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Claims  |
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What is claimed is:
1. A method for patient care comprising the steps of:
(a) localizing a target within a patient;
(b) subjecting the patient to an imaging system with a display to generate
patient reference data in the form of x,y,z coordinates of the target and
adjacent portions of the patient;
(c) supplying the patient reference data to a dosimetric computer connected
to said display;
(d) inputting into the dosimetric computer a proposed plan for applying at
least one beam of radiation to the patient from at least one source and
from at least two different directions;
(e) selecting a plane of the patient for display of a distribution of
radiation which would result from the proposed plan;
(f) displaying on said display a selected plane patient image corresponding
to the selected plane by operation of the imaging system;
(g) determining the distribution of radiation within the selected plane by
calculating the radiation dose at points spaced by distance D1 within a
fine dose grid relatively close to the target and calculating the
radiation dose at points spaced by distance D2, greater than D1, within a
coarse dose grid outside said fine dose grid, the dosimetric computer
performing the calculations for the radiation dose at a greater density of
points in the fine dose grid and at a lower density of points in the
coarse dose grid; and
(h) displaying data on the display from the radiation dose calculations.
2. The method of claim 1 wherein some data resulting from the radiation
dose calculations is placed on the patient image in the display.
3. The method of claim 2 wherein the proposed plan includes at least a
first arc through which the beam is applied to the patient, said first arc
being in a first arc plane, and wherein said inputting step includes the
substeps of:
selecting the first arc plane; displaying a first arc plane patient image
corresponding to the first arc plane on the display;
creating a bit map of pixels on the display to said x,y,z coordinates of
the patient reference data;
identifying each pixel which makes up an outside contour of the patient;
storing the beginning and end of the first arc;
drawing a line between the source at a given location and an isocenter
within the target;
finding the intersection pixel of the line and the pixels corresponding to
the outside contour;
referencing the bit map to identify the x,y,z coordinates of the
intersection pixel as a beam entrance point;
repeating the drawing, finding, and referencing substeps to identify
additional beam entrance points corresponding to different source
locations; and
storing the beam entrance points, these beam entrance points corresponding
to the center of the beam at different source locations.
4. The method of claim 3 wherein the stored beam entrance points are used
to perform the radiation dose calculations.
5. The method of claim 4 wherein the radiation dose calculations are
performed for each of said different source locations based on radiation
entering only at said beam entrance points such that edge effects are
ignored.
6. The method of claim 5 wherein the width of each beam used in the
proposed plan is less than 5 cm such that ignoring edge effects will not
introduce substantial errors.
7. The method of claim 6 further comprising the step of:
subjecting the patient to radiation in accord with the proposed plan.
8. The method of claim 4 further comprising the step of:
subjecting the patient to radiation in accord with the proposed plan.
9. The method of claim 1 further comprising the step of:
subjecting the patient to radiation in accord with the proposed plan.
10. A method for patient care comprising the steps of:
(a) localizing a target within a patient;
(b) subjecting the patient to an imaging system with a display to generate
patient reference data in the form of x,y,z coordinates of the target and
adjacent portions of the patient;
(c) supplying the patient reference data to a dosimetric computer connected
to said display;
(d) inputting into the dosimetric computer a proposed plan for applying at
least one beam of radiation to the patient from at least one source and
from at least two different directions;
wherein the proposed plan includes at least a first arc through which the
beam is applied to the patient, said first arc being in a first arc plane,
and wherein said inputting step includes the substeps of:
selecting the first arc plane;
displaying a first arc plane patient image corresponding to the first arc
plane on the display;
creating a bit map of pixels on the display to said x,y,z coordinates of
the patient reference data;
identifying each pixel which makes up an outside contour of the patient;
storing the beginning and end of the first arc;
drawing a line between the source at a given location and an isocenter
within the target;
finding the intersection pixel of the line and the pixels corresponding to
the outside contour;
referencing the bit map to identify the x,y,z coordinates of the
intersection pixel as a beam entrance point;
repeating the drawing, finding, and referencing substeps to identify
additional beam entrance points corresponding to different source
locations; and
storing the beam entrance points, these beam entrance points corresponding
to the center of the beam at different source locations.
11. The method of claim 10 further comprising:
selecting a plane of the patient for display of a distribution of radiation
which would result from the proposed plan;
displaying on said display a selected phase patient image corresponding to
the selected plane by operation of the imaging system;
determining the distribution of radiation within the selected plane; and
displaying data on the display resulting from the distribution
determination.
12. The method of claim 11 wherein the determination of the distribution is
accomplished by calculating the radiation dose at points spaced by
distance D1 within a fine dose grid relatively close to the target and
calculating the radiation dose at points spaced by distance D2, greater
than D1 within a coarse dose grid outside said fine dose grid; the
dosimetric computer performing the calculations for the radiation dose at
a greater density of points in the fine dose grid and at a lower density
of points in the coarse does grid.
13. The method of claim 12 wherein the stored beam entrance points are used
to perform the radiation dose calculations.
14. The method of claim 13 wherein the radiation dose calculations are
performed for each of said different source locations based on radiation
entering only at said beam entrance points such that edge effects are
ignored.
15. The method of claim 14 further comprising the step of:
subjecting the patient to radiation in accord with the proposed plan.
16. The method of claim 15 wherein the beam or beams applied to the patient
are less than 5 cm in width.
17. The method of claim 10 further comprising the step of:
subjecting the patient to radiation in accord with the proposed plan.
18. The method of claim 17 wherein the beam or beams applied to the patient
are less than 5 cm in width.
19. A method for patient care comprising the steps of:
(a) localizing a target within a patient;
(b) subjecting the patient to an imaging system with a display to generate
patient reference data in the form of x,y,z coordinates of the target and
adjacent portions of the patient;
(c) supplying the patient reference data to a dosimetric computer connected
to said display;
(d) inputting into the dosimetric computer a proposed plan for applying at
least one beam of radiation to the patient from at least one source and
from at least two different directions;
(e) selecting a plane of the patient for display of a distribution of
radiation which would result from the proposed plan;
(f) displaying on said display a patient image corresponding to the
selected plane by operation of the imaging system;
(g) determining the distribution of radiation within the selected plane by
calculating the radiation dose at points spaced by distance D1 within a
fine dose grid relatively close to the target and calculating the
radiation dose at points spaced by distance D2, greater than D1, within a
coarse dose grid outside said fine dose grid, the dosimetric computer
performing the calculations for the radiation dose at a greater density of
points in the fine dose grid and at a lower density of points in the
coarse dose grid; and
(h) outputting data from the radiation dose calculations.
20. The method of claim 19 further comprising the step of subjecting the
patient to radiation in accord with the proposed plan using a stereotactic
radiosurgery apparatus comprising:
a gantry supported for rotation about a gantry axis, the gantry having a
radiation-emitting head for movement in an arc in a radiation plane about
a center point corresponding to an intersection of the gantry axis and the
radiation plane, said gantry axis being normal to said radiation plane;
a collimator disposed to focus radiation from said radiation-emitting head
on said center point; and
collimator linking means for linking movement of said collimator to said
radiation-emitting head for automatic rotation of said collimator in said
radiation plane and about said gantry axis upon rotation of said gantry,
said collimator linking means allowing said collimator to track rotation
of said gantry with no or minimal transfer of positioning inaccuracies
from said gantry to said collimator. |
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Claims |
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Description |
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A portion of the disclosure of this patent document contains material
subject to copyright protection. The copyright owner has no objection to
facsimile reproduction by anyone of the patent document or the patent
disclosure, as it appears in the Patent and Trademark Office patent file
or records, but otherwise reserves all copyright rights whatsoever.
REFERENCE TO THE MICROFICHE APPENDIX
This application was filed with a computer program listing printout which
has been submitted in the form of a "microfiche appendix". The microfiche
appendix consists of four sheets of microfiche consisting of a total of
237 frames and is not a part of the printed patent.
BACKGROUND OF THE INVENTION
This invention relates generally to dosimetry for a radiosurgery system
employing multiple beams of radiation focused onto a stereotactically
localized target, and more particularly to a dosimetry technique which
quickly provides useful data for planning patient treatment.
In 1951, Dr. Lars Leksell coined the term "radiosurgery", to describe the
concept of focusing multiple beams of external radiation on a
stereotactically localized intracranial target. After experimentation with
standard X-ray treatment devices, proton beam, and linear accelerators, he
and his collaborators developed a device which is called the GAMMA KNIFE
(currently marketed by the Electra Corporation, Stockholm, Sweden). The
device consists of a hemispheric array, currently containing 201 Cobalt-60
sources. The radiation from each of these sources is collimated and
mechanically fixed, with great accuracy, on a focal point at the center of
the hemisphere. When a patient has a suitable lesion for treatment
(usually an intracranial arteriovenous malformation), it may be precisely
localized with another device called a stereotactic frame. Using the
stereotactic apparatus, the intracranial target is positioned at the focal
point of the GAMMA KNIFE. Since each of the 201 radiation pathways is
through a different area of the brain, the amount of radiation to normal
brain tissue is minimal. At the focal point, however, a very sizable dose
is delivered which can, in certain cases, lead to obliteration of the
lesion. This radiosurgical treatment is, in some instances, a much safer
treatment option than conventional surgical methods.
Several GAMMA KNIFE devices are currently being used worldwide for
stereotactic radiosurgery and have been used to treat approximately 1500
patients. The results of treatment, as well as many technical issues, have
been discussed in multiple publications. Several factors, however, have
impeded the widespread usage of this device. First, the device costs about
$2.2 Million Dollars, U.S. Second, the Nuclear Regulatory Commission has
ruled that this device cannot be shipped loaded in the U.S.A.
Consequently, loading must be done on site, necessitating the construction
of a portable hot cell. Third, the half life of Cobalt-60 is 5.2 years,
which requires reloading the machine, at great expense, every 5-10 years.
Fourth, the dosimetry system currently marketed with the device is
relatively crude, especially when utilized with more modern imaging
modalities such as CT scan and MRI scan.
An alternative method for radiosurgery involves irradiation of intracranial
targets with particle beams (i.e., proton or helium). In this instance,
one does not rely solely on multiple cross-fired beams of radiation. A
physical property of particle beams, called the "Bragg-peak effect",
allows one to deliver the majority of the energy of a small number of
beams (approximately 12) to a precisely predetermined depth. Multiple
publications regarding particle irradiation of intracranial lesions
(especially pituitary tumors and arteriovenous malformations) have
appeared in the literature. The results have not generally been as good as
those obtained with the GAMMA KNIFE. This may, however, be solely a
consequence of patient selection criteria. Particle beam devices require
the availability of a cyclotron. Only a few such high energy physics
research facilities exist in the world.
A third current radiosurgical method uses a linear accelerator (LINAC) as
the radiation source. As mentioned above, Leksell rejected the LINAC as
mechanically inaccurate. More recently, groups from Europe have reported
their methods for radiosurgery with LINAC devices. In the United States,
researchers at the Peter Bent Brigham Hospital in Boston have developed a
prototype LINAC system using highly sophisticated computer techniques to
optimize dosimetry. Thus far, approximately 12 patients have been treated
with good results. This LINAC system, however, suffers from certain
mechanical inaccuracies which have limited its use. In addition, the
computer dosimetry system employed is very time consuming, rendering the
treatment program inefficient.
Currently, there is great interest in radiosurgery. Although the GAMMA
KNIFE represents the "gold standard", its great expense and requirement
for frequent replenishment of radiation sources have discouraged most
potential users. The proton beam devices are never likely to be widely
available because of the requirement for high-energy particle beam source
(cyclotron). The linear accelerator offers an attractive alternative to
such devices. However, a major disadvantage of known linear accelerator
based systems is the need for time consuming (e.g., several hours)
computer calculations for determining the radiation distributions.
Before subjecting a patient to stereotactic radiosurgery, the tumor or
other target area within the patient must be localized. This may be
accomplished by stereotactic angiography or by CT (computer tomography)
localization. After the localization of the tumor or other target area, a
CT localizer (or an NMR imaging system) should be used on the patient,
even if the original localization was using stereotactic angiography. The
data from the CT scan and the angiographic films, if any, should be
transferred to a computer system used for calculating the dosage.
When applying radiation to a patient, it is important that the radiation be
concentrated on the target area and minimized for the patient's healthy
tissues. It is especially important that the radiation be minimized on
certain critical structures. For example, if using radiation treatment on
a patient's brain, it may be important that the radiation dosage applied
to the patient's optic nerves is minimized.
Before a physician applies the radiation to the patient, the physician may
decide on two or more arcs which will be used for applying the radiation
to the patient. In particular, the physician decides upon the plane in
which the radiation beam will be applied in an arc to the patient's target
area. The localization data and the proposed treatment arcs are input into
a dosimetric computer system. That computer system generates a value for
the radiation at each point in a grid extending throughout the patient's
skull (assuming that the radiation is for the treatment of a target area
within the brain). It is this process that is very time consuming and may
require over four hours of computer time. Specifically, the process
usually generates the value of the radiation dose at over 250,000 points
within the patient's skull. After the doctor has received the radiation
distributions from the computer, the doctor may decide that one or more
critical structures is receiving too much radiation. Alternately, the
doctor may decide that the target area is not receiving sufficient
radiation. At any rate, the doctor may be required to revise the arcs
through which the radiation source will travel in order to apply radiation
to the tumor. It would then be necessary to repeat the very time consuming
process of recalculating the radiation distribution.
Some prior dosimetric computer systems have been designed in which the
radiation distribution may be calculated and shown or supplied for a
smaller volume than the complete volume of the patient's skull. These type
of systems require that one repeatedly indicate the area or volume for
which the radiation distribution is desired. Although this may give faster
results than the process giving the complete radiation distribution, the
results are somewhat incomplete unless the doctor repeatedly selects
numerous areas or zones for which the radiation distribution is requested.
Each radiation distribution that is generated shows only a portion of the
plane of view illustrating the radiation distributed within the patient.
The time-consuming nature of prior dosimetric systems is at least partly
due to the generally used technique for calculating where the beam goes
into the patient's skull. Specifically, the patient's skull may be
simulated by thousands (often hundreds of thousands) of tiles and the
usual "tiling" technique uses a series of simultaneous equations in order
to calculate where the beam of radiation enters the patient's skull.
A further reason for the time-consuming nature of prior dosimetric
procedures is that the resolution must be sufficiently high to give
adequate details of the radiation distribution. In other words, the points
at which the radiation dosages are given must be sufficiently close
together that the doctor will have enough information to make proper
decisions. On the other hand, this requirement for high resolution causes
one to use so many data points that the calculations will, on most
computers, take a tremendous amount of time.
A further reason for the time-consuming nature of previous dosimetric
techniques is that such techniques require radiation distribution
calculations based upon relatively complex mathematical models. The models
require that the entrance width of the beam be taken into account because
the width of the beam is generally large compared to the curvature of the
patient's skull. In other words, the center of the radiation beam might be
perpendicular to the patient's skull, but the beam is sufficiently wide
compared to the curvature of the patient's skull that the edge of the beam
is entering the patient's skull at a significantly different angle than at
the beam center. Since the portion of the beam entering at the edge has a
significantly different angle than the center of the beam, prior systems
have generally taken into account this edge effect. This increases the
complexity of the calculations. A further reason for the complexity of
calculating the radiation distribution is that prior techniques usually
require calculation of the primary radiation and the scattered radiation.
The primary radiation is radiation which reaches a point inside the target
volume with few interactions with the overlying material, whereas the
scattered radiation is the radiation distribution resulting from the
interaction of the primary radiation with the overlying structures or
materials away from the primary path. The scattered radiation does not
proceed along the same directional path as the primary radiation or the
beam.
A further disadvantage of prior dosimetric systems is that they lack
flexibility in terms of providing requested data.
OBJECTS AND SUMMARY OF THE INVENTION
A primary object of the present invention is to provide a new and improved
dosimetric technique for stereotactic radiosurgery.
A more specific object of the present invention is to provide an improved
dosimetric technique which avoids or minimizes the problems associated
with the prior dosimetric techniques as discussed above.
The present invention uses localization in order to determine the target
area (such as a tumor or other area to be treated) within a patient. An
imaging system is used on the patient, such as a CT scanner, angiography,
or NMR system. Using the localization data obtained, an operator supplies
a computer with a series of arcs corresponding to the manner of applying
radiation to the target area within the patient. The arcs are input into a
computer system which has been programmed to perform the present
dosimetric analysis. The computer system will very quickly generate data
for the doctor showing the radiation distribution within the patient from
the proposed arcs.
Advantageously, the present invention avoids many of the calculations of
the prior dosimetric systems by providing a different resolution within a
zone close to the isocenter (i.e., a location within the target zone at
which the radiation will be most concentrated) and a zone removed from the
isocenter. In other words, there might be a grid in the high-resolution
zone where the radiation distribution is calculated every one millimeter,
whereas the radiation distribution would be calculated every five
millimeters in a grid outside of the high resolution zone or area.
Accordingly, the number of data points may be significantly reduced
without lowering the useful information supplied to the doctor since the
low-resolution zone or area corresponds to locations where the radiation
distribution changes only very slowly.
Another advantageous technique of the present invention is to use a thin
beam of radiation such that the scattered radiation may be ignored and the
beam may be modeled as though it strikes the patient's skull at a single
point. In other words, the beam may be sufficiently thin compared to the
curvature of the patient's skull so that one may ignore the edge effects
discussed above.
A further advantageous feature of the present invention is that one avoids
the tiling technique to determine where the beam enters the patient's
skull. Instead of performing the simultaneous equations, the present
dosimetric technique uses a computer to graphically proceed along the
radiation beam from the target area towards the source of the radiation.
The computer can recognize whether the beam is inside the patient's skull
or has just transversed into the outside of the patient's skull.
A further significant feature of the present invention is that it allows
the user to arbitrarily select key planes for display of the radiation
distribution.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features of the present invention will be more readily
understood when the following description is considered in conjunction
with the accompanying drawings wherein like parts have the same number
throughout and in which:
FIGS. 1 and 2 are a side elevation view and an end elevation view,
respectively, of conventional linear accelerator apparatus which may be
employed for stereotactic radiosurgery, the figures illustrating possible
misalignments of a radiation-emitting head of the apparatus;
FIGS. 3 and 4 are a side elevation view and a top view, respectively, of
stereotactic radiosurgery apparatus useful in implementing the invention;
FIG. 3A is a side exploded view of a linking arrangement for linking a
collimator to a radiation-emitting head;
FIG. 4A is a top view showing parts of a floorstand support arrangement:
FIG. 4B shows a side exploded view with some parts in cross-section of
parts of FIG. 4A;
FIGS. 5 and 6 are a side elevation view and a top view, respectively, of
guiding structure;
FIG. 5A is a top exploded view of parts from FIG. 5;
FIG. 7 is a perspective view illustrating conceptually a preferred form of
a main arcing bearing;
FIG. 8 is a perspective view illustrating conceptually a preferred form of
a gimbal bearing;
FIG. 9 shows an alternate arrangement for supporting a collimator;
FIG. 10 shows an alternate arrangement for supporting a floorstand;
FIG. 11 shows a further alternative arrangement for supporting both the
collimator and a floorstand by way of a common support;
FIG. 12 shows a side view of an arrangement for linking rotation of a
floorstand to rotation of a treatment table;
FIG. 13 shows a cross-section view of the connection between the floorstand
and treatment table of FIG. 12;
FIG. 14 shows a side view of an alternate arrangement for linking a
floorstand to a table;
FIG. 15 shows a simplified flow chart of the overall patient treatment
process according to the present invention;
FIG. 16 shows a simplified flow chart of a method of setting a beam
treatment arc and determining beam entrance points according to the
present invention;
FIG. 17 is a view of the display produced during a portion of the present
process;
FIG. 18 is a simplified flow chart showing how the present process provides
for displaying the radiation distribution in an arbitrarily selected
plane;
FIG. 19 is a simplified drawing showing a radiation beam entering a patient
and illustrating several principles of operation of the present invention;
FIGS. 20A and 20B show beam intensity distributions respectively as a
function of the distance from the center of a beam and the depth in tissue
of the beam;
FIG. 21 is a simplified flow chart showing how the present process computes
the radiation dose resulting from the radiation beam being swept through a
particular arc;
FIG. 22 is a simplified flow chart illustrating the computation of the
radiation dose at a particular point from a beam;
FIG. 23 shows a view screen produced by the process of the present
invention;
FIG. 24 shows a view screen as produced without any windows;
FIG. 25 shows how the present invention includes radiation dose information
on the display of a portion of the patient's body;
FIG. 26 shows a view screen produced by the present invention wherein the
vertical and horizontal distributions of radiation are plotted; and
FIG. 27 shows a simplified schematic of a system for implementing the
present technique.
DETAILED DESCRIPTION
The process of the present invention is especially well adapted for use in
conjunction with a specific linear accelerator structure which will be
initially described herein. However, it should be noted that the present
dosimetric technique has more general application.
FIGS. 1 and 2 illustrate a conventional LINAC device which comprises a
fixed base 10 and an L-shaped gantry 12 which is rotatable with respect to
the base about a horizontal axis 14. The gantry carries a
radiation-emitting head 16, and rotation of the gantry causes the head to
sweep through an arc R located in a substantially vertical plane which is
perpendicular to the horizontal axis. The dotted lines in the figures
indicate potential misalignments caused by mechanical inaccuracies or sag
of the gantry in any of the directions indicated in the FIGS. as A, B or
z. These misalignments result in misfocusing of the radiation from the
head 16 and are intolerable in radiosurgery, for the reasons noted
hereinafter.
In order to best understand the invention, the three principle components
of a stereotactic radiosurgery procedure will first be explained. These
components are localization, dose computation and optimization, and
execution of treatment. The ultimate accuracy of the procedure is
dependent on each of these components.
The first component in the procedure involves the localization of the
tumor. This is accomplished by one of two means. Currently, the method of
choice is through stereotactic angiography. The procedure begins with the
stereotactic ring being fitted to the patient. An angiographic localizing
device is then attached to the ring. This device is known and consists of
four sets of fiducial alignment markers. Two sets of these markers project
onto each of two orthogonal angiographic x-rays. By location of the
fiducial points and the target on each x-ray, the precise, x, y, z
coordinates of the target (to an accuracy of 1 mm) relative to the
stereotactic ring can be derived. While this part of the procedure allows
the coordinates of the target relative to the localization ring to the
determined, more anatomical information is needed for dosimetric analysis.
The next step replaces the angiographic localizing device with another
localizer specially designed for localization in computer tomography. This
is the standard BRW CT Localizer. The patient is aligned in the CT gantry
and contiguous 5 mm slices, beginning at the level of the localization
ring and advancing superiorly past the top of the patient's skull, are
obtained. If the target volume can be identified in the computerized
tomography image, then the x, y, z coordinates of the target volume are
again calculated. (This can provide a double check of the x, y, z
coordinates relative to the stereotactic ring.) If not, then the target
obtained from the angiographic procedure can then be superimposed onto the
CT scan data.
With the digitally encoded data from the CT scan and the two angiographic
films, the data may be then transferred to a dosimetry computer system.
The CT scan provides three dimensional anatomical information of the
patient allowing a solid patient model to be constructed. The coordinates
of the target volume from the angiogram and the CT scan data are then
merged.
Computation and Dose Optimization: In order for the high single fractions
of radiation to be delivered to the target volume, a technique to
concentrate the radiation at the target while spreading out the radiation
to lesser concentrations throughout the normal tissues must be utilized.
Moving the radiation source through multiple arcs achieves this objective.
It is important for the radiotherapist and neurosurgeon to be able to
examine the consequence of each portion of the arc. The computer system
which computes the dosimetry must have the ability to display each arc
segment. In the routine stereotactic procedure, it is anticipated that
four arcs, three at 100 degrees and one at 240 degrees, will be utilized.
The computer must allow the CT scan to be reformatted in each of these arc
planes (relative to the patient's skull) so that each individual arc's
dose distribution can be examined. If any particular arc results in an
extensive dose to a critical structure, the therapist can alter the arc
parameters to avoid the anatomical area of concern. The dosimetry system
discussed in detail below will allow dose optimization through operator
control. For as yet undeveloped more sophisticated versions, the operator
will identify the target region and the areas where dose should be
minimized. The computer will then, through use of an optimization
algorithm, design the treatment which best concentrates the radiation over
the tumor volume while minimizing the dose to normal tissues. The spacing
between arcs, the size of the collimator, and the variation in arc length
and weight will be parameters used in the optimization.
The method necessary for dose computation and optimization using a CT scan
is complicated by the high resolution necessary in the procedure. The
stereotactic targets can be identified to plus and minus a millimeter. The
treatment portals can range anywhere from 1 to 3 cm in diameter. The
spatial coordinates of the computational grid, in the area of the target,
must be in the 1 mm range. However, there is little need for 1 mm accuracy
outside about a 5 cm radius of the target itself. A 0.5 grid is adequate
in this region. By working with both the 1 mm and 5 mm grids, the number
of computation points at which a dose must be evaluated for the complex
arcs can be vastly reduced.
Once the acceptable treatment scheme has been derived, the coordinates of
the isocenter (focal point of the radiation), the collimator size, and the
arc parameters are then transferred to the operator of the linear
accelerator.
FIGS. 3 and 4 illustrate the stereotactic treatment setup. As shown, a
patient is placed on a treatment table 20 which is supported by a member
22 on a rotating plate 24 positioned in the floor. The patient's head is
immobilized by a stereotactic ring 26 which is connected to a BRW
stereotactic floorstand 28 which has been modified in accordance with the
invention (as will be explained shortly) so that the patient's head is at
a predetermined location with respect to the radiation-emitting head 16 of
the LINAC. As shown in FIG. 4, rotating plate 24 may be rotated, to
position the table at different locations 20' as indicated by the dotted
lines. Gantry 12 of the LINAC may be rotated about base 10 to swing
treatment head 16 in an arc located in a vertical plane indicated by the
dotted line 30 in FIG. 3. The radiation from head 16 is collimated by a
collimator 32 and is confined to the vertical plane 30 in which the
treatment head moves. FIG. 4 shows the gantry 12 swung over to one side
such that the radiation enters the left side of the patient's head, and
FIG. 3 shows the gantry in an upright position such that the radiation
enters through the forehead of the patient. Collimator 32 focuses the
radiation at an isocenter or center point 34 corresponding to the
intersection of the horizontal axis 14 of rotation of the gantry and
vertical plane 30. Center point 34 corresponds to the origin of the arc
through which th | | |
