U.S. patent number 3,886,367 [Application Number 05/434,458] was granted by the patent office on 1975-05-27 for ion-beam mask for cancer patient therapy.
This patent grant is currently assigned to The United States of America as represented by the United States Energy. Invention is credited to John G. Castle, Jr..
United States Patent |
3,886,367 |
Castle, Jr. |
May 27, 1975 |
Ion-beam mask for cancer patient therapy
Abstract
An ion-beam mask has been provided for spatially distributing
fast ions used to irradiate tumors so that the region near the
Bragg peak receives a uniform radiation dose while the tissue at
the point of entrance of the beam has a web of unirradiated volume
to promote healing of the surface tissue following radiotherapeutic
treatment of embedded tumors.
Inventors: |
Castle, Jr.; John G.
(Huntsville, AL) |
Assignee: |
The United States of America as
represented by the United States Energy (Washington,
DC)
|
Family
ID: |
23724320 |
Appl.
No.: |
05/434,458 |
Filed: |
January 18, 1974 |
Current U.S.
Class: |
250/505.1;
378/145; 976/DIG.429; 976/DIG.442 |
Current CPC
Class: |
A61N
5/10 (20130101); G21K 5/04 (20130101); G21K
1/025 (20130101); A61B 6/10 (20130101); A61N
2005/1087 (20130101); A61N 2005/1095 (20130101) |
Current International
Class: |
A61B
6/10 (20060101); G21K 5/04 (20060101); G21K
1/02 (20060101); A61N 5/10 (20060101); G21f
001/00 () |
Field of
Search: |
;250/494,496,505,511-514,520 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Assistant Examiner: Nelms; D. C.
Attorney, Agent or Firm: Horan; John A. Zachry; David S.
Breeden; David E.
Government Interests
The present invention was made during the course of, or under, a
contract with the United States Atomic Energy Commission.
Claims
What is claimed is:
1. A radiation mask for mitigating the surface tissue effects of a
radiotherapeutic treatment of embedded tumors in a patient by means
of a beam of fast ions from a beam source, comprising:
a block of radiation shielding material positioned in said beam
intermediate the patient and said beam source, said block having a
surface area larger than the cross section of said beam and a
thickness greater than the range of ions of said beam in said
shielding material, said shielding material block having a
plurality of circular cross section apertures extending
therethrough in a direction parallel to the rays of said beam and
arranged in an orderly, evenly spaced array, each of said apertures
having a diameter (d) and spaced with a wall thickness (w)
therebetween defined by d = w = F.sigma..sub.e, with f being in the
range of from 1 to 2 and where .sigma..sub.e is the effective
dispersion of the ions passing through said apertures at the depth
of said embedded tumor to attain sufficient overlap of discrete
beam portions passing through adjacent ones of said apertures at
the depth of the Bragg peak for said beam source coincident with
the tumor region, thereby providing a lateral uniformity of
radiation dose over the irradiated area of said tumor while
shielding a substantial are of skin and intervening tissue.
2. The radiation mask as set forth in claim 1 wherein said beam of
fast ions is protons in the range of from 50 to 200 MeV and said
shielding material is carbon.
3. The radiation mask as set forth in claim 2 wherein the apertures
in said shield are arrayed in a hexagonal pattern.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to radiotherapy and more
specifically to a shielding mask for use in ion-beam radiotherapy
which allows distributed irradiation of an embedded tumor while
shielding a substantial portion of the surface volume tissue.
A treatment of choice for cancer patients having embedded tumors is
irradiation of the tumors with a beam of fast ions such as by a
beam of 65 MeV protons produced by the Oak Ridge Isochronous
Cyclotron (ORIC). Such a treatment frequently results in
destruction of the malignant cells with a consequent remission of
the cancer. The dose to be delivered to a specified target tumor
region is usually prescribed to be uniform throughout the target
volume to well within .+-.5 percent. The desired uniformity of
accumulated dose laterally across the target volume is readily
attained, as indicated, for example, by R. S. Bender's measurements
on the proton beam at ORIC as reported in the Annual Progress
Report of the Electronuclear Division of the Oak Ridge National
Laboratory, Oak Ridge, Tenn., for 1967, on pages 101-102, by using
only the control portion of a wide beam of fast ions, masked to
cover the lateral extent of the target volume. The desired
uniformity longitudinally through the depth of the target volume is
readily obtained by modifying the range of the ions for certain
fractions of the total exposure time. Unfortunately, healthly
tissues between the outer epidermis and the tumor itself must be
exposed to the damaging fast protons as they seek out the target
tumor.
SUMMARY OF THE INVENTION
In view of the above, it is an object of this invention to provide
an ion beam mask for mitigating surface tissue effects of
radiotherapeutic treatment of embedded tumors which will permit a
beam of fast ions, such as protons to penetrate to the tumor and
deliver with the desired uniformity over the lateral extent of the
target volume at each exposure any prescribed dose level but which
will, at the same time, leave a reasonable fraction of skin area
either unirradiated or with a dose level low enough to enable the
skin and dermal regions to more readily recover from the
deterimental effects produced by the heavy charged particles.
Other objects and many of the attendant advantages of the present
invention will be obvious from the following detailed description
taken in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a radiotherapy system
employing the ion-beam mask according to the present invention.
FIG. 2 is a planar view of the ion-beam mask shown in FIG. 1.
DETAILED DESCRIPTION
The subject invention is best illustrated by referring to the
drawings. FIG. 1 shows a cross section of one example of the
subject ion-beam mask positioned over tissue having a tumor
embedded therein. The mask is shown generally at 10. A beam of fast
ions, such as protons from a source 8 in the range of from 50 to
200 MeV, enters from the direction of arrows 12. Mask 10 is
preferably made of carbon (graphitized and purified) and has an
array of holes 14 drilled therethrough to permit passage of the
ions to the surface of the skin 16 whereupon the ions proceed to
the target tumor 18. The masked portions of the surface of skin 16
will receive less of a dose of ionizing radiation with the mask in
place than they would if no carbon shield were interposed between
the patient and the beam. The purpose, therefore, of breaking the
beam into many discrete portions whereby a large area of skin and
intervening tissue are protected from ionizing radiation can be
achieved by means of the subject design provided that the thickness
of the mask 10 exceeds sufficiently the range of the ions in the
mask material. Although a carbon mask is preferred, other suitable
masking materials, such as dense plastics, aluminum, and denser
metals, may be used.
FIG. 2 is a planar view of the device showing that the device
consists simply of a shield 10 of a material, such as carbon,
having parallel holes 14 drilled therein for collimation of the
beam which passes therethrough.
The stated object of this invention can be fulfilled by an
appropriate choice of the diameter (d) of the holes through the
mask and of the wall thickness (w) between the holes to attain
sufficient overlap between beamlets passing through adjacent holes
at the depth of the Bragg peak, approximately the ion range
(R.sub.o) in the patient's tissue. Optimal overlap between adjacent
beamlets and thereby optimal lateral uniformity of dose will occur,
for the example of each beamlet having a Gaussian spread whose
effective dispersion is .sigma..sub.e at the target depth of
interest, when the sum of d and w is made equal to 1.5
.sigma..sub.e. The lateral Gaussian spread of each beamlet may be
seen from the expression for the beam intensity I(x,y) at depth y
being I(x,y) = R(y) exp (-x.sup.2 /.sigma..sub.e.sup.2) where x is
the lateral coordinate (from beamlet center) at which I(x,y) is
specified. It should be noted that reasonable uniformity may be
attained with factors differing considerably from this ideal
Gaussian value of 1.5. For large depths, y, the effective
dispersion, .sigma..sub.e, is related to the hole diameter, d, and
the dispersion of the Gaussian spread associated with each ion's
motion through the tissue, .sigma..sub.o, by the expression from
the work of A. Koehler and W. Preston of the Harvard Cyclotron,
.sigma..sub.e = (.sigma..sub.o.sup.2 + d.sup.2 /4).sup. 1/2
where .sigma..sub.o gives the Gaussian spread for each fast ion's
lateral position as it moves through toward the end of its range R.
Measurements show .sigma..sub.o to be 0.031 R for protons moving
through water and 0.045 R for protons moving through aluminum. This
.sigma..sub.0 is large enough for ions reaching depths such as 20
cm to permit d and w to be optimized. The hole 14 illustrated in
FIG. 2 can be arrayed in a variety of configuration but it has been
found that the hexagonal pattern, as illustrated in FIG. 2,
provides a close packed array for the best uniformity at target
depth near R.
Consider the illustration of planning to deliver a certain dose at
depth R = 12 cm. The proton beam energy needed is 130 MeV for which
each proton's spread has .sigma..sub.o = 0.031 R = 0.37 cm in soft
tissue. For the optimal lateral uniformity, the sum d + w is chosen
equal to a factor F times the effective Gaussian dispersion of each
beamlet .sigma..sub.e at depth R to have the value of F = 1.5 for
Gaussian beams (d + w = F.sigma..sub.e). For round holes the
optimum uniformity at the proton range R.sub.o = R is attainable by
setting F = 1.5 and specifying d/w by the expression ##EQU1##
acceptable uniformity may be attainable with F values in the range
of about 1 to 2. The variety of consistent values of d and w
include one set of d = 0.60 cm and w = 0.12 cm and another set of d
= 0.18 cm hole diameter and w = 0.36 cm wall thickness.
To further illustrate the way in which d and w may be determined
from the above expressions, suppose the radiotherapist were to
relax his requirement of lateral uniformity from F = 1.5 (the
ideal) to F closer to 2, say F = 1.73, then the design above of a
suitable carbon mask gives a thickness of at least 8 cm and a
choice of hole diameter and corresponding wall thickness including
the sets of d = 0.037 cm for w = 0.37 cm and d = 0.76 cm for w =
0.12 cm.
This design procedure makes the subject invention compatible with
accurate location control, i.e., the beam can be accurately
repositioned during a series of exposures to effectively spare a
web of skin and still deliver a uniform dose to a deeply embedded
tumor.
In summary, the subject development is shown to break a beam of
fast ions given to cancer patients for therapeutic purposes into
separate and discrete rays whereby the region near the Bragg peak
(the end of the range of ions, in tissue, for example) has a
lateral dose uniform to within a few percent and whereby the skin
surface area and underlying tissues receive a discontinuous dosage
of irradiation thereby enabling the skin to heal more readily than
it would if exposed to a massive dose covering the entire area
interposed between the radiation source and the tumor.
* * * * *