U.S. patent number 3,917,954 [Application Number 05/414,363] was granted by the patent office on 1975-11-04 for external x-ray beam flattening filter.
This patent grant is currently assigned to Gundersen Clinic, Ltd.. Invention is credited to Raymond J. Boge.
United States Patent |
3,917,954 |
Boge |
November 4, 1975 |
External x-ray beam flattening filter
Abstract
An x-ray beam flattening filter used in radiation therapy for
providing uniform radiation intensity distribution across large
linear accelerator produced x-ray fields, at any depth of treatment
within a patient. The filter includes a solid generally planar
filter member, of material semi-permeable to x-rays, and having an
accurately defined surface configuration symmetrically oriented
about a central axis. The filter member is affixed to a base which
is designed for mounting external of the x-ray beam emergence
outlet of a linear accelerator for completely intercepting the
x-ray beam produced thereby, to accurately and selectively filter
x-rays of the beam for producing uniform radiation intensity across
the x-ray therapy fields of the accelerator apparatus.
Inventors: |
Boge; Raymond J. (LaCrosse,
WI) |
Assignee: |
Gundersen Clinic, Ltd.
(LaCrosse, WI)
|
Family
ID: |
23641124 |
Appl.
No.: |
05/414,363 |
Filed: |
November 9, 1973 |
Current U.S.
Class: |
378/159; 378/156;
976/DIG.435 |
Current CPC
Class: |
G21K
1/10 (20130101); A61B 6/4035 (20130101); A61B
6/032 (20130101) |
Current International
Class: |
A61B
6/03 (20060101); G21K 1/00 (20060101); G21K
1/10 (20060101); H01J 005/16 () |
Field of
Search: |
;250/510,511,505 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Anderson; B. C.
Attorney, Agent or Firm: Merchant & Gould
Claims
What is claimed is:
1. An external filter for flattening a plurality of large x-ray
fields of an x-ray beam produced by a linear accelerator of the
type having means for producing x-rays, means for collimating the
x-rays into an x-ray beam having a longitudinal axis, and first
filter means in said collimating means for roughly shaping said
fields of the beam, said external filter comprising:
A. a single solid disc-shaped member semi-permeable to said x-rays
and peripherally symmetrically shaped in the configuration of said
x-ray field about a central axis, said disc-shaped member
comprising:
i. a planar lower surface lying in a plane perpendicular to the
central axis; and
ii. a geometrically shaped upper surface defined relative said
lower surface by:
a. a first ring-shaped surface parallel to said lower surface,
defining a washer shaped volume therebetween about said central
axis;
b. a second surface linearly radially extending from said lower
surface to an outwardly directed peripheral edge of said first
surface, defining a ring having a geometrical volume of triangular
cross-sectional area about said central axis;
c. a third surface radially extending from an internally directed
edge of said first surface toward said central axis and terminating
at a circle thereabout, defining a ring having a geometrical volume
of polygon shaped cross-sectional area about said central axis;
and
d. a fourth surface forming a radially directed extension of said
third surface in the direction of said central axis and terminating
at said lower surface, defining a ring having a geometrical volume
of triangular cross-sectional area about said central axis; and
B. mounting means of x-ray permeable material, having a planar
mounting surface to receive the lower surface of said disc-shaped
member for mounting said disc-shaped member thereon, said mounting
means being adapted for mounting alignment with said linear
accelerator such that said longitudinal beam axis and said disc
central axis are colinear.
Description
DEFINITIONS
The following definitions will be employed throughout this
document:
X-ray Beam: The projected volume of collimated x-rays of polyhedron
or conical shape infinitely extending from an x-ray target along a
longitudinal axis.
X-ray Field: That cross-sectional area of an x-ray beam defined by
a plane perpendicularly intersecting the longitudinal axis of the
x-ray beam and whose peripheral border is defined by the polygon or
circular shaped sides of the collimated x-ray beam.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to radiation beam shaping devices
for use in radiation therapy and in particular to an externally
mounted x-ray filter for providing uniform radiation intensity over
large x-ray treatment fields.
2. Description of the Prior Art
Treatment of certain types of skin and organ diseases, most notable
of which is cancer, by exposure to predetermined doses of x-rays
has become commonplace in today's medical technology. In
therapeutic radiology, control of the characteristics of the
treating x-ray beam is all-important. Since generally high levels
of radiation intensity are employed in radiation therapy,
precautions must be taken to minimize exposure of and damage to
those areas or tissue layers of a patient which are not directly
involved in the treatment process.
Although many of the considerations and precautions in controlling
an x-ray beam are identical for both diagnostic radiology and
therapeutic radiology applications, the two generally differ
considerably in their requirements of radiation dose distribution
across any given x-ray field of the beam within a patient. In
diagnostic radiology, radiographs are produced by directing the
x-ray beam from an x-ray tube or port through the subject for
action on and against a suitably sensitized surface. The surface is
affected to a varying degree, as controlled by the attenuation of
the subject to the passage of x-ray therethrough. The resulting
radiograph thus varies in exposure across the x-ray field. Since
the permeability to x-ray radiation of the composition of matter
being radiograph varies, diagnostic radiology requires non-uniform
radiation intensity distributions over specific x-ray fields for
providing greater radiation intensity to certain areas of the
subject being x-rayed (for example, bones, etc.), while
simultaneously subjecting more x-ray permeable or sensitive tissues
to less radiation intensity.
Radiation filters of varied construction have been employed for
providing the required radiation intensity nonuniformity across an
x-ray field in diagnostic radiology. These types of filters have
included: filter members having custom shaped curvilinear surfaces
for accomodating individual patients, wedge shaped filters
positionally adjustable across the x-ray beam, multi-band type
filters wherein adjacent bands comprise materials having different
x-ray permeability properties, and the like. Since the radiation
intensity levels and time durations thereof employed in diagnostic
radiology are generally low and short enough respectively so as to
minimize damage by the x-rays to the subject being x-rayed, the
control problem of preciseness in uniformity of radiation dose
distribution over large x-ray fields is not as acute as in the area
of therapeutic radiology. Therapeutic radiology, requires the x-ray
beam to define x-ray fields having uniform radiation intensity
distributions across the entire area of the field at any depth of
treatment within a patient. The problem is further complicated in
those cases requiring treatment over large x-ray fields, for
example in the treatment of Hodgkins disease. In such situations
the required x-ray field of treatment may be 30cm .times. 30cm or
larger. In general, therefore, it is important in radiation therapy
to expose the treatment area to exact, known uniform radiation
intensities across a treatment field at any given treatment depth
with the patient. Such uniformity in radiation intensity over the
entire field is necessary to prevent radiation burns of the
patient's skin or to prevent overdose of sensitive normal tissues
within and adjacent to the treatment area during the treatment
process.
In filtering the x-ray beam produced by a linear accelerator, two
general techniques with respect to positioning of the filter have
been employed. A first of such techniques, which to date has
provided the greatest measure of success in "flattening" (providing
uniform distribution to) the radiation intensity across a field of
a collimated x-ray beam, has placed a filter within the collimating
apparatus of the linear accelerator, relatively close to its x-ray
target. Use of this technique, while theoretically capable of
producing the desired result of flattened/uniform fields at all
depths of treatment, is in practice impossible to construct, due to
the extreme preciseness in shape and dimensional tolerances that
would be required. A minute tolerance variation in any dimension of
such a filter projects significant and undesired radiation
intensity differences to the remotely positioned x-ray treatment
field.
The second general technique for filtering therapeutic x-rays
requires placement of the filter between the x-ray beam outlet of
the linear accelerator and the field of treatment. One such filter
used in radiation treatment of tumors employs a layer of metal
having a grid pattern therein and placed in the path of the x-ray
beam for absorbing a portion of the "soft" rays of the beam which
would damage the skin tissue while permitting the "hard" rays of
the beam to pass therethrough to the treatment area. This type of
filtering technique, however, does not compensate for any basic
non-uniformity in the radiation intensity of x-ray beams across the
various fields of treatment. Another approach provides a set of
standardized compensating filters for mantle-field therapy, which
is intended to provide non-uniform field intensities at various
treatment depths within the patient, and is similar in function to
the diagnostic radiology filters previously described.
The beam flattening filter of my invention overcomes the practical
inaccuracies and inadequacies of the prior art beam flattening
filters, by simultaneously providing uniform radiation intensity
over entire large area x-ray fields of an x-ray beam, at any
practical treatment depth. While my invention will be described in
conjunction with its use in a specific linear accelerator
apparatus, it will be understood that it is not limited to this
use, but can be employed in any linear accelerator x-ray producing
apparatus. Further, while the present invention is described herein
as particularly applicable to use is radiation therapy, it will be
understood that my invention is not limited to this use but could
well be employed in diagnostic radiology and other applications
requiring a uniform radiation intensity distribution across a x-ray
field. It will further be understood that the specific geometrical
surface configurations (including dimensions and angles) of the
preferred embodiment beam flattening filter described herein apply
directly to use with the specific linear accelerator described, and
that such angles, dimensions and geometric relationships could be
appropriately modified within the spirit and intent of my invention
to apply to other accelerator apparatus.
SUMMARY OF THE INVENTION
The x-ray beam flattening filter of this invention comprises a
solid, generally planar filter member semi-permeable to x-rays and
peripherally shaped to the cross-sectional configuration (field) of
the x-ray beam to be filtered. At least one of the planar-like
surfaces of the filter member is precisely geometrically configured
to provide predetermined varied attenuation across the x-ray field
to x-rays passing therethrough. The filter member is designed for
placement within an x-ray beam which is generally symmetrically
collimated about a longitudinal axis such that the longitudinal
beam axis is perpendicular to the general plane of the filter
member and passes through a geometrical center of the configured
surface of the filter member. The filter member is designed for
intercepting the x-ray beam intermediate the collimating apparatus
of the linear accelerator and the treatment fields for a patient,
to provide uniform radiation intensity completely across the x-ray
treatment fields at all depths of treatment.
The filter member may be mounted upon a shadow tray, mounted
externally of the linear accelerator head, or may be accurately
affixed to a mounting base of material having uniform x-ray
permeability thereacross for mounting on a bracket adjacent the
x-ray emergence outlet of the linear accelerator.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the Figures wherein like numerals and letters
represent like parts throughout the several views;
FIG. 1 is a diagrammatic representation of a typical linear
accelerator x-ray producing and collimating apparatus employing the
beam flattening filter of the present invention;
FIG. 2 is a top plan view of a preferred embodiment of the beam
flattening filter of this invention, disclosed in FIG. 1;
FIG. 3 is a cross-sectional view of the beam flattening filter
disclosed in FIG. 2, generally taken along the line 3--3 of FIG.
2;
FIG. 4 is a graph of typical isodose distribution curves referenced
to the radiation intensity at the dose maximum of an of a
megavoltage x-ray beam as they would typically appear for a linear
accelerator apparatus of the type disclosed in FIG. 1, without the
benefit of the beam flattening filter of this invention; and
FIG. 5 is a graph of the typical isodose distribution curves of
FIG. 4 as they would appear when "flattened" by the beam flattening
filter of this invention as disclosed in FIGS. 2 and 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a linear accelerator is generally
diagrammatically illustrated at 20. That portion of the linear
accelerator which produces x-rays comprises: an electron gun 21 for
projecting electrons therefrom, an electron accelerator section 22
having an RF input 23, and an x-ray target 24. The principles of
x-ray production by linear acceleration apparatus are well known in
the art, and will not be belabored herein. In general, however, the
accelerator section 22 directs and accelerates electrons produced
by the electron gun 21 toward the x-ray target 24 at high
velocities. In the preferred embodiment, the x-ray target 24 is
comprised of tungston. In the process of deceleration, the
accelerated electrons bombarding the x-ray target 24 give up energy
in the form of x-rays which are directed toward a primary
collimator 25. The primary collimator 25 is supported by the
support assembly generally designated at 26. The primary collimator
25 generally comprises a mass of depleted uranium having a
truncated conical hole 27 symmetrically formed therein about a
longitudinal axis 30 which extends through the center of the x-ray
target 24.
An ion chamber 35 having a primary filter 36 connected thereto are
operatively connected in spaced relationship with the primary
collimator 25 such that the primary filter 36 is symmetrically
aligned about the longitudinal axis 30. In the preferred
embodiment, the primary filter 36 has a symmetrical spherically or
hyperbolically shaped surface directed into the oncoming path of
the x-rays.
A pair of upper collimating jaws 40 (one of which is illustrated in
FIG. 1) are symmetrically opposed about the longitudinal axis 30
for providing secondary collimation of the x-rays. The upper
collimating jaws 40 are operatively connected (not illustrated) for
movement toward and away from the longitudinal axis 30 in a
direction normal to the plane of the paper when viewed as in FIG.
1. A pair of lower collimating jaws 42 are symmetrically disposed
about the longitudinal axis 30 for providing further collimation of
the x-rays. The lower collimating jaws 42 are operatively connected
by apparatus (not illustrated) for movement in the plane of the
paper as viewed in FIG. 1 at right angles to the longitudinal axis
30. The upper and lower collimating jaws 40 and 42 respectively,
are typically comprised of depleted uranium and define the outer
periphery of a beam of the x-rays, projected downwardly therefrom
and generally designated by the number 50 in FIG. 1.
A housing cover 51 encloses the ion chamber 35, the primary filter
36, and the collimating jaws 40 and 42 and the movement and
alignment apparatus (not illustrated) associated therewith. An
opening 52 in the bottom portion of the housing 51 defines an
emergence outlet therefrom for the x-ray beam 50. That portion of
the linear accelerator apparatus, sequentially including the
electron gun 21 through the emergence outlet 52, is often referred
to as the "head" of the linear accelerator.
A pair of mounting brackets 55 are connected to the housing 51
adjacent the outlet 52 for holding accessories. In FIG. 1, the
brackets 55 are illustrated as holding a first shadow tray 56 and
an external beam flattening filter apparatus, generally designated
at 58, comprising my invention.
The linear accelerator apparatus 20 illustrated in FIG. 1 and that
for which the external beam flattening filter apparatus 58 of the
preferred embodiment was specifically designed, is the "Clinac 4"
manufactured by Varian Associates (Radiation Division) and
described in their brochure "Rad 1568C 3M 4-69". The primary filter
36 employed within the "Clinac 4" was described in a paper by
Henning Hensen at the annual meeting in Houston, Texas, in 1971 of
the American Association of Physicists in Medicine.
A more detailed illustration of the external beam flattening filter
apparatus 58 is illustrated in FIGS. 2 and 3. Referring to FIGS. 2
and 3, there is generally illustrated a mounting base 60, of
plexiglass material in the preferred embodiment, sized for accurate
alignment and positioning within the mounting brackets 55 of the
linear accelerator 20. The mounting base is of sufficient thickness
and rigidity to prevent bending under its own weight when held by
its edges in the mounting brackets 55. In the preferred embodiment,
the filter member 62 is comprised of brass. However, other solid
materials having known x-ray permeability properties may be
employed within the spirit and intent of this invention.
A filter member, generally designated at 62, is accurately
positioned and affixed to the base 60. In the preferred embodiment,
the filter member 62 is generally disc shaped and is symmetrical
about a central axis 64 extending perpendicular to the general
plane of the filter member 62. The filter member 62 is accurately
positioned upon the mounting base 60, such that the central axis 64
will be aligned colinear with the longitudinal axis 30 of the x-ray
beam 50 when the base 60 is operatively secured by the mounting
brackets 55 of the linear accelerator 20.
The filter member 62 generally has a planar lower surface 62a
directly attached to and forming an interface with the upper
surface of the base member 60, and a geometrically precisely
configured upper surface 62b. The upper surface 62b is
symmetrically configured about the central axis 64, and in the
preferred embodiment is configured to precisely define a thickness
of brass between the upper and lower surfaces 62b and 62a for
providing uniform radiation intensity across an entire field of the
x-ray beam 50, for the "Clinac 4" type linear accelerator.
In the description to follow, it will be understood that although
the filter member 62 will be described in terms of a plurality of
segments or portions thereof, this manner of description is for the
purposes of defining the particular unique goemetrical
configuration of the filter member's upper surface 62b, and that
the entire filter member 62 comprises a single integral unit.
The upper geometrically configured surface 62b of the filter member
62 generally comprises a first flat ring-like surface 65, defining
a washer shaped volume of uniform cross-sectional thickness
relative the lower surface 62a and symmetrically disposed about the
central axis 64. A second downwardly sloping surface 66 radially
extends from the externally directed upper peripheral edge of the
ring-like portion 65 of the filter member 62 to the interface 62a
with the mounting base 60. The second surface 66 defines a second
volume of triangular cross-sectional area relative the lower
surface 62a. The external radially directed edge of the second
surface 66 of the filter member 62 also defines the external
periphery of the filter member. In the preferred embodiment, the
radially slanting peripherial surface 66 is formed at an angle of
25 degrees 15 minutes in the radial direction with respect to the
lower surface 62a of the filter member 62.
A third distinct segment of the upper surface 62b of the filter
member 62, generally designated at 67, comprises a downwardly
sloping surface radially extending from the internal peripheral
edge of the upper surface of the ring-shaped portion 65 of the
filter member, and forms an angle of 1 degree 49 minutes therewith.
The third surface 67 downwardly slopes in the direction of the
central axis 64 and defines, with the lower surface 62a of the
filter member 62, a symmetrical volume thereabout having a
trapezoidal cross-sectional area.
A fourth distinct surface area 68 of the upper surface 62b of the
filter member 62 is contiguous with the internally directed edge of
the third surface 67 thereof, and extends radially inward therefrom
toward the central axis 64. The fourth surface slopes downwardly
toward the central axis 64 forming an angle of 2 degrees 4 minutes
with the plane of the first surface 65 of the filter member 62. The
fourth surface 68 defines with the lower surface 62a, a volume of
triangular cross-sectional area symmetrically disposed about the
central axis 64, whose internally directed peripheral edge defines
a hole 69, of the filter member 62. The hole 69 is axially aligned
with the central axis 64.
The dimensions for that preferred embodiment of the beam flattening
filter apparatus 58 which have been formed to be specifically
adapted for use with the "Clinac 4" linear accelerator are detailed
in FIGS. 2 and 3.
A graph of the typical isodose distribution curves within the
treatment area for that type of linear accelerator 20 illustrated
in FIG. 1 which employs the primary filter 36, is illustrated in
FIG. 4. The use of isodose curves is conventionally employed in the
art to define radiation intensity distributions, and will not be
detailed herein. The isodose curves represent the convention of
defining 100% of dose on the longitudinal central axis of the x-ray
field and at a point on the axis spaced a known distance from a
phantom surface 72, and called "dose maximum". In FIG. 4, the dose
maximum is designated at 76.
In the preferred embodiment, the dose maximum 76 is located on the
longitudinal central axis 30 a distance of 81.2 cm from the x-ray
target 24. All further percentages of dose are referenced to the
100% of dose position (i.e., to dose maximum 76). Therefore, that
point designated at 5 cm in FIG. 4 on the axis 30, is in reality 85
cm from the x-ray target 24, and 5 cm below the phantom surface
72.
Several of the x-ray fields used in treatment of a patient are
represented by horizontal dashed lines at 77 in the graph of FIG.
4. It will be understood that an infinite number of such fields
exist within the treatment area. The peripheral edge of each
isodose curve is defined by the x-ray beam edge 50, and has
associated therewith a penumbra, generally designated at 75,
representing the typical shadowing effect of incomplete collimation
of the x-ray beam 50. It will be understood that the isodose curves
illustrated in FIGS. 4 and 5 represent the variation in radiation
intensity distribution across the x-ray beam 50 at those various
distances (in cm) from the dose maximum 76, and that the isodose
curves are merely a two-dimensional representation of the radiation
intensity distribution across the three-dimensional x-ray beam.
It will also be understood that the size of the x-ray field
(designated as 30 cm .times. 30 cm in FIG. 4) is that
cross-sectional area of the x-ray beam 50 as measured at the x-ray
field passing through the phantom surface 72. The size
(cross-sectional dimensions) of those x-ray fields 77 placed
downstream in the x-ray beam 50 from the dose maximum 76 will
necessarily increase in size, according to the inverse square law,
with respect to their axial distance from the x-ray target 24.
In radiation therapy, it is highly desirable to have uniform
radiation intensity distribution across an entire x-ray field at
any depth of treatment. Without the primary filter 36 intercepting
the x-ray beam 50, the isodose curves would be highly non-uniform
across any field 77 of the beam within the treatment area. The
primary filter 36 provides some degree of uniformity in radiation
intensity distribution across the fields 77 in the treatment area.
However, it does not provide that degree of uniformity (flatness)
required across large x-ray fields in the treatment area. Referring
to FIG. 4, it will be noted that while the top (100%) isodose curve
has a radiation intensity of 100% of dose at the position of dose
maximum 76 on the longitudinal central axis 30, the radiation
intensity near the edges of the beam 50 of an x-ray field passing
through the dose maximum 76 is approximately 117% of dose.
Therefore, any tissue exposed to the higher radiation at these
outer edges of the field could be subjected to radiation burns. A
similar non-uniformity in the radiation intensity distribution
across the x-ray fields, at all treatment depths will be noted from
FIG. 4. Besides endangering surface burns of skin and subcutaneous
tissue of a patient being treated, such an isodose curve
distribution causes non-uniform radiation intensity to be applied
to the specific tissues being treated at the lower treatment
depths. Submaxillary nodes, salivary glands, the thyroid gland and
sternal bone marrow could receive excessive radiation doses
expecially if the radiotherapist were accustomed to think in terms
of 100% of dose occurring on the longitudinal central axis of the
large field.
FIG. 5 is a graph of the same isodose curves for that type of
linear accelerator 20 illustrated in FIG. 1, with the beam
flattening filter apparatus 58 of this invention employed
therewith. The precisely contoured upper surface 62b of the filter
member 62 of the beam flattening apparatus 58 removes that
non-uniformity of radiation intensity across the x-ray treatment
fields 77 otherwise present at all depths of treatment, as
previously illustrated in FIG. 4.
Referring collectively to FIGS. 2, 3 and 5, and assuming that the
beam flattening filter apparatus 58 of this invention has been
properly mounted in the path of the x-ray beam external of the
emergence outlet 52, it will be noted that the hole 69 at the
center of the filter member 62 allows unimpeded passage of x-rays
therethrough along and adjacent the longitudinal axis 30.
Therefore, the 100% of dose radiation intensity characteristic of
the linear accelerator 20 will remain at the same dose maximum
position as previously described. The fourth and third surfaces 68
and 67 respectively of the upper surface 62b of the filter member
62, are respectively sloped at predetermined angles to compensate
for that portion of increasing radiation intensity across the
treatment fields 77, generally designated at 80a in FIG. 4. The
first washer-shaped portion 65 of the filter member 62 is of
uniform cross-sectional thickness to provide uniform attenuation to
the x-ray beam 50 at that portion thereof generally designated at
80b in FIG. 4. The second portion 66 of the filter member 62 is
shaped to compensate for the rapid decrease in radiation intensity
near the peripheral edges of the x-ray beam, generally designated
at 80c in FIG. 4. The result of placing the beam flattening filter
apparatus 58 of the preferred embodiment so as to intercept the
x-ray beam 50 of a "Clinac 4" linear accelerator 20, is that the
radiation intensity distribution across the entire beam is
"flattened" to provide uniform radiation intensity across an entire
x-ray field at any depth of treatment as illustrated by the isodose
curves of FIG. 5. In the preferred embodiment, the beam flattening
filter apparatus 58 is spaced approximately 50 cm. from the x-ray
target 24. The beam flattening filter apparatus of the preferred
embodiment, above described, produces less than 3% radiation
intensity variation over a 30 cm .times. 30 cm field at the
position of dose maximum.
While I have disclosed a specific embodiment of my invention, it
will be understood that this is for the purpose of illustration
only, and that my invention is to be limited soley by the scope of
the appended claims.
* * * * *