiation Facility located in Grenoble, France. The storage ring associated with this synchrotron is 300 m in diameter, and the facility cost approximately $900M to construct. These characteristics of a synchrotron source prohibit widespread use of microbeam radiosurgery. 
       SUMMARY 
       [0021]    In accordance with the presently claimed invention, microbeam radiosurgery is performed by irradiating target tissue within a patient with high energy electromagnetic radiation from an inverse Compton scattering radiation source via microbeam envelopes. 
         [0022]    In accordance with one embodiment of the presently claimed invention, a method of performing microbeam radiosurgery on a patient includes irradiating a target tissue, within a patient, with high energy electromagnetic radiation from an inverse Compton scattering radiation source via a plurality of microbeam envelopes which are mutually spatially distinct. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  depicts an energy level diagram for radioactivity for the nuclide  60 Co. 
           [0024]      FIG. 2  depicts a conventional radiosurgery system using the radioactive nuclide  60 Co. 
           [0025]      FIG. 3  depicts the physical process of bremsstrahlung. 
           [0026]      FIG. 4  depicts a conventional radiosurgery system using bremsstrahlung. 
           [0027]      FIG. 5  depicts the generation of radiation via the oscillation of a charged particle. 
           [0028]      FIGS. 6A-6B  depict a conventional radiosurgery system using the oscillation of charged particles to produce high energy radiation. 
           [0029]      FIGS. 7A-7C  depict three types of radiation spatial patterns typical of conventional radiosurgery systems. 
           [0030]      FIG. 8  depicts inverse Compton scattering. 
           [0031]      FIG. 9  depicts one example of a radiation source using inverse Compton scattering. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
         [0033]    Referring to  FIG. 8 , radiosurgery using microbeam radiation in accordance with a preferred embodiment uses the physical process of inverse Compton scattering in which a high energy electron  30  collides with a low energy photon  80 . Emerging from the collision is a high energy photon  81  and a reduced energy electron  82 . 
         [0034]    Referring to  FIG. 9 , in accordance with an exemplary embodiment, a radiation source utilizing inverse Compton scattering useful for microbeam radiosurgery includes a linear accelerator  40  which injects pulses of high energy electrons  42  into a small storage ring  62 . The electron beam path along a portion of the storage ring  62  is substantially collinear with an optical cavity established by two mirrors  90 ,  92 . Light from a pulsed, mode-locked laser  94  is injected into the optical cavity. The repetition rate of the laser  94  is set such that the pulses of laser light arrive at an interaction region  96  at the same time as the pulses of high energy electrons  42 . As the high energy electrons collide with the low energy laser photons  80 , high energy photons  81  are generated. 
         [0035]    The high energy photons  81  can be arranged into the desired pattern of one or more microbeams (e.g., as depicted in  FIGS. 7A-7C ) in accordance with various techniques. For example, they can be passed through a collimator  28  which segments the radiation into the desired one or more simultaneous microbeams. For another example, the track of the electron beam  42  circulating in the storage ring  62  ( FIG. 9 ) and/or the track of the low energy photon beam  80  circulating in the optical cavity defined by the mirrors  90 ,  92  can be manipulated to produce a beam of high energy photons  81  which scans through the desired regions of space as a function of time. 
         [0036]    An inverse Compton scattering source of radiation such as described above should achieve a dose delivery rate of 1×10 4  Gy/s. The diameter of the storage ring associated with such a source is expected to be less than 10 m, and the cost of such a source is expected to be less than $15 M.