Miniaturized source of ionizing radiation and method of delivering same

A method and apparatus of creating a miniaturized source of radiation and delivering radiation to a location such as therapy location. The radiation source comprises a member made of a material emitting electrons when energy is supplied to the member. There is an electron retarding member disposed opposite the electron emitting member, and the electron retarding member is made of a material emitting ionizing radiation when electrons are retarded therein. The radiation source is further provided on an elongated member in the distal region thereof, and the elongated member is insertable into the body.

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION 
The invention relates to a method and apparatus for the use of radiation 
sources in therapy, and in particular to a miniaturized radiation source 
having the capability of being switched on and off at the operator's 
discretion. 2. Related Art 
The manufacture of radiation devices has been developing during the last 30 
years. The primary applications for these devices are in microelectronics, 
diods and transistors. However more recently, devices that generate 
radiation in the visible frequency region have been used for displays of 
the Flat Panel Display type, and this technology has become a separate 
research area for applications in the television field, etc. The primary 
effort has been to decrease the anode to cathode voltage, so that these 
devices can be used in general purpose electronic circuits. A more 
detailed description of this research can be found in an article entitled 
"Vacuum Microelectronic Devices", in Proceedings of the IEEE, Vol. 82 no. 
7, July 1994. 
In this article there are disclosed the principles and basic construction 
of micro field-emission sources. It is stated therein that it is necessary 
to have emission areas no larger than 10.sup.-2 cm.sup.2 in order to 
obtain uniform field emission. Therefore, it is necessary to form the 
emitter in the shape of a needle with a tip having an end radius less than 
1 .mu.m. A specific design of the emitter is a metal cone, 10.sup.-4 cm 
tall with a tip radius of 30 nm. Also, there is disclosed the provision of 
an accelerating electrode (gate) spaced 60 from the tip. 
In this application, these field emitting devices, among others, can be 
utilized to emit ionizing radiation with energies high enough to be used 
for medical therapy. 
Radiation therapy is a well established method for treatment of several 
serious diseases, including cancer. Either alone or combined with other 
forms of therapy, the irradiation of human or animal tissue with ionizing 
radiation has proven to be very effective, and is used throughout the 
world and at several levels in health care organizations from specialized 
university clinics to regional and county levels. However, complications 
and side effects are often present. Ionizing radiation is biologically 
destructive in the sense that the structure of biomolecules is 
irreversibly changed, frequently leading to cellular disorganization, 
functional damage and even death. The result is also non-specific. A 
common problem is to limit the radiation exposure to areas of disease, to 
avoid destruction of healthy tissue. 
Traditional radiation therapy makes use of radioactive nuclei, particle 
accelerators or high voltage generators to create radiation with such a 
high energy that it penetrates the patient's body. The radiation source is 
usually located outside the body, and means for collimating the radiation 
is used to concentrate it on the tissue where therapy is required. A 
difficult compromise is to maximize the therapeutic dose while minimizing 
the radiation exposure to healthy tissue. 
In recent years, miniaturized radiation sources consisting of radioactive 
substances contained at the tip of a metal wire have been introduced. With 
such a localized radiation source it is possible to concentrate the dose 
to a small region. However, the use of radioactive substances is 
impractical for several reasons. First, the source must be properly 
shielded during introduction into the body to avoid exposure to healthy 
tissue. Second, all handling procedure must be carefully controlled to 
avoid exposure by mistake. Third, the dose and energy of radiation are not 
easily controlled. 
SUMMARY OF THE INVENTION 
The present invention provides an adequate solution to these problems. It 
has now been ascertained that the principle of field emission and 
thermionic emission is possible for use in medical procedures, namely for 
delivering radiation to a therapy location in a living body. One aspect of 
the invention comprises a miniaturized radiation source which is 
electronically controllable to generate exactly the required energy or 
wavelength of radiation. It can be switched on and off as desired. 
Furthermore, the delivered intensity and dose can be independently 
controlled, and the source can be manufactured with extremely small 
dimensions. For certain purposes it will have a volume of less than 
10.sup.-3 mm.sup.3, whereas for other purposes it may be as large as 1 
cm.sup.3. 
Thus, in one aspect of the invention there is provided an apparatus for 
delivering radiation to a therapy location in a living body, comprising a 
miniaturized source of ionizing radiation, the radiation source ionizing 
radiation, the radiation source comprising a member made of a material 
emitting electrons when energy is supplied to the member, an electron 
retarding member disposed opposite the electron emitting member, the 
electron retarding member being made of a material emitting ionizing 
radiation when electrons are retarded therein, the radiation source being 
provided on an elongated member in the distal region thereof, and the 
elongated member being insertable into the body. 
In another aspect of the invention there is provided a method of delivering 
radiation to a therapy location in a living body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The basic physical principle behind the radiation source is well known from 
the literature of modern physics. When high energy electrons are retarded 
by nuclei having a large atomic weight, electromagnetic radiation is 
emitted. The primary radiation, denoted "bremsstrahlung", has a continuous 
spectrum with a peak corresponding to a given fraction of the electron 
energy. The emitted radiation can have an energy peak from a few electron 
volts (eV) to several million electron volts (MeV) depending on the energy 
of the incident electrons. In terms of wavelength, this corresponds to a 
range from ultraviolet light (10-4000 .ANG.) via X-rays (0.1-100 .ANG.) to 
gamma radiation (&lt;0.10 .ANG.). Thus, by varying the energy of the 
electrons, the wavelength peak can be displaced accordingly. In addition 
to bremsstrahlung, which basically has a continuous spectrum, absorption 
or emission peaks corresponding to atomic electron transition may be 
embedded in the spectrum, depending on the materials contained in the 
transmission medium. 
The details of the radiation source and its function will be described with 
reference to FIGS. 1 and 2. Basically, the source is built up from two 
plates 2, 3 with a recessed region forming a microcavity 1 at one or 
several localities. An anode material 5 and a cathode 4 with extremely 
small dimensions, and having the form of a sharp tip 10 are located within 
this microcavity. The radius of curvature of the tip 10 of the cathode is 
preferably in the nanometer range. If a voltage is applied between the 
anode 5 and cathode 4, the electric field strength will be extremely high 
at the cathode. A positive voltage on the anode will cause electrons to be 
emitted from the cathode by the phenomenon known as field emission. 
Alternatively, the cathode may be heated to high temperatures, giving rise 
to thermal emission of electrons. This will be further discussed below 
with reference to FIG. 6. The electrons are accelerated by the electric 
field, until they are retarded by the impact at the anode. The anode 5 
preferably consists of a metal having a high atomic weight, corresponding 
to an atomic number exceeding 50. In a preferred embodiment, the anode 5 
is made of tungsten which is an endurable metal that can be deposited in 
the form of thin films either by physical or chemical deposition 
techniques. Other metals include cobalt, molybdenum and aluminium. The 
cathode preferably consists of a thin deposited film of a material having 
a low work function, i.e. the energy required for an electron to be 
emitted from the surface into the ambient. Materials with this property 
are oxides of metals from Groups I and II in the periodic table, including 
cesium, barium and magnesium. 
The anode 5 and cathode 4, may be connected to a voltage source by 
electrically conducting leads 6, 7, which may, at least partly, be an 
integral part of the plates 2, 3. This can be achieved by deposition of 
stripes by evaporation, sputtering or chemical vapor deposition. 
Alternatively, if the plates 2, 3 are semiconductors, the leads 6, 7 may 
be doped regions according to well-known technology. In a preferred 
embodiment, a third electrode 11 is also present within the microcavity 1. 
This electrode 11 acts as a gate, controlling the electron current emitted 
toward the anode 5. The gate electrode has a separate lead 12, enabling a 
separate voltage source to be connected. According to the well-known 
theory of vacuum tubes, the anode current is controlled by the gate 
voltage. This will directly influence the intensity of the emitted 
radiation which is approximately proportional to the anode current. The 
emitted dose is simply the time integral of this intensity. By separate 
and independent control of the gate and anode voltages, it is thus 
possible to independently control the emitted dose and energy, 
respectively. 
The leads 6, 7 and 12 must be properly isolated to avoid short circuit or 
current leakage. If the plate materials by themselves are not isolating 
themselves, passivating films may be necessary to ensure proper isolation. 
Furthermore, the lateral location of the leads is preferably chosen to 
minimize the electric field across material barriers. The voltage to the 
anode and cathode should preferably be in the kV range in order to obtain 
radiation of sufficient energy. 
With reference to FIG. 6, there is schematically shown an implementation 
wherein the thermionic emission principle is employed. Through a thin wire 
601 or filament disposed in a microcavity 602, such as the one disclosed 
above, a current I is passed. The temperature will be so high that 
electrons will be emitted and accelerated by an electronic voltage imposed 
across the filament 601 and an anode 603, also disposed in microcavity 
602. 
There are two principally different ways of fabricating the radiation 
source according to the invention. One way is to use two separate solid 
substrates and define the structures containing the cathode 4, the gate 
11, with their leads 7 and 12, and the recess or microcavity 1 in one 
substrate. The anode 5 and its lead 6 are defined in the second 
substrates. Lithographic techniques according to well-known art are 
preferably used in defining these structures. Then finally the two 
substrates, corresponding to plates 2 and 3, are bonded together, using 
techniques such as solid-state bonding. If the bonding is performed in a 
vacuum, the microcavity 1 will remain evacuated, since the bonded seal is 
almost perfectly hermetic, provided that no organic materials are used. 
Absolute vacuum is not a necessity, but the density of gas molecules 
inside the microcavity must not be so high that the accelerating electrons 
are excessively impeded. 
A requirement for successful bonding is that the bonded surfaces 8 and 9 
are flat and smooth with a precision corresponding to a few atomic layers. 
A second requirement is that all structures are able to withstand a 
relatively high annealing temperature, approximately 600-1000.degree. C., 
without damage. This first fabrication technique is basically known as 
bulk micromachining, in contrast to its alternative, surface 
micromachining. According to this, all structures are formed by 
depositions on one single substrate, again using lithography to define the 
two-dimensional pattern on the surface. The microcavity 1 is formed by 
first depositing a sacrificial layer which is etched away after the 
uppermost layers have been deposited. Closing the microcavity can be done 
by depositing a top layer, covering openings which are required for the 
etching of the sacrificial layer. 
Both described methods of fabrication are feasible and lead to similar 
device performance. Indeed, from examining a final device, it may be 
difficult or even impossible to conclude which fabrication procedure has 
been used. An important characteristic of the proposed fabrication 
techniques is that the manufacturing cost per unit becomes very small when 
the source elements are fabricated in large numbers. This is due to the 
fact that batch fabrication with thousands of units per batch is feasible. 
In FIG. 3 an embodiment is shown where the source and its leads 6, 7 are 
mounted inside a tubular element, such as a cannula 100, consisting of a 
material which is transparent to the emitted radiation. 
Preferably, the tubular element (or the hollow portion where the source is 
mounted in the case of a needle), is made from elements having a low 
atomic number. As shown in the cross section A--A the leads 6, 7 are 
connected to wires 101, 102 having isolated mantles 103, 104. In a 
preferred embodiment, the outer diameter of the tubular element is smaller 
than 2 mm. The cannula is then sufficiently small to penetrate tissue in 
order to reach a certain location where radiation therapy is required. 
FIG. 4 shows a further embodiment where the source 200 is located near the 
distal end of a wire 201, having high bending flexibility in order to 
prevent organs and tissue from perforation or penetration by mistake. 
Instead, the wire 201 can be guided to the tissue where radiation therapy 
is required by insertion through a catheter which has previously been 
inserted in the tissue by well-known techniques. A cross section B--B of 
the wire 201 shows that it consists of a tubular member 202, and power 
transmitting leads 203, 204. The leads 203, 204 are proximally connectable 
to an external power source by connecting elements 205, 206. 
Geometrically, the connecting elements 205, 206 have a diameter 
approximately equal to the diameter of the wire to allow insertion of the 
wire into a catheter. 
Referring now to FIG. 7a and b, other vehicles for the radiation source are 
conceivable, e.g a needle 700 with a solid distal portion 701 having a 
sharp tip for the easy penetration of soft and hard tissue, and a hollow 
portion 702, proximal to the solid tip, wherein the radiation source 703 
is mounted. In still another embodiment the radiation source may be 
mounted in a tube 704, the distal end of which, 705, has been bevelled to 
render it sharp enough for penetration purposes. The open end of the tube 
may be plugged at 706 so that the interior of the tube housing the source 
will not be soiled by tissue. 
The power leads supplying power to the radiation source can either be 
electrical or fiberoptic leads, according to well-known technology. In the 
case of optical power transmission, it is necessary to convert the optical 
power into electrical voltage to provide voltage supply to the source. 
This may be done by providing optical energy through the fiberoptic leads 
and letting the light impinge onto a photodiode which converts the light 
into a voltage. 
FIG. 5 shows an electronic circuit element M capable of multiplying an 
input voltage 305 to its output terminals 307, 308 by a factor of 
approximately two. The circuit operates with two switching elements, for 
example diodes 301, 302, and two capacitors 303, 304. If two circuit 
elements as that shown in FIG. 5 are cascaded, the input voltage will be 
multiplied by a factor of approximately four. Even larger multiplication 
factors are possible by cascading more circuit elements of a similar type. 
The diodes 301, 302 may be replaced by other switching elements, such as 
transistors. 
Preferably, electronic circuitry M such as that shown in FIG. 5 may be 
integrated with one of the plates 2, 3 accommodating the source 
(schematically shown in FIG. 8a). Alternatively, the circuitry consists of 
a separate electronic chip located close to the source (schematically 
shown it FIG. 8b). 
The high voltage generation may of course alternatively be disposed outside 
the body, e.g in the external power supply. 
The method of providing a controlled dose of radiation is carried out as 
follows. 
The physician localizes the region of interest, e.g. a tumor to be treated. 
Depending on the site and type of tissue, various vehicles for the 
radiation source may be employed, e.g. a needle for penetrating through 
soft tissue, or a guide wire possibly in combination with a catheter, or 
the insertion may be made through blood vessels or other body channels, 
such as intestines. When the radiation source has been correctly located 
inside the body, the radiation source is activated and the required dose 
is given. The device is switched off and the source is withdrawn from the 
patient. This procedure may be repeated frequently until the desired 
clinical result has been achieved. 
While several embodiments of the invention have been described, it will be 
understood that it is capable of further modifications, and this 
application is intended to cover any variations, uses, or adaptations of 
the invention, following in general the principles of the invention and 
including such departures from the present disclosure as to come within 
knowledge or customary practice in the art to which the invention 
pertains, and as may be applied to the essential features hereinbefore set 
forth and falling within the scope of the invention or the limits of the 
appended claims.