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Description  |
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DESCRIPTION
1. Technical Field
The present invention relates to radiation dose calculations used in
determining a radiation therapy treatment plan for a patient.
2. Background Art
When radiation is absorbed by cancerous tissue, in many instances the
tissue is damaged to the point it becomes dormant and disappears. In other
instances, radiation treatment is used as a follow-up measure in
conjunction with radical, surgical removal of cancerous tissue. Within the
past few years, computed tomography scanners have been utilized for
diagnosing a patient condition and also utilized for simulating
x-radiation treatment of that condition.
In an actual radiation treatment a patient is selectively irradiated by a
radiation source in accordance with a precisely defined treatment plan.
The radiation source produces either x-radiation or gamma radiation
depending on whether an x-ray tube or radioactive element such as cobalt
is used as the source. In such a therapy treatment session, the intent is
that a volume containing the cancerous tissue, i.e. the tumor, receive a
fatal dose of radiation and that other regions within the patient receive
minimal doses to reduce the damage caused by radiation absorption.
Studies conducted on the effectiveness of radiation treatment of cancer
patients indicate that radiation dose levels must be precise if the
radiation treatment is to have beneficial effects. To control radiation
dose at a region within the patient, prior art techniques use beam
definition devices of differing shapes positioned within a radiation beam
to selectively block radiation from reaching the patient. The intensity of
radiation reaching the patient is also controlled. The planning of a
radiation treatment for a patient is an interactive process whereby the
therapist determines a desired dose for a particular patient region and
then devises one or more radiation beams to irradiate that region with the
prescribed dose while minimizing radiation absorption at other patient
regions.
In the prior art, the task of determining desired dose and constructing an
appropriate radiation beam to provide that dose is done with the aid of a
computer. The computer calculates beam doses for specified conditions and
allows the therapist to change those conditions to most accurately achieve
a desired treatment. In the prior art, this process typically takes about
one hour for each patient. This time is required for the therapist to plan
a treatment, construct a beam configuration, allow the computer to
calculate dosage based upon that configuration, make adjustments in the
beam configuration based upon an initial calculation and once an
appropriate beam is defined and confirmed, actually conduct the radiation
treatment. Much of the time in a prior art radiation treatment session is
spent waiting for the computer to calculate patient dose based upon user
inputs.
Total radiation dose results from not only primary irradiation, i.e.
radiation passing directly from a radiation source to the cancerous region
but also includes a scatter contribution caused by Compton scattering of
radiation from other regions in the patient. In order to accurately
calculate dosage, the computer must therefore take into account direct
irradiation as well as scatter contributions from other regions. This is a
time consuming process and significantly slows patient through-put. This
is especially true if irregualr beam geometrics are constructed using beam
blocking devices.
In one prior art technique for determining absorbed dose, scatter
calculations are performed using a differential scatter-air ratio
technique proposed by J. R. Cunningham in two printed publications
entitled "Scatter-air Ratios", Phys. Med. Biol., 1972, Vol. 17, No. 1,
42-51, and "Calculation of Dose From Irregularly Shaped Radiation Beams",
Computer Programs in Biomedicine 2 (1972) 192-199. North-Holland
Publishing Company. The subject matter of these two prior art references
is expressly incorporated herein by reference.
In accordance with procedures disclosed in these two printed publications,
scatter radiation to a particular region within a subject is calculated
using an integration technique which accesses differential scatter-air
ratio tables stored in the computer. Incremental scatter doses are
calculated for pie shaped regions extending from the dose calculation
point to the boundry of the radiation beam. This integration is performed
in polar coordinates wherein a radial distance from the point of interest
to the beam boundry is first determined and then used to access a
scatter-air ratio corresponding to that radial distance. This prior art
procedure, while theoretically correct, results in long calculation times
with diminished patient through-put.
DISCLOSURE OF THE INVENTION
A principal feature of the invention is more rapid therapy treatment
planning. The planning and implementation of radiation therapy that takes
an hour or more using prior art procedures can be performed in much less
time by use of the invention. This results in greater patient through-put
and, perhaps more importantly, allows greater precision in therapy
planning.
The invention utilizes differential scatter-air ratios to determine scatter
contributions to total dose, but in a way that maximizes speed in dose
calculation. Whereas in the prior art, dose calculations are based upon
lengthy polar coordinate integrations, the present invention uses
rectangular coordinate integrations which can, to an extent, be
precalculated outside an integration loop.
In accordance with the invention, total radiation dose for therapy
treatment is calculated by combining primary and secondary dose
contributions from a radiation source to a specific region within the
patient. The scatter contribution is calculated by integrating scatter
contributions for other regions within the patient. A differential
scatter-air ratio method is used where differential scatter-air ratios for
radial distances within a plane of the patient are stored in a one
dimensional matrix and a differential scatter-air ratio contribution for
each subregion within the plane is accessed by converting a two
dimensional coordinate within the plane to an offset or position within
the one dimensional matrix.
Through an interactive process similar to that conducted in the prior art,
a therapist plans a radiation treatment. The use of a two dimensional
coordinate for determining an offset into the one dimensional matrix
speeds integrations for the scatter component of the dose calculation.
This procedure also results in a reduction in memory required to perform
the scatter calculation.
In accordance with a preferred embodiment in the invention the calculation
is performed in rectangular coordinates. In the prior art, the calculation
was performed in polar coordinates so that for each boundry point of a
beam configuration a new differential scatter-point air ratio figure was
required, scatter contributions for a rectangular sequence of elements
allow offsets into the one dimensional array of differential scatter-air
ratios to be precalculated. Stated another way, differential scatter-air
ratios for regions within a plane are precalculated in advance and then a
rapid integration scatter summation performed by a computer for each
subregion within the plane.
In accordance with another aspect of the invention, a beam profile matrix
takes into account beam geometries built into the therapy plan by the
therapist. Wedges, blocks, etc. used to define the radiation beam have an
effect on both primary and scatter contributions to the region of
interest. The primary effect due to these beam defining elements is
precalculated and used to generate a one dimensional beam transmission
matrix. Values for this matrix are convolved with the scatter contribution
of a given region to correct for beam inhomogenieties. By reviewing the
two aforementioned prior art publications, one sees that beam
irregularities are ineffectually taken into account by the prior art and
lengthen an already slow calculation.
One object of the invention is a rapid method of determining scatter
contributions during a therapy planning session. This method uses the
differential scatter-air ratio calculation procedure of the prior art but
in a much more effecient manner that speeds the interactive planning
process of the therapist. This and other objects, advantages and features
of the invention will become better understood when a detailed description
of a preferred embodiment of the invention is described in conjunction
with the accompanied drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic perspective view of a radiation treatment
accelerator;
FIG. 2 is a perspective view of a representative slice of tissue with
radiation impinging upon it;
FIGS. 3-5 are plan views of the FIG. 2 tissue showing certain features of a
radiation dose determination;
FIG. 6 is a view of a patient cross-section showing radiation impinging
from two directions and mappings of isodose regions in the cross-section;
and
FIGS. 7 and 8A-8E are flowcharts of method steps for calculating radiation
dose.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to the drawings, FIG. 1 depicts a radiation accelerator 10
including a radiation source 20 for irradiating a patient. The source 20
is mounted to an accelerator gantry 30 that rotates the source relative
the patient. During treatment, the source 20 irradiates the patient in a
manner such that x-radiation passes into the patient and is absorbed by
patient tissue. Radiation treatment is performed using both gamma
radiation from a radioactive source such as cobalt and can similarly be
accomplished by x-radiation treatment from the accelerator 10. The
discussion of a preferred radiation dose calculation procedure will be
described in conjunction with x-radiation treatment but it should be
appreciated that similar steps are utilized when utilizing radioactive
materials.
A computer 40 helps a radiation therapist plan a treatment session. A
mapping of radiation dose in a paitent cross-section is displayed on a
cathode ray tube 45 to help the radiation therapist determine radiation
dose within the patient.
A patient couch 50 helps position the patient relative the x-ray beam. A
top 60 can be moved relative the source 20 to precisely position a
particular patient cross-section within a radiation beam.
Once a tumor or cancerous region within the patient has been identified
using a computed tomography or nuclear magnetic resonance scan, the
radiation treatment therapist must plan a therapy session whereby specific
doses of radiation are directed at the cancerous tissue in an effort to
fatally damage the tumor while leaving other regions of the patient
relatively unaffected.
By way of introduction, a cross-sectional image of a patient region is
illustrated in FIG. 6. This region includes a cancerous tumor
schematically illustrated by reference character 100 that has been
identified by the diagnostician from three dimensional computed tomography
scans of the patient. Accepted treatment indicates a dosage of X rads of
radiation when directed at the center of this tumor will have the
therapeutic effect of fatally damaging the tumor 100.
Two oblique x-radiation beams 110, 120, having centerlines 110a, 120a, are
seen impinging upon the tumor from different directions. The beams 110,
120 have different beam widths as generated by beam defining structure
mounted relative the radiation source 20. The two beams 110, 120 are
generated at different times. The patient is moved relative the
accelerator gantry 30 until a particular cross-section, including the
tumor 100, is positioned properly and the source 20 rotated about its
circular path until it coincides with a position for generating the first
beam 110. The patient is irradiated with the first x-ray beam 110 for a
first time period and then the source 20 is rotated to a position such
that the beam 120 is produced. The patient is then irradiated for a second
period. To produce the total dose of X rads, the patient is typically
radiated a number of times over an extended time period so that in a given
session less than the total dose is received.
Prior to this radiation treatment, the x-radiation therapist must plan the
irradiation sequence. To facilitate this planning, an interactive dose
calculation procedure is followed, whereby the diagnostician chooses
certain parameters and the computer 40 utilizes these parameters to
produce a dosage mapping for patient regions. The radiation dose depends
upon a number of variables, and in particular, varies with the depth of
the region within the patient, position within a cross-section
perpendicular to the x-radiation beam, patient contour at the region of
x-radiation impingement upon the patient, x-radiation intensity, and
position and orientation of various beam defining apparatus such as blocks
(total or nearly total attenuation) and wedges (partial attenuation). The
computer 40 must account for each of these variables in calculating
patient dose and must do so as rapidly as possible.
FIG. 2 shows a perspective view of a representative volume of patient
tissue having x-radiation impinging upon the volume parallel to a "Y"
axis. The calculation of radiation dose for a particular point P having
rectangular coordinates i, j, k relative a coordinate system origin 0 is
conducted using the differential scatter-air ratio technique proposed and
discussed by Cunningham in his prior art publications. The radiation dose
is divided into primary and scatter contributions. The primary dose
depends upon the x-radiation intensity from the source 20 and the
attenuation the radiation experiences passing through the patient prior to
reaching the point P. This is affected by patient contour and in
calculating primary dose contributions it is assumed a uniform patient
density is encountered once the x-radiation enters the patient. This
primary dose calculation is also affected by the presence of radiation
defining wedges and blocks, but so long as these contributions to
radiation strength or intensity are known, the primary dose calculation is
straightforward and can be quickly calculated by the computer 40.
As noted by Cunningham, the total dose calculation for the point P is also
dependent on a scatter contribution resulting from radiation scattering
from other regions within the patient to the point P. As pointed out by
Cunningham, this scatter contribution can be computed with the aid of a
computer using a summation technique which approximates an integration.
Refering to FIG. 5, Cunningham's prior art technique of summing scatter
contributions relative to the point P will be summarized and can be
reviewed in detail in the referenced and incorporated prior art
publications to Cunningham.
In FIG. 5, the radiation is directed perpendicular to the planar section
illustrated. As a first example, consider a situation in which a uniform
intensity beam forms a rectangle bounded by the four points q, r, s, t. In
accordance with the Cunningham procedure, scatter contribution for regions
within the patient volume (FIG. 2) are calculated using a summation
technique wherein pie-shaped regions of radiation scatter are added
together to form a total scatter dose. One such pie-shaped region is shown
having an apex at the point P and a boundry at the point d. The pie-shaped
region illustrated contributes a scatter dose to the point P given by the
relationship:
S=S(d,r.sub.e).DELTA..theta..sub.e /2.pi.
where:
d=depth within tissue to point;
r.sub.e =distance from point to boundary of pie-shaped slice;
.DELTA..theta..sub.e =angular extent of slice; and
S (d, r.sub.e)=scatter-air ratio from look-up table.
To calculate the scatter contribution for the entire rectangle q, r, s, t,
similar pie-shaped regions from the boundry of the rectangle to the point
P are calculated and summed to produce a scatter contribution which is
added to the primary contribution to produce a total dose. This procedure
becomes particularly involved when blocks or wedges affect the total dose
impinging upon the region. Stated another way, when a nonuniform radiation
is used for therapy treatment, the differential scatter calculations
become not only a summation of positive terms but a subtraction for
regions irradiated with a nonuniform intensity. In FIG. 5, for example,
when a block is inserted within the x-radiation beam to totally prevent
primary radiation from reaching the rectangle defined by points t, u, v, w
of the figure, the scatter contribution for the pie-shaped region
extending from the point P to the point d must take into account this beam
inhomogeneity. Cunningham suggests a subtraction of the scatter for the
blocked off region extending from the two points d', d" in FIG. 5.
The prior art method of calculating scatter contribution is inefficient
since a new differential scatter-air ratio S (d, r.sub.e) for each point
along the rectangle perimeter q, r, s, t must be determined for each
pie-shaped segment. This is a time consuming operation for the computer
and results in long calculation times which adversely affect patient
throughput. The scatter calculations of the present invention will be
described in overview with respect to FIGS. 3 and 4 and described in more
detail in the flow charts of FIGS. 7 and 8A-8E.
Turning to FIG. 3, the rectangular region q, r, s, t of FIG. 5 has been
reproduced showing the calculation point P. As in the prior art, the
present method breaks the dose contribution in two parts. The primary
calculation is performed and a scatter contribution added to produce a
total dose for a particular subregion P within the patient. For the point
P, each of the rectangular elements dx, dz (FIG. 3) contributes to scatter
dosage. In accordance with the present invention, the scatter contribution
for each small region dx, dz is summed in a iteration loop by the computer
40 by accessing a table of radial scatter-air ratios (RSAR) that have been
precalculated based upon initial parameters such as radiation intensity.
The calculation of the dose at point P involves a summation of all small
elements dx, dz shown in FIG. 3. In accordance with one procedure, scatter
contribution for each element within a row is calculated in turn and then
the row is indexed until the scatter contribution for an entire region
bounded by the points q, r, s, t has been calculated. In a disclosed and
preferred embodiment, the dimensions dx, dz are one centimeter and a
particular plane within the patient is divided into a grid matrix of these
one centimeter square elements.
The specifics of a calculation involve determining the radial distance from
the point P to the point dx, dz and determining an offset into the one
dimension radial scatter-air ratio matrix that points to a value which is
added to the scatter contribution for the point P. This one dimensional
radial matrix is precalculated based upon the beam characteristics chosen
by the therapy diagnostician.
Each region dx, dz within the rectangle q, r, s, t has an offset into the
one dimensional radial scatter-air ratio matrix. The memory requirements
for storing the one dimensional matrix are an order of magnitude less than
the requirements for storing a scatter-air ratio for each element dx, dz
in the plane. In a representative dose calculation where dose to a
128.times.32.times.32 centimeter patient region is determined this storage
savings (assuming 1 cm increments in calculations) reduces the storage
requirements from 524k bytes (128.times.32.times.=131,072) to
approximately 32k bytes (128.times.64=8,192) if four bytes of data are
stored per table entry.
Once an entire plane of dose values are calculated, the y dimension is
indexed and doses are calculated for each other slab or patient
cross-sections perpendicular to the radiation I.
If more than one radiation beam (FIG. 6) impinges on the patient, the dose
contributions to a point P must be combined for a total dose mapping for
the entire patient region of interest.
Turning to FIG. 4, the uniform rectangular radiation pattern of FIG. 3 is
modified using a block which completely attenuates radiation from passing
to a quadrant of the rectangular region. A second region identified with
stipling is a region where a wedge has been used to attenuate radiation
passing through the patient but wherein the wedge has a transmission
factor which allows a certain percentage of radiation to impinge upon the
patient.
The Cunningham procedure of selectively subtracting scatter contributions
based upon inhomogeneities in the radiation pattern impinging on the
patient is replaced in the invention by a procedure wherein the modified
intensity is made part of a one dimensional transmission matrix in advance
of the computer summations. Both scatter and primary dose to the point P
take into account the different beam intensities impinging upon the region
q, r, s, t stored in this matrix.
FIGS. 7 and 8A-8E illustrate a preferred implementation of the dose
calculations of the invention. In the algorithms discussed in FIGS. 7 and
8A-8E, a reference system consistent with that defined in FIGS. 2-5 is
used in calculating radiation dose. In the frame of the patient, the
positive X direction is to the right looking towards the paitent's head,
the positive Y direction is towards the patient anterior or front and the
positive Z direction is towards the patient's feet.
A plan initialization step 210 (FIG. 7) is an interactive process with a
computer 40, wherein the user enters various specification data to
determine parameters for the dose calculation. Certain of these parameters
are available from the three dimensional computed tomography scan, for
example, the patient contour can be accessed by the computer 40 based upon
an initial patient scan.
At step 212 (FIG. 7), a process of obtaining radial indices as a function
of scatter point coordinates begins. Returning to FIG. 3, each
representative area dx, dz can be characterized as an offset into a one
dimensional radial scatter-air ratio matrix so that once one knows the X
and Z coordinates of the point, one can directly access a location in that
matrix. The steps 214, 216, 218, 220 represent the generation of a matrix
FLR indicating an offset for a particular X and Z coordinate (I and J in a
FORTRAN implementation of the flowchart). The two loops 214, 218 index the
X and Z coordinates within a plane (constant Y coordinate). For each X and
Z combination an entry in the matrix FLR is calculated. The formula in
method step 220 is the FORTRAN representation of the pythagorean theorem
for the initial conditions defined in step 210. As seen in method step
222, the calculation loops end when radial offsets for a 64 by 64 array of
elements dx, dz (FIG. 3) has been calculated so that for each coordinate
within the plane an appropriate address into the matrix FLR is determined.
This address stores an offset into RSAR, a radial scatter-air ratio matrix
to be calculated below.
Turning now to FIG. 8A, the algorithms for calculating dose to a particular
point are discussed. The theory of calculation separates the scatter and
primary contributions which are added at the end of the calculation. This
algorithm begins with an initialization step 250 which primarily relates
to parameters defining the beam for which the dose calculation must be
performed. The beam is defined in terms of coordinates within the X, Y, Z
reference frame at which the beam intersects the surface of the patient,
the limits of the beam as defined by a beam defining apparatus (not shown)
and the orientation of the patient. Additionally, the initialization step
250 includes information relating to beam intensity including
inhomogeneity in that intensity due to the existence of blocks and wedges
placed in the beam path between the x-ray source 20 (FIG. 1) and the
paitent. At a next step 252, a primary dose matrix BWP for each point in
the region of the beam bounded by inside edges of a beam penumbra is
generated. This matrix is an initial primary dose matrix used as a
starting point for dose calculations. A next step 254 in the FIG. 8A
algorithm corrects the primary matrix based upon wedge information since
the presence of a wedge defining member in the beam path affects the
primary at certain points within the beam cross-section. In the FIG. 4
representation, the beam quadrant that is attenuated (stipled) has a
transmission factor entered into a matrix WDGC to account for this
attenuation.
At a next step 256 in the algorithm, a block correction matrix BLC is
generated based upon the presence of beam blocking apparatus inserted into
the x-ray beam. At a next step 258 the three matrices generated in the
previous steps 252, 254, 256 are convolved into a single matrix BWP for
scatter calculations. This is accomplished in the remaining method steps
260-266 in FIG. 8A. The matrix BWP is a one dimensional matrix formed from
the initial BWP matrix 252, the wedge WDGC matrix 254 and the block
correction matrix BLC 256. The value of BWP for a given X, Z coordinate
(I, J index in FORTRAN) is a beam profile at the region X, Z. Subsequent
to the step 266, the computer has defined the beam in terms of the output
from the x-ray tube, beam blocking geometry and any wedges inserted within
the beam.
A tissue-air ratio DNOR for a normalization point is calculated at a next
step 270 (FIG. 8B) in the dose calculation algorithm. This reference
tissue-to-air ratio is based upon the beam characteristics and is used as
a reference dose to which the remaining points in the region under
scrutiny are referenced. DNOR is calculated in accordance with Day's
method for calculating dosage at any depth within a rectangular beam.
At a next step 272, a contour correction matrix CCOR is generated. CCOR
stores distances from points the beam enters the patient to a plane
intersecting the point the beam center enters the patient. This data
allows distances into the patient from a point of entry of the x-ray beam
to dose calculation points to be determined. This distance is used in
generating scatter-air ratios in accordance with the Cunningham technique.
Radial scatter-air ratios (RSAR) are generated in the steps 274-278 in FIG.
8B. Scatter-air ratios (SAR) for a particular depth within the patient are
used to generate the radial scatter-air ratios (RSAR).
In these RSAR calculations the index J corresponds to a radial distance
from a calculation point to a scatter point and the index I corresponds to
a depth within the patient. These indexes refer only to the steps 274-278.
The calculation at method step 277 is a determination of the radial
scatter-air ratios as a function of depth and radial distance. RSAR is the
difference between the scatter-air ratio for the radius J and the
scatter-air ratio for the radius J+1 divided by the area of the ring
encompased by circles having the radii J and J+1. The scatter-air ratios
SARs are known values read into computer memory at the initialization step
250.
The algorithm next determines 279 if the radiation beam is rectangular and
symmetric about the z axis. If the beam is not symmetric, the computer
skips to FIG. 8C and primary dose calculations begin. If the beam is
symmetric and uniform about the z axis, the scatter dose is the same for
elements symmetric with respect to the z axis and a simplification in the
RSAR matrix is performed at steps 280-286. This simplification
precalculates scatter for a slice parallel to the z axis so that during a
scatter summation, the scatter to a given point is conducted by summing
across the beam width.
Method steps 310-313 (FIG. 8C) begin the dose calculations in the x, y, z
coordinate space with calculation loops that access each point in each
calculation plane. At step 314 the primary dose TAROD for each calculation
point J, I, K is determined. At the next step 315, the scatter
contribution at the point is determined using a constant increment
cartesian coordinate summation.
The scatter calculation begins with calculations 316 of distances of the
point J, I, K from the x, y and z axes. This sets the limit for
integration for each point. Two decision steps 317, 318 determine the
parameters of the scatter calculation in terms of beam geometry and
whether a contour correction is to be performed. Step 319 indicates a
blocked beam with contour correction scatter calculation is performed.
Steps 320-322 set up a rectangular coordinate integration where scatter
contributions of each point dx, dz (FIG. 3) for the beam geometry are
added in turn until a total scatter contribution is determined.
Since contour correction is performed, the depth (PA in the flow diagram)
from the point the beam enters the patient to the integration point dx, dz
is calculated 232. The next step 325 is the scatter calculation. In the
equation of step 325 (FIG. 8D) the incremental scatter from the region dx,
dz is determined by the three variables XOFFB, PA, and LW. PA is the depth
variable in the RSAR table and LW is the radial distance from the point J,
I, K to the region dx, dz. XOFFB is an offset into the matrix BWP
(calculated at step 265) based upon the position of the incremental region
dx, dz within the beam.
The calculation of the three variables XOFFB, PA, and LW is straightforward
yet powerful. These variables are well ordered and not succeptible to the
lengthy calculations involving complex exponential terms necessary in the
prior art. The matrix BWP defines the beam characteristics so that
multiplication of the BWP matrix element with the RSAR value convolves the
scatter characteristics of the beam into the region dx, dz RSAR value.
After the scatter contribution SCAT for the region dx, dz is added, the
loop variables are incremental 330-333 until the scatter contributions for
all incremental regions in the plane of the point J, I, K have been summed
334.
Returning to FIG. 8C, at decision step 317, had the beam been rectangular,
a branch to step 340 (FIG. 8D) occurs. At step 342, the decision step
(318) regarding the contour correction is made and at the next step 344 a
rectangular beam scatter calculation with contour correction is made.
Method steps 350-356 correspond to the scatter calculation steps discussed
above, but for a rectangular beam. The computer only loops over the beam
width since the RSAR table for a rectangular beam already includes the
scatter contribution to the point J, I, K from a lengthwise slice (see
step 283) a given perpendicular distance from the point at specified
depths within the patient. By looping across the beam width and changing
only the primary offset XOFCOF and the contour correction variable PA the
scatter calculation is simpler and faster for the rectangular beam.
To obtain a rapid dose calculation, the therapist may wish to obtain a dose
mapping without patient contour correction or the use of blocks and
wedges. This choice at step 342 (FIG. 8D) causes the computer to calculate
dose for a rectangular beam with no contour correction. This condition
causes the computer to branch to this calculation 360 for each point J, I,
K.
To complete the calculation, the primary and scatter doses are added 362
and stored in memory. At steps 364-366, the J, I, K variables of regions
within the patient are indexed and radiation dose for other regions
calculated and stored in memory. These values are used to create the
mapping of FIG. 6 on the cathode ray tube 45 as an aid to the therapist in
planning a patient treatment.
Turning to FIG. 6, the tumor 100 is intersected by the two beam centerlines
110a, 120a. One isodose delineating indicator 130 of an intensity of
x.div.n rads is bounded by other isodose indicators 125 of lesser
intensity. By modifying the beam geometries the therapist can more nearly
isolate the indicator 130 about the tumor 100 to minimize dose to other
regions. The new and improved dose calculation embodied by the FIGS. 7 and
8A-8E flow diagram increase the speed of this process by rapidly
presenting a mapping similar to FIG. 6 as the beam geometry is altered.
The equal increment integration (summation) of the disclosed method
suggests the possibility of implementing the scatter loop using an array
processor to decrease the calculation time even further, potentially by an
order of magnitude.
The flow diagrams discussed above represent FORTRAN implementation of the
improved dose calculations. The steps of convolving beam characteristics
into the scatter and printing calculations, use of a well ordered
summation for incremental regions within the body, and the data
organization using radial scatter-air ratios in a one dimensional matrix
of values for each beam geometry can be implemented using other languages.
The method also has applicability to electron rather than photon beam
calculations. Although the disclosed implementation has been described
with a degree of particularity, it is therefore the intent that the
invention include all alterations and modifications falling within the
spirit or scope of the appended claims.
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