Method and apparatus to control photon beam dose enhancements

A photon beam dose enhancement is controlled by configuring a topical magnetic field, the magnetic field configuration having a magnetic field component across the beam path and having a magnetic field gradient component along the beam path which cause the dose enhancement, the dose enhancement being changeable during beam use by changing the magnetic field configuration during beam use, wherein the topical magnetic field can be produced by an array of magnet coils.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED 
RESEARCH AND DEVELOPMENT 
BACKGROUND OF THE INVENTION 
The invention is used to control dose enhancements along a photon beam path 
by control of the magnetic field configuration of a topical magnet, the 
magnetic field configuration having a magnetic field component across the 
photon beam path and having a magnetic field gradient component along the 
photon beam path. 
Since the advent of radiation systems workers have long been seeking 
methods and devices to control dose enhancements, where a dose enhancement 
is the ratio of radiation dose in a target volume relative to the 
radiation dose outside of the target volume. For example, one of the 
fundamental problems in the treatment of many forms of cancer using beams 
of high energy photons (mainly in the range 1 MEV to 60 MEV) from 
accelerators and other sources is the limited success of current 
techniques for delivering appropriate levels of dose to a diseased region 
while sparing surrounding healthy tissue. 
The dose generated by a high energy photon beam comes from the loss of 
energy of Compton and pair production electrons in an electron-photon 
cascade generated by the photon beam. (The differences in charge and 
particle interactions between electrons and positrons are minimal in the 
phenomena relied on here, so positrons created by pair production are 
called simply electrons here.) The electron-photon cascade follows the 
photon beam progression, and scattering into the penumbral region around 
the beam is usually acceptably small. Thus, healthy regions lying in 
directions transverse to the beam direction can usually be protected by 
shaping the photon beam cross-section by means of absorber blocks and 
related techniques. In addition, the targeted region, when possible, is 
irradiated from various directions so as to spare any particular region of 
surrounding healthy or especially radiosensitive tissue from the full 
destructive impact of the treatment. For a photon beam incident from any 
given direction, however, there heretofore has been no effective means of 
minimizing damage to healthy tissue not in the target volume. No 
suggestions have been made that dose enhancements along uncharged photon 
beams could be controlled by control of the magnetic field configuration 
of a topical magnet, the magnetic field configuration having a magnetic 
field component across the photon beam path and having a magnetic field 
gradient component along the photon beam path. 
Suggestions for improving the dose distribution along a charged particle 
beam by use of magnetic fields have been made. In C. C. Shih, "High Energy 
Electron Radiotherapy in a Magnetic Field," Medical Physics, Vol. 2, No. 
1, January/February 1975 calculations are reported which suggest that an 
electron beam dose distribution could be improved in the uniform magnetic 
field of a large magnet. In Whitmire, D. P., Bernard, D. L., Peterson, MD, 
and Purdy, J. A., "Magnetic Enhancement of Electron Dose Distribution in a 
Phantom," Medical Physics, Vol. 4, No. 2, March/April 1977 measurements of 
dose in a phantom in the uniform magnetic field of a large magnet are 
reported which also suggest that an improved dose distribution could be 
achieved by these means. 
Similar work is reported in Nath, R. and Schulz, R. J., "Modification of 
Electron-beam Dose Distributions by Transverse Magnetic Fields," Medical 
Physics, Vol. 5, No. 3, May/June 1978; in Whitmire, D. P. Bernard, D. L. 
and Peterson, M.D., "Magnetic Modification of the Electron-Dose 
Distribution in Tissue and Lung Phantoms," Medical Physics, Vol. 5, No. 5, 
September/October 1978; in Paliwal, B. R., Wiley, Jr., A. L., Wessels, B. 
W. and Choi, M. C., "Magnetic Field Modification of Electron-beam Dose 
Distributions in Inhomogeneous Media," Medical Physics, Vol 5, No. 5 
September/October 1978; and in Paliwal, B. R., Thomadsen, B. R. and Wiley, 
Jr., A. J., "Magnetic Modification of Electron Beam Dose Distributions," 
Acta Radiological Oncology, Vol. 18, 1979 Fasc. 1. 
None of these workers suggest that dose enhancements along a photon beam 
could be controlled by controlling the configuration of a topical magnet 
magnetic field having a magnetic field component across the photon beam 
and having a magnetic field gradient component along the photon beam. The 
1978 Whitmire paper mentions an increase in dose from a photon beam at the 
surface of a phantom in their magnetic field and a decrease in dose at the 
bottom of their phantom. Their discussion of this observation teaches away 
from control of dose enhancements by control of the configuration of the 
magnetic field of a topical magnet. 
In Weinhous, M. S., Nath, R. and Schuylz, R. J., "Enhancement of Electron 
Beam Dose Distributions by Longitudinal Magnetic Fields: Monte Carlo 
Simulations and Magnet System Optimization," Medical Physics, Vol. 12, No. 
5 September/October 1985 and in Bielajew, A. F., "The Effect of Strong 
Longitudinal Magnetic Fields on Dose Deposition from Electron and Photon 
Beams," Medical Physics, Vol. 20, No. 4, July August 1993 calculations are 
reported to suggest that large uniform magnetic fields along the beam axis 
would reduce the scattering of electrons laterally out of the beam. In the 
case of the photon beam, the electrons in the electron-photon cascade 
which are scattered transverse to the beam are kept in the beam, thereby 
somewhat reducing the dose in the penumbral region around the beam. Their 
discussion of this effect teaches away from using a topical magnet with a 
gradients along a photon beam path. 
SUMMARY OF THE INVENTION 
Objects of this invention comprise requirements listed in the following 
imperatives. Control dose enhancement along a photon beam by control of 
the configuration of a magnetic field produced by a topical magnet. 
Configure the magnetic field to a magnetic field configuration with a 
magnetic field component across the photon beam and with a magnetic field 
gradient component along the beam which cause the dose enhancement. During 
use of the beam configure the magnetic field to a second magnetic field 
configuration with a second magnetic field component across the beam path 
and with a second magnetic field gradient component along the beam path 
which cause a second dose enhancement. Use an array of magnet coils to 
make the topical magnet. Make the array of magnet coils an array of magnet 
coils on a planar surface and alternatively on a non-planar surface. Make 
the topical magnet support a method where a dose enhancement is chosen and 
the magnetic field is configured to cause the dose enhancement and where a 
second dose enhancement is chosen and during use of the beam the magnetic 
field configuration is changed to cause the second dose enhancement. 
Other objects will be comprehended in the drawings and detailed 
description, which will make additional objects obvious hereafter to 
persons skilled in the art. 
In summary, one embodiment of this invention is a dose enhancement device 
used with a photon beam source which produces a photon beam which produces 
an electron-photon cascade along the beam, the dose enhancement control 
device being at least one topical magnet having a magnetic field 
configuration with a magnetic field component across the beam and with a 
magnetic field gradient along the beam which cause a dose enhancement, the 
dose enhancement being controlled by control of the magnetic field 
configuration. 
Other equivalent embodiments will be comprehended in the drawings and 
detailed description, which will make additional equivalent embodiments 
obvious hereafter to persons skilled in the art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The radiation system shown in FIG. 1 has a photon beam source 21 which 
produces an incident photon beam along a beam path, the beam path being 
defined by all of the paths of the incident photons in the beam. Though 
this beam path, can have a complicated cross section, a beam vector 101 
can be chosen to represent the beam path. The photon beam is indicated by 
the point 22 on the beam vector 101. The beam vector 101 enters a body 23 
at the point 24 and the incident photons generate an electron-photon 
cascade along the beam path, the electron-photon cascade being indicated 
by the point 25 on the beam vector 101 in the body. 
At the energies of interest here the path of the electron-photon cascade, 
being the collection of the paths of the particles in the electron-photon 
cascade, can be considered to follow along the incident photon beam path. 
Thus, the electron-photon cascade can also be represented by the beam 
vector 101, so that beam path here means both the incident photon beam 
path and the beam path of the electron-photon cascade. 
The topical magnet 11 shown in FIG. 1 is also shown in FIG. 2 with a cross 
section of the magnetic field lines indicated. The component 10 of the 
magnetic field which is in the plane including the magnet central axis 102 
is shown in FIG. 3 along a vector 104 which passes 4.5 cm in the 102 
direction from the magnet center. Subtracting 1 cm of magnet along 102 and 
0.5 cm of magnet cryostat along 102 puts axis 104 at 3 cm into the body 
23. The magnetic field component 10 has a magnetic field gradient 
component 10a along axis 104, which is shown in FIG. 4. These curves were 
calculated using well validated code. 
When the beam vector 101 coincides with the vector 104 then 10 is a 
magnetic field component across the beam vector 101, and thus across the 
beam path, and 10a is a magnetic field gradient component along the beam 
vector 101, and thus along the beam path. Across the beam path means 
perpendicular to the velocities of the incident photons. Along the beam 
path means perpendicular to across the beam path. 
In the case where the beam vector 101 coincides with the 104 vector, an 
electron-photon cascade proceeding along the 104 vector encounters a steep 
positive magnetic field gradient component 10a along the beam path, the 
steepest point of which is indicated by 13d in FIG. 4. The location of 
this steepest positive gradient is also indicated as point 13c in FIG. 3, 
as point 13b in FIG. 2, and, in this case where the beam vector 101 
coincides with the 104 vector, is indicated as 13a in FIG. 1. 
The dose produced by the electrons in the electron-photon cascade can be 
calculated for the case with no magnetic field and for the case with a 
magnetic field using the standard and well validated EGS-4 code. The ratio 
of the dose for the case with a magnetic field relative to the dose for 
the case with no magnetic field is the relative dose. A relative dose 
profile is all the values of the relative dose along a beam vector such as 
101. The relative dose profile 30 from a 2 cm diameter, 24 MEV photon beam 
along the beam vector 101 coinciding with the 104 vector is shown in FIG. 
5. 
As a photon beam enters a body an electron-photon cascade is generated. The 
dose builds up as more and more electrons enter the cascade by the Compton 
effect and by pair production. (Again, the differences in charge and 
particle interactions between electrons and positrons are minimal in the 
phenomena relied on here so positrons created by pair production are 
called simply electrons here.) This build up can reach a quasi-equilibrium 
where the energy carried by electrons into given volume is equal to the 
energy carried by electrons out of the volume. 
As the electrons near the location of the steepest positive magnetic field 
gradient 13a, at 9.5 cm in FIG. 5, then the increasing Lorentz force from 
the magnetic field component across the beam path (10 in this case) causes 
the electron paths to rapidly tighten into decaying spirals and these 
electrons contribute to the increased relative dose, indicated by the peak 
31 in the relative dose profile 30. The Lorentz force from the magnetic 
field component across the beam path also causes electrons which enter the 
cascade to rapidly tighten into decaying spiral paths and also contribute 
to the increased relative dose. 
The dose then declines along vector 101 because only electrons just 
entering the electron-photon cascade are available to contribute to the 
dose. A minimum in the relative dose, indicated by the valley 32, in the 
relative dose profile 30 occurs in the vicinity of the steepest negative 
magnetic field gradient component 14. Further along the beam vector 101 in 
the valley 32 the decreasing Lorentz force releases electrons from the 
spiral paths and the electron-photon cascade again builds up. 
At a point along a beam vector, 101 for example, chosen to represent the 
beam, the dose comes from the electron-photon cascade in the beam passing 
the point, the beam being defined by all of the paths of the incident 
photons. The relative dose at a point along a beam vector chosen to 
represent the beam is the ratio of the dose at the point with a magnetic 
field relative to the dose at the point without any magnetic field. 
The relative dose profile, 30 for example, is the curve showing the 
relative dose at all points along the beam vector in the body. In the 
relative dose profile 30 the dose received in a target volume located 
about 10 cm into the body where the relative dose peaks 31 is about 45% 
greater with the magnetic field than without any magnetic field. 
Similarly, the dose received in a protected volume located about 19 cm 
into the body where the dose bottoms 32 is about 35% less with the 
magnetic field than without any magnetic field. (The central axis 102 of 
the magnet passes through a point located at 15.5 cm into the body.) 
The dose enhancement is the net ratio of the highest dose in a target 
volume relative to the lowest dose in a protected volume, which is the 
difference between the highest relative dose, 31 for example, and the 
lowest relative dose, 32 for example. A relative dose profile thus 
comprises the dose enhancement and the locations of the greatest relative 
dose and the least relative dose. 
The dose enhancement shown in FIG. 5 is about two. This means that the dose 
received in a target volume located about 10 cm into the body along the 
beam vector 101 where the relative dose peaks 31 is about twice the dose 
received in a protected volume located about 19 cm into the body along the 
beam vector 101 where the relative dose bottoms 32. 
When the beam vector 101 coincides with the 104 vector, then the magnetic 
field component across the beam path is that shown as 10 in FIG. 3, the 
magnetic field gradient component along the beam path is that shown as 10a 
in FIG. 4, and the relative dose profile is that shown as 30 in FIG. 5. 
The portion of the relative dose profile which is greater than unity can 
be located outside a target body in a second body in front of the target 
body so that the protected volume is at the surface of the target body. 
Also, the portion of the relative dose profile less than unity can be 
chosen within in the target body. 
When the beam vector does not coincide with the 104 vector, such as the 
beam vector 103 shown in FIG. 1 which is skewed relative to the 104 
vector, but there is still a magnetic field component across the beam path 
and a magnetic field gradient component along the beam path, then the 
relative dose profile can have a peak followed by a valley like that shown 
in FIG. 4, can have a peak only (for example when the negative magnetic 
field gradient component occurs outside the body), can have a valley only 
(for example when the peak occurs in a second body located before the beam 
enters the body), and even can have a valley followed by a peak (for 
example when the beam axis is parallel to the 102 axis). 
The topical magnet depicted in FIG. 2 has a thickness of 2 cm and an 
overall radius of 5.5 cm. It is an array of five concentric coils on a 
planar surface, with the inner and outer radii of the first coil being 1 
cm and 1.5 cm, of the second coil being 1.5 cm and 2.5 cm, of the third 
coil being 2.5 cm and 3.8 cm, of the fourth coil being 3.8 cm and 4.5 cm, 
and of the fifth coil being 4.5 cm and 5.5 cm. The magnetic field and 
magnetic field gradients depicted in FIG. 3 and FIG. 4 and the dose 
enhancement depicted in FIG. 5 were calculated for this magnet with the 
coils fabricated of Nb3Sn wire which can carry 2,000 Amps per square 
millimeter at 2.2 degrees Kelvin and can sustain 14 Tesla fields without 
quenching. This magnet can produce a magnetic field component across the 
beam path of slightly over 2 Tesla and with a magnetic field gradient 
component along the beam path of just under 1 Tesla per centimeter at the 
point 13b in FIG. 2 which is 3 cm into the body 23 along the magnet 
central axis 102. 
In tissue, the range in centimeters of an electron is approximately 1/2 the 
kinetic energy of the electron in MEV. Thus a 10 MEV electron has a range 
of 5 cm and a 20 MEV electron has a range of 10 cm. The Lorentz force from 
a magnetic field perpendicular to the velocity of an electron deflects the 
electron to a decaying spiral path with an initial radius which is 
approximately the kinetic energy of the electron divided by three times 
the magnetic field in Tesla. Thus the 10 MEV electron which had a range of 
5 cm with no magnetic field has a decaying spiral path with an initial 
radius of about 1.67 cm in the 2 Tesla field. 
For a given energy photon beam, given beam cross section area, and given 
beam divergence, the relative dose profile, such as 30, is determined by 
the magnetic field configuration of a topical magnet, such as the topical 
magnet 11. This magnetic field configuration is determined by the shape of 
the topical magnet, by the location of the topical magnet relative to the 
beam path, by the orientation of the topical magnet relative to the beam 
path, by the shapes, locations, and orientations of magnet coils 
comprising the topical magnet, and by the currents in the coils. 
Thus a relative dose profile from a photon beam can be controlled by 
control of a magnetic field configuration relative to the beam path. This 
magnetic field configuration is controlled by control of the position of a 
topical magnet, control of the orientation of the topical magnet, by 
control of the currents in magnet coils comprising the topical magnet, and 
by control of shapes, locations, orientations, and currents of the coils. 
Thus, photon beam users can choose a relative dose profile and configure a 
topical magnet magnetic field to produce that relative dose profile. 
The relative dose profile can be changed to alternative relative dose 
profiles during beam use by changes of the magnetic field configuration. 
For example, a second relative dose profile can be caused by a second 
magnetic field configuration having a second magnetic field component 
across the beam path and having a second magnetic field gradient component 
along the beam path. This can be done during a beam exposure and between 
exposures when the beam use comprises a series of beam exposures. Control 
of the relative dose profile by control of the configuration of a topical 
magnet magnetic field and change of the relative dose profile during beam 
use does not interfere with any other devices and methods used with a 
photon beam. Thus, photon beam users can now choose a relative dose 
profile and can choose changes of the relative dose profile during beam 
use, both choices specifically tailored to the needs of the beam use. 
Results similar to those shown and described above for topical magnet 11 
are obtained with other topical magnets. Topical magnets can be made of 
single coils and of various arrays of magnet coils to produce useful 
magnetic field configurations. Magnet coils can be arrayed in concentric 
arrays as shown in FIG. 2 and can be arrayed in non-concentric arrays. 
Coils can be arrayed in concentric and non-concentric arrays on planer 
surfaces such as in FIG. 2 and on non-planar surfaces. Coils in these 
arrays can be spatially separated. Arrays of superconducting magnet coils 
small enough so that, along with their cryostats, they can be placed 
inside a living body have been devised. 
Other equivalent forms for the topical magnet and other equivalent ways to 
configure the magnetic field will be obvious hereafter to persons skilled 
in the art. Therefore this invention is not limited to the particular 
examples shown and described here.