 |
Description  |
|
|
FIELD OF THE INVENTION
This invention pertains to an apparatus and method for radiation treatment
employing shaping and dynamic control of spatial distribution of intensity
of the radiation field in a radiotherapy machine and in the application of
such radiation in a selective manner to living biological materials
including human patients in patient therapy for cancer treatment.
BACKGROUND OF THE INVENTION
Conventional x-ray treatment of a tumor in a patient is carried out by
planning the radiation angles and dosage by taking into consideration
safety factors in respect to the patient's organs which would be in the
path of the beam. The treatment plan assumes that the treatment equipment
has certain capabilities. Accordingly, the current treatment practice
assumes that the machine can cause a beam of selected rectangular shape
and intensity to intersect a central fixed point in space from any solid
angle. Therefore, the positioning of the patient and the use of multiple
positions and multiple beam directions enable one to obtain integrated
high doses on selected areas while maintaining low irradiation of other
organs. Heretofore, control of the outline of the cross-section of the
x-ray beam was accomplished by using jaw devices and control of the
intensity of the beam was possible by using absorber plates or accelerator
energy controls which provide uniform intensity across the beam
cross-section. Irregular shape field boundaries are then obtained by
mounting shadow blocks on a shadow tray and irregular intensity across the
cross-section is obtained by use of wedge filters or compensating filters
(which are shaped pieces of metal), all of which are inserted between the
jaws and the patient. These devices naturally have to be changed at every
angle.
My invention permits an entirely new method of treatment which eliminates
the need for shadow blocks, wedge filters and compensating filters of the
prior art and reduces the workload for the radiation technologists in
treatment of the patient, while at the same time permits much improved
precision in the two dimensional intensity distribution shaping of the
resulting dose distribution in the patient. Furthermore, since my
invention enables this beam shaping and intensity distribution control to
be accomplished dynamically, it enables use of more effective treatment
programs which would have been impractical in the prior art.
In conventional therapy, rectangular field shapes are formed by four motor
driven jaws in the radiation head. Irregular field shapes for individual
portals are then produced by mounting shadow blocks on a shadow tray
between the jaws and the patient. The shadow blocks shield critical organs
not invaded by the tumor. The radiation beam can be directed at the
prescribed treatment volume from a single direction (single port therapy),
from two or more directions (multi-port therapy), or the beam can be swept
through an arc (arc or rotation therapy), all by rotating an isocentric
gantry, for example. A cylindrical-shaped region of high dose is produced
by a rectangular field in multi-port, arc or rotation therapy.
In multi-port therapy, the shadow blocks are changed for each beam angle.
If the beam angle is not vertical, the shadow blocks must be locked to the
shadow tray to avoid their falling off. Handling these blocks individually
or on shadow trays is time-consuming. The shadow blocks are typically made
by pouring a heavy metal into a pre-cut mold, which is also
time-consuming. The shadow blocks can be heavy, difficult to handle, and
dangerous if they fall on the patient or the radiotherapy personnel. In
arc or rotation therapy, it is not practicable to change the shadow blocks
continually or in small steps of beam angle. Also, this can require that
the technologist go back into the shielded treatment room for each
treatment field, a time-consuming process.
The usual treatment field shapes result in a three-dimensional treatment
volume which includes segments of normal tissue, thereby limiting the dose
that can be given to the tumor. The irradiation dose that can be delivered
to a portion of an organ of normal tissue without serious damage can be
increased if the size of that portion of the organ receiving such
radiation dose can be reduced. Avoidance of serious damage to the organs
surrounding and overlying the tumor determines the maximum dose that can
be delivered to the tumor. Cure rates for many tumors are a steep function
of the dose delivered to the tumor. Techniques are reportedly under
development to make the treatment volume conform more closely to the shape
of the tumor volume, thereby minimizing the product of volume and dose to
normal tissue, with its attendant effects on the health of the patient.
This other technique could possibly permit higher dose to tumors or can
result in less damage to normal tissue. These techniques reportedly
involve moving the x-ray jaws during treatment, scanning the x-ray beam or
using multileaf collimators. Generally, in the prior art, multileaf
equipment has not been capable of shaping internal regions of the field,
e.g., islands and longitudinal peninsulas.
In a technique called dynamic therapy, one set of jaws is set to form a
narrow (e.g., 4 cm) fan x-ray beam and the spread of the fan beam is
varied by the second set of jaws to conform to the boundaries of the
prescribed treatment volume as the beam is swept or stepped in angle
around the patient and as the patient and associated table top are moved
through the fan beam. A computer controls the movements of the table top
in x, y and z, the gantry angle, the upper jaws during start and stop of
the scan, the lower jaws throughout the scan, and the dose rate. The
complexity is such that great care must be exercised in preparing for such
treatments, which consumes considerable time.
A technique has also been proposed in which a narrow collimated lobe of
x-rays is scanned over the treatment field, permitting production of
irregular field shapes at selected beam angles. Because only a small
fraction of the x-ray output is within the narrow lobe, the effective dose
rate is low and the time to produce a portal field is hence long and
multi-port treatment times are excessively long. Also, scanning individual
fields is not readily applicable to arc and rotation therapy modes.
Machines have been built in which each of the lower pair of jaws is divided
into a number (e.g., 5 to 32) of narrow bars called leaves. Each leaf may
be about 8 cm thick (in the beam direction) to provide adequate
attenuation of the x-ray beam (down to about 1%), about 0.5 to 1.5 cm wide
and about 14 cm long physically (not SAD). Each leaf can be moved
independently by a motor drive. This permits the production of irregularly
shaped fields with stepped boundaries, thereby avoiding shadow blocks for
many situations in portal therapy. The shape can be changed as the beam
direction is swept in arc or rotation therapy. The disadvantage of this
technique of replacing the lower jaws by a multiplicity of leaves is that
each leaf is quite large and heavy, requiring a motor drive system which
consumes considerable space. There is limited room in the radiation head
for all these components so either sacrifices in performance are made
(such as fewer leaves, limited field size) or the construction costs
become large.
In a different technique, the conventional upper and lower pairs of jaws
are retained and a set of leaves is mounted between the jaws and the
patient. Each leaf moves in a plane, driven by a rotating cam or pushed by
a form corresponding to the desired irregular field shape. In one early
concept, each leaf was thick enough to attenuate the x-ray beam to the
required level (to about 5% of unattenuated beam intensity), the ends and
sides of the leaf forming a rectangular parallelpiped, hence the ends and
sides were not aimed toward the x-ray source. In a recent concept, a
multiplicity of small diameter rods forms a stack sufficiently thick to
provide the required beam attenuation. Each rod can slide with respect to
its neighbors. A form corresponding to the desired field shape boundary is
used to push the assembly of rods so that their ends form a similar beam
boundary. Since the rods are small in diameter, the radiation field
boundary can be relatively smooth (very small steps) and tapered (focused)
toward the source. However, varying the field shape as a function of beam
angle without entering the treatment room can require a quite complex
drive system because the large number of rods requires that they be driven
enmasse instead of individually.
Wedge filters are pieces of metal which are tapered in one direction but of
constant thickness in the orthogonal direction. They are used to produce a
more uniform dose distribution in a treatment volume when it is irradiated
from two directions which are less than 180.degree. apart. And they are
used at any gantry angle as a crude compensation for the variation in
depth from the patient's surface to the plane at treatment depth. In both
cases, only an approximate correction of dose distribution in the
treatment volume is achieved. Typically, standard wedges are used, with
wedge angles of 15.degree., 30.degree., 45.degree. and 60.degree..
Intermediate angles are achieved by using two exposures per field, one
with wedge filter, one without. Since manual insertion and retraction of
wedges is laborious, fixed angle (typically 60.degree.) auto-retractable
wedge filters have been developed. Essentially all wedged fields then
require two exposures, one with the wedge filter, one without. This is a
time-consuming process, especially in rotational therapy, since an extra
gantry rotation is required.
Compensators, often termed compensation filters, are formed or assembled
pieces of metal which are shaped to match the patient's demagnified
anatomical shape so as to attenuate the x-ray beam by the amount that
would have occurred if the patient thickness to depth of treatment plane
were uniform. However, their use has been more limited because of the
needs for custom shaping for each patient and manual insertion for each
field.
Computed tomography (CT) images for treatment planning are typically
obtained in successive planes which are normal to the patient axis. After
transfer of these images, internal structures, target volume and patient
surface can be outlined directly on the treatment planning computer
display. However, in conventional radiotherapy, correction is required for
divergence of the x-ray beam in the direction through the successive CT
planes. This is a computation chore (beam's eye view) for the treatment
planner and a mental visualization chore for the radiation therapist.
OBJECT OF THE INVENTION
An object of the invention is to provide an improved method of radiation
treatment enabling more resolution and precision in treatment by more
precisely enabling control of the radiation intensity distribution across
the fan beam cross section.
A further object is to enable dynamic, real time changes in the cross
section intensity distribution of the fan beam to provide more effective
patient treatment.
A further object of the invention is to provide a new system or an
accessory to conventional medical electron accelerators and to radiation
treatment and like techniques to permit dynamic control of
three-dimensional spatial distribution of radiation dose in a treatment
volume of arbitrary external and internal shape employing a fan x-ray beam
which can be delivered, for example, in the same parallel planes in the
patient as the computed tomography (CT) imaging planes.
These objects of the invention and other objects, features and advantages
to become apparent from the following descriptions.
SUMMARY OF THE INVENTION
A fan x-ray beam, such as is produced by employing a slit aperture in
conjunction with an x-ray source, is established. This could be
accomplished using the collimator jaws of a conventional medical linac to
produce a rectangular slit field at normal treatment distance. A multileaf
collimator (MLC) is positioned in the fan beam including a first set of
leaves which can be individually moved into or out of the fan x-ray beam
to block or pass individual radiation pixels. Continuous monitoring of
alignment of the patient's anatomy with both inner and outer edges of the
fan beam is obtained with a linear detector array retractably mounted on
the opposite side of the patient from the x-ray source. Tapered
extensions, added to a second opposite set of leaves of the MLC are
variably positionable to attenuate the dose rate in individual radiation
pixels of the fan x-ray beam. The patient scan is obtained by moving the
patient perpendicularly to and through the fan x-ra field while the dose
delivered in each radiation pixel is dynamically controlled. Normal tissue
is protected by the positions of the first set of leaves of the MLC, which
attenuate transmission to less than 5% of open field dose. Depth
variations from the patient surface to the plane at treatment depth are
compensated at each radiation pixel of the field by the positions of the
tapered extensions of the second, opposite, set of leaves of the MLC,
providing variable transmission from 50% to 100% of open field dose, for
example. Reduced dose to critical organs such as the spinal cord can
thereby be delivered in each treatment fraction.
To compensate for the fact that the treatment beam is now a fan shape, one
can operate, for example, the klystron at higher than conventional RF
power. The RF pulse length and the ratio of beam pulse length to RF pulse
length are increased so that the beam duty cycle is increased. The purpose
of this combination is to achieve preferred treatment times with the fan
x-ray beam. For example, an open field dose of 300 cGy at depth of dose
maximum (D-max) can be delivered to a 40.times.40 cm field in 240 seconds
(4 minutes), with individual control of dose in each of 1600 1.times.1 cm
radiation pixels.
The MLC can be constructed as an accessory to a standard conventional
radiotherapy machine wherein by retraction of the compensator fingers to
their storage positions on the MLC leaves, multileaf collimation of
irregular fields is retained. By retracting the MLC leaves to their
support frames, conventional x-ray therapy with the four jaws in the
radiation head is retained using shadow blocks for irregular fields.
Conventional electron therapy is also retained.
Because other modes of therapy may be retained, interlock sensors for
excess electron beam current and collapsed electron beam lobe are
installed in the radiation head. Since the MLC could be installed in the
space normally occupied by the conventional wedge filter tray, an
automatic retractable support tray system for opposed angle wedge filters
and for custom compensators would be mounted inside the radiation head.
Advantages of the invention are:
1. Elimination of prior art shadow blocks, wedge filters, and conventional
compensators. Wedge tilt in any direction relative to the field is
obtainable without mechanical rotation.
2. One-to-one match of treatment geometry and CT slice images. Avoidance of
beam's eye view treatment planning computation.
3. Increased depth dose for a given x-ray beam energy.
4. Reduced penumbra longitudinally at depths other than SAD with multiple
ports.
5. Field sizes to 40 cm width and any length.
6. Scanning movement of patient table only longitudinally, eliminating need
for lateral and vertical scanning movement of patient table by use of
dynamic compensation and dynamic field shaping.
7. Continuous monitoring of alignment of patient's anatomy with treatment
beam during every treatment, with image contrast sensitivity superior to
conventional port films. When implemented as accessory, retains the
capability of conventional electron therapy and of conventional x-ray
therapy with collimator jaws and shadow blocks and with multileaf
collimator.
These and further constructional and operational characteristics of the
invention will be more evident from the detailed description given
hereinafter with reference to the figures of the accompanying drawings
which illustrate a current envisioned preferred embodiment and
alternatives. Clearly, there are many other ways one might build the
embodiments of the hardware to carry out the inventive method and these
examples should not be considered as the only way considered to carry out
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view from the x-ray source of multiple-leaf fields according to
the invention.
FIG. 1a shows the leaves in the configuration for a right oblique treatment
of the region of FIGS. 2-5.
FIG. 1b shows the leaves in the configuration for a right lateral treatment
of the region of FIGS. 2-5.
FIG. 2 is an illustration of a complex target region for use of the
invention, the region of cervix-pelvic nodes-para-aortic lymph nodes
region based on: Chin, L. M., et al, "Int. J. Radiation Oncology, Biol.,
Phys" Vol. 7, pp 61-70.
FIG. 3 is a section of the target region in the patient mid-saggital
section plane 3--3 of FIG. 2.
FIG. 4 is a section of the target region in the section plane 4--4 of FIG.
3.
FIG. 5 is a section of the target region in the section plane 5--5 of FIG.
3.
FIG. 6 is a sectional view of the collimator according to the invention as
shown in the section plane 6--6 of FIG. 7.
FIG. 7 is a sectional view of the collimator according to the invention as
shown in the section plane 7--7 of FIG. 8.
FIG. 8 is a view of the collimator of the invention as seen from the
patient treatment region looking toward the x-ray source.
FIG. 9 is a view from the bottom of a fan x-ray beam flattening filter with
inherent shielding.
FIG. 10 is a sectional view of the filter of FIG. 9 along the section line
10--10 in FIG. 9.
FIG. 11 is a sectional view of the filter of FIG. 9 along the section line
11--11 of FIG. 9.
FIG. 12 is an end view of the assembly showing attachment of collimator
fingers to MLC leaves.
FIG. 13 is a side view of the assembly of FIG. 12 along the section line
13--13 of FIG. 12.
FIG. 14 is a section of an alternate embodiment of the MLC leaves shown in
FIG. 12.
FIG. 15 is a section of a second alternate embodiment of the MLC leaves
shown in FIG. 12.
FIG. 16 shows a gantry mounted linear array detector.
FIG. 17 is a sectional view of the detector array shown in FIG. 16 along
the section line 17--17.
FIG. 18 is a sectional view of the array shown in FIG. 17 along the section
line 18--18.
FIG. 19 is a sectional view of the array shown in FIG. 18 along the section
line 19--19.
FIG. 20 is a block diagram of the electronics system for the linear array
detector of FIGS. 16-19.
FIG. 21 is a diagram defining the parameters for calculating the multileaf
penumbra for various shaped leaf ends.
FIG. 22 is a plot of the penumbra for the configurations defined in FIG.
21.
FIG. 23 shows a forty-leaf collimator showing support motor drive with
compensator fingers attached as viewed from isocenter.
FIG. 24 is a sectional view of the collimator of FIG. 23 along the section
line 24--24 from the side with compensator fingers attached.
FIG. 25 is a sectional view of the collimator of FIG. 23 along the section
line 25--25.
FIG. 26 is a sectional view of the collimator of FIG. 23 along the section
line 26--26 to show the curved end tapered MLC leaves.
FIG. 27 is a sectional view of the collimator of FIGS. 23-26 along section
line 27--27, showing frames, lead screws, ball bearings and support rods.
FIG. 28 is a sectional view of the collimator of FIGS. 23-27 along section
line 28--28.
FIG. 29 is a diagram of a radiation treatment plan which is possible using
the invention.
FIG. 30 is a longitudinal section through the subject of the diagram of
FIG. 29.
FIG. 31 is a cross section diagram through the subject of the diagram of
FIG. 29.
FIG. 32 shows control and monitoring electronics for MLC and compensator
fingers.
FIG. 33 shows a schematic diagram of a toroid beam pulse sensing system.
FIG. 34 shows a top view of a pressurized and interlocked dual foil
electron scatterer.
FIG. 35 is a sectional view of the device of FIG. 34 along the section line
35--35.
FIG. 36 is a top view of an evacuated and interlocked dual foil electron
scatterer.
FIG. 37 is a sectional view of the device of FIG. 36 along the section line
37--37.
FIG. 38 shows a radiation head with insert system for conventional static
compensator and automatic wedge filter and with toroid beam sensor.
FIG. 39 is a sectional view of the system of FIG. 38 along the section line
39--39.
FIG. 40 is a sectional view of the system of FIG. 38 along the section line
40--40.
LEXICON
The following is a listing of terms, abbreviations, units, and definitions
used throughout this specification.
cGy: centiGray, 10.sup.-2 Joules per kilogram of absorbed dose, a unit of
mean energy imparted by ionizing radiation to matter.
compensator filter: device which modifies the distribution of absorbed dose
over the radiation field.
depth dose: absorbed dose at a specified depth beneath the entrance surface
of the irradiated object.
D-max: depth of maximum absorbed dose.
dynamic changing with time in accord with a radiation plan as the radiation
dose progresses.
flattening filter: device which homogenizes the absorbed dose over the
radiation field.
imaging pixel: rectangular elements which together add to form an image.
isocenter: the position around which the radiation x-ray therapy source
moves to achieve optimum treatment of a tumor in a patient.
MeV: million electron-volts.
MLC: multileaf collimator.
penumbra: fringe at edges of the radiation field, where the radiation
intensity falls off rapidly with distance from the full intensity region
of the field.
radiation pixel: rectangular elements of radiation which together add to
form the radiation field.
SAD: source-axis distance, the distance from the x-ray source to the
isocenter.
SSD: source-skin distance, the distance from the x-ray source to the skin
of the patient.
tomography: radiography of layers (slices) within the patient.
Other standard terminology is defined in Medical Radiology--Terminology,
Pub.788, International Electrotechnical Commission, Geneva , Switzerland,
1984.
Glossary
The following is a glossary of elements and structural members as
referenced and employed in the present invention.
10--collimator
11--flat cylinder
12--leaves
14, 16--multileaf half assemblies
18, 20--leaf support frames
22, 23--lower jaws
24--electrical drive motor for half frame
25--threaded shaft
26--rod
27--threaded bushing
28--upper sub-leaves
29--lower sub-leaves
30, 32--rods
34, 36--bushings
38--threaded shaft
40--threaded hole
42--flexible cable
44--motor
46, 48--spur gears
50, 52--subframes
54--correction motor
56--chain
58--sprocket
60--rods
62--upper plate
64--side wall
66--lower plate
68--lip
70--jaw frame
72--bearing
80--flattening filter assembly
82--slit aperture
84--flattening filter piece
86--cylindrical tungsten shield piece
88--aluminum mounting plate
90--MLC leaves with multiple notch
92--compensator fingers
94--slide bar
96--mating slot
98--support rod
100--lead screw
102--detent
104--notch in the MLC leaf
106--ridge in the MLC leaf
108--alternate MLC leaf with simple notch
110--second alternate MLC leaf
112--linear array of detectors
114--gantry
116--patient treatment table
118--MLC housing
120--fan beam
122--detector crystals
124--shielding strips
126--photodetectors
128--lead strips
130--collimator slit
132--electronics
134--telescoping support
136--analog multiplexer
138--preamplifier
140--integrator
142--sample and hold circuit
144--integrator
146--A/D convertor
148--computer memory
150--clock and timing controls
152--control logic
154--video monitor
156--leaves
158--drive system
160--support frame
162--motors
164--threaded shafts
166--collimator jaws
168--TV camera
170--lens
172--mirror
174--toroid
200--reflective surface
202--flange
204--fiber optics for light
206--fiber optics for detector
208--first foil
210--second foil
212--button
214--foil holder
216--spring
218--bellows
220--lip on foil holder
222--pinch off
224,226--trays
230--upper jaws
231--drive apparatus for upper jaws
232--ionization chamber
234--x-ray target
236--electron window
238--carousel for filter and scatterer
240--lower jaws
242,244--wedge filters
246,248--support bars
250,252--lead screws
254--motors
256--lead shielding
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings wherein reference numerals are used to
designate parts throughout the various figures thereof, there is shown in
FIG. 1 an example of multileaf field shapes of the collimator 10 mounted
in a flat cylinder 11 for a complex shaped clinical site, the region of
cervix-pelvic nodes-para aortic nodes, as illustrated in FIG. 2. In this
example, the field is 36 cm long. Its irregular width is defined by 24
pairs of leaves 12, each producing a 1.5 cm strip shadow in the radiation
field at SAD (source-axis distance). The fields are presented for only two
gantry angles but they illustrate the range of field shape variation
during essentially full gantry rotation.
FIG. 1 is drawn assuming that both upper and lower conventional jaws are
used to define the field rectangular limits (36 cm long, 15 cm wide at
30.degree. gantry angle, 13.5 cm wide at 90.degree. gantry angle) and that
the multileaf system simply provides the extra shadow blocking required
within the rectangle. This permits shallow leaves 12 of 4.5 cm (1.77 inch)
thickness tungsten (18.2 g/cm.sup.3) for 5% transmission, the usual
shielding criterion for shadow blocks, instead of 7 cm or more thickness
tungsten for 1% transmission, the usual criterion for jaws. The maximum
extension of any leaf into the field in FIG. 1 is only 9 cm at SAD and
only 2 cm beyond centerline. Assuming a more extreme case of 5 cm
extension beyond centerline from a field edge 7 cm from field center; 2 cm
beyond center for a 20 cm wide field; and allowing for about 1 cm jaw
overlap, the leaves would need to be only 13 cm long projected to SAD,
about 6.84 cm (2.7 inches) actual length.
About 95% of all treatment field | | |
