Electric lamp and discharge devices: systems – High energy particle accelerator tube – Magnetic field acceleration means
Reexamination Certificate
1999-03-09
2001-07-24
Berman, Jack (Department: 2881)
Electric lamp and discharge devices: systems
High energy particle accelerator tube
Magnetic field acceleration means
C315S501000, C313S359100, C250S505100, C250S492300
Reexamination Certificate
active
06265837
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a charged-particle beam irradiation method and system for performing a medical treatment such as a cancer treatment through irradiation with a charged-particle beam, and more particularly to a charged-particle beam irradiation method and system in which an affected part can be irradiated with a charged-particle beam in conformity of the shape of the affected part.
In the case where a cancer treatment is performed by use of a charged-particle beam such as a proton beam with a high energy generated by an accelerator or the like, it is required that an area having a diameter of about 20 cm should be irradiated with a proton beam having an energy of about 230 MeV at the highest. The conventional method for realizing this has been disclosed by W. T. Chu et al, “Instrumentation for treatment of cancer using proton and light-ion beams”, Review of Science Instrument, Vol. 64, No. 8 (August 1993), pp. 2055-2122. In the disclosed method, an affected part is divided into a plurality of layers in the direction of depth in a body and is scanned layer by layer through irradiation with a charged-particle beam in conformity to the shape of each layer.
FIG. 9
shows the construction of a charged-particle beam irradiation system disclosed by the Chu et al's article. Referring to
FIG. 9
, a charged-particle beam
90
ejected from an accelerator is adjusted in energy by a degrader
17
so that the irradiation of a plurality of layers
210
to
212
in an affected part
202
of a body
201
with the adjusted beam is made in a sequence from a deeper layer to a shallower layer. The beam is scanned by use of first and second scanning electromagnets
31
a
and
31
b
which are disposed in the irradiation system so that the directions of deflection are orthogonal or vertical and horizontal in the plane of each layer.
The Chu et al's article has disclosed charged-particle scanning methods including a wobbler scanning method in which a beam is circle-wise scanned, a raster scanning method in which a beam is zigzag-wise scanned, and a pixel scanning method in which a beam is pixel-wise scanned.
FIG. 10
shows a charged-particle beam irradiation method based on the raster scanning method. As shown in
FIG. 10
, a charged-particle beam
220
is zigzag-wise scanned in the first layer
210
in conformity to the shape of the first layer
210
. A similar scanning is made in the n-th layer
212
.
FIG. 11
shows a dose profile
230
(or a relationship between depth and dose) in the case where the irradiation is made with a charged-particle beam having a high energy and a dose profile
231
in the case where the irradiation is made with a charged-particle beam having a high energy. As shown in
FIG. 11
, the dose profile of the charged-particle beam has the value
240
or
241
of a dose peak called Bragg peak. A beam penetration depth providing the Bragg peak becomes larger as the energy is higher. It is also shown in
FIG. 11
that the irradiation with the charged-particle beam is made with a small dose even at depth portions shallower than the Bragg peak providing portion. Referring to
FIG. 10
, this shows that when the irradiation with the charged-particle beam
220
is made for the first layer
210
, a region
222
of the n-th layer
212
is also subjected to the irradiation with the same charged-particle beam
220
. Accordingly, in the case where the irradiation with a charged-particle beam
221
is made for the n-th layer
212
, it is required that the dose of a beam portion (indicated by dotted line) for irradiation of the region
222
should be reduced. Though only the first layer and the n-th layer are shown in
FIG. 10
for simplification of illustration, the actual irradiation of the n-th layer amounts to the superimposed irradiation for the first to (n−1)th layers. Therefore, when the irradiation is to be made for the n-th layer, it is necessary that a dose for the beam portion indicated by dotted line in the n-th layer should be equal to or smaller than, for example, one tenth (at the largest ratio) as compared with a dose for a beam portion indicated by solid line.
For such requirements, the Chu et al's article has proposed two irradiation methods as follows. In a first method, the scanning speed of a charged-particle beam at the time of irradiation of each layer is constant while the intensity of the charged-particle beam is reduced when the region
222
is irradiated. In a second method, the intensity of a charged-particle beam at the time of irradiation of each layer is constant while the scanning speed of the charged-particle beam is increased when the region
222
is irradiated. With each of the first and second methods, it is possible to reduce the radiation dose of the charged-particle beam in the region
222
.
In the first method, however, it is required that while one layer is being irradiated with a beam, the intensity of the beam should be changed greatly in accordance with an irradiation position. Namely, there is a problem that a large change in intensity of each charged-particle beam, for example, from 1 to {fraction (1/10)} is needed in the period of 0.1 to 2 seconds when one layer is irradiated, which complicates the control of the accelerator ejecting the beam.
In the second method, it is required that the scanning speed of a beam at the time of irradiation of the region
222
should be increased to, for example, 10 times, which needs a large change in magnetic field intensity of the scanning electromagnet with time. Accordingly, there is a problem that a power supply voltage of the scanning electromagnet becomes high, thereby increasing the cost of a power supply for the scanning electromagnet.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a charged-particle beam irradiation method and system in which the control of an accelerator ejecting a charged-particle beam is simplified and the cost of a power supply for a scanning electromagnet can be reduced.
A first invention for attaining the above object is characterized in that in a charged-particle beam irradiation method in which while a charged-particle beam ejected from an accelerator is scanned by an electromagnet, each layer resulting from the division of an affected part into a plurality of layers in the direction of progression of the charged-particle beam is irradiated with the charged-particle beam, wherein the intensity of a charged-particle beam for irradiation of a first layer is made lower than the intensity of a charged-particle beam for irradiation of a second layer existing at a position deeper than the first layer in the beam progressing direction, and a scanning speed in the first layer is changed between a portion of the first layer subjected to irradiation at the time of irradiation of the second layer and a portion of the first layer subjected to no irradiation at the time of irradiation of the second layer.
With the construction of the first invention in which the intensity of the charged-particle beam for irradiation of the first layer is made lower than the intensity of the charged-particle beam for irradiation of the second layer, the scanning speed of the charged-particle beam for irradiation of the first layer can be lowered, thereby making it possible to lower a voltage to be applied to the electromagnet. As a result, it is possible to reduce the cost of a power supply for the electromagnet. Also, with the construction in which the scanning speed is changed between the portion of the first layer subjected to irradiation and the portion of the first layer subjected to no irradiation, it is possible to adjust the accumulative dose amount of a portion subjected to superimposed irradiation. Further, since there is no need to make a large change of the intensity of the charged-particle beam in a short time, the control of the accelerator is simplified.
A second invention for attaining the above object is characterized in that in a charged-particle beam irradiation method in which while a charged-part
Akiyama Hiroshi
Hiramoto Kazuo
Matsuda Koji
Antonelli Terry Stout & Kraus LLP
Berman Jack
Hitachi , Ltd.
Wells Nikita
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