X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2000-03-15
2001-03-06
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S119000, C378S065000, C378S015000, C378S137000, C378S113000
Reexamination Certificate
active
06198804
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
REFERENCE TO MICROFICHE APPENDIX
Not Applicable.
BACKGROUND OF THE INVENTION
This invention relates to a miniaturized, programmable X-ray treatment system having an electron beam source and an X-ray emitting probe for use in delivering substantially constant or intermittent levels of X-rays to a specified region and, more particularly, to the electron beam generating components of the X-ray treatment system.
In the field of medicine, radiation is used for diagnostic, therapeutic and palliative treatment of patients. The conventional medical radiation sources used for these treatments include large fixed position machines as well as small, transportable radiation generating probes. The current state of the art X-ray treatment systems utilize computers to generate complex treatment plans for treating complex geometric volumes.
Typically, these systems apply doses of radiation in order to inhibit the growth of new tissue because it is known that radiation affects dividing cells more than the mature cells found in non-growing tissue. Thus, the regrowth of cancerous tissue in the site of an excised tumor can be treated with radiation to prevent the recurrence of cancer. Alternatively, radiation can be applied to other areas of the body to inhibit tissue growth, for example the growth of new blood vessels inside the eye that can cause macular degeneration.
One type of X-ray treatment system used for such applications is disclosed in U.S. Pat. No. 5,153,900 (“'900 patent”) issued to Nomikos et al., owned by the assignee of the present application, which is hereby incorporated by reference. The system disclosed in the '900 patent uses a point source of radiation proximate to or within the volume to be radiated. This type of treatment is referred to as brachytherapy. One advantage of brachytherapy is that the radiation is applied primarily to treat a predefined tissue volume, without significantly affecting the tissue in adjacent volumes.
A brachytherapy X-ray treatment system includes an X-ray source
10
shown in
FIG. 1
, generally comprised of an electron beam (“e-beam”) source
12
and a miniaturized insertable probe assembly
14
capable of producing low power radiation in predefined dose geometries or profiles disposed about a predetermined location. The probe assembly
14
includes a shoulder
16
which provides a rigid surface by which the X-ray source
10
may be secured to another element, such as a stereotactic frame used in the treatment of brain tumors. The probe assembly
14
also includes an X-ray emitting tube
18
, or “probe”, rigidly secured to shoulder
16
. A typical probe of this type is about 10-16 cm in length and has an inner diameter of about 2 mm and an outer diameter of about 3 mm.
Typical brachytherapy radiation treatment involves positioning the insertable probe
18
into the tumor or the site where the tumor or a portion of the tumor was removed to treat the tissue adjacent to the site with a “local boost” of radiation. In order to facilitate controlled treatment of the site, it is desirable to support the tissue portions to be treated at a predefined distance from the radiation source. Alternatively, where the treatment involves the treatment of surface tissue or the surface of an organ, it is desirable to control the shape of the surface as well as the shape of the radiation field applied to the surface.
A typical e-beam source
12
of the prior art includes a single 50 kV drift tube acceleration stage or accelerator
20
, as shown in FIG.
2
. The accelerator
20
includes a cylindrical body
22
comprised of a ceramic portion
24
and a metal portion
26
. The accelerator
20
houses an electron gun assembly, including pins
30
which generate electrons and gun
32
which direct the electrons along a central axis
42
of the system through metal tube
38
, tube opening
36
, probe interface
40
, and probe
18
(not shown). The metal portion
26
includes an electrically conductive ring shaped end
28
, which includes Node X. The voltage at Node X is ideally 0V during operation and the voltage at Node Y is ideally −50 kV (near the electron gun), thereby providing an acceleration field for the e-beam.
In operation, the e-beam is directed to a target at the distal end of probe
18
. The e-beam thus establishes a current along the axis
42
. A return current path along the metallic probe
18
couples the distal end of the probe back to Node X at ring
28
. The current present at Node X is ideally the beam current (I
B
), which is measured and an indication of the measurement is communicated to a radiation controller (not shown). The radiation controller adjusts the power supplied to the electron gun
32
as a means of adjusting the beam current and, ultimately, achieving the desired output radiation level at the end of probe
18
. As a means of testing and stressing the system, a voltage of 75 kV is applied across Nodes X and Y; this is called “over-voltaging” the system.
A consequence of the single stage 50 kV accelerator
20
of a typical X-ray source
10
, is that during operation stray electrons leak out from pins
30
and “avalanche” along ceramic wall
24
and metal wall
26
, as shown by arrow
44
. These stray electrons are typically emitted form the “triple point”
34
, i.e., the point where the negative electrode
30
, the vacuum within the accelerator, and the insulator
24
meet. Eventually, the stray electrons strike the end of the accelerator
20
at end
28
. At very high voltages, e.g., 75 kV test voltage and 50 kV operational voltage, the electrons incident on metal end
28
have sufficient energy to cause X-rays to be emitted, resulting in unshielded X-rays which may be hazardous to those present. Additionally, the risk of high voltage hazards, such as arcing, are undesirably high while using test voltages the magnitude of 75 kV applied across a single stage. A performance related problem also results from the incident stray electrons on metal
28
. The stray electrons cause a “leakage current” to be present at Node X along with the beam current. This stray current is combined with the beam current, leading to an erroneous beam current measurement and resulting calibration of the output radiation by the radiation controller. Consequently, the radiation controller erroneously adjusts the power delivered to the electron gun
32
, which alters the beam current and changes the characteristics of the radiation from probe
18
. The ability for the system to operate safely and perform adequately at relatively high voltages is a reflection of its “high voltage standoff” capability.
It is an object of the present invention to provide an X-ray source with improved high voltage standoff capability.
It is another object of the present invention to provide electron beam accelerator with improved high voltage standoff capability.
It is a further object of the present invention to provide a modular electron beam accelerator which requires lower voltage stress across the accelerator stages, for testing purposes and to reduce the overall size of an equivalent system.
It is a further object of the present invention to provide an X-ray source with improved radiation accuracy, by achieving improved X-ray source calibration based on accurate beam current measurements.
SUMMARY OF THE INVENTION
These and other objects are achieved by the modular multi-stage accelerator of the present invention. The accelerator includes a first stage, which houses the electron gun used to produce an electron beam. The first stage of the accelerator is cylindrical in shape about a central axis which is common to the system, including an X-ray emitting probe of the system. A header is brazed to one end of the cylindrical first stage, forming a vacuum seal. At the opposite end of the first stage cylinder multiple modular accelerator stages may be added. In the preferred embodiment, the first stage, and each modular stage thereafter positioned along the axis, accelerates an electron using a differenc
Bruce David V.
Hobden Pamela R.
McDermott & Will & Emery
Photoelectron Corporation
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