Note: Descriptions are shown in the official language in which they were submitted.
CA 02611834 2007-11-21
SCINTILLATING FIBER DOSIMETER ARRAY
FIELD OF THE INVENTION
This invention relates generally to the field of dosimetry and, more
particularly, to rapid,
high-resolution dosimeters for advanced treatment technologies.
BACKGROUND OF THE INVENTION
Treatment modalities such as Intensity Modulated Radiotherapy (IMRT), helical
tomotherapy and radiosurgery are pushing current dosimeter technologies to
their limits. Two
major driving forces are involved: 1) complex, two-dimensional field patterns
encountered in
IMRT and tomotherapy require high resolution (i. e. , small volume) dosimeters
that can be
stacked or arrayed to provide a rapid but precise two- or three-dimensional
dose measurement,
which in turn requires that the dosimeters be water-equivalent in order not to
disturb the fluence
in the measurement plane and 2) small fields used in all three treatment
modalities, down to lx1
2 i
cm n IMRT and just a few millimeters in radiosurgery, give rise to volume
effects such as
spatial averaging in most dose detectors with detecting volumes larger than a
few cubic
millimeters.
Radiographic films possess high spatial resolution and are used for two-
dimensional dose
measurements. They are, however, subject to drawbacks. The need to develop the
films before
reading makes their use for online assessment impossible. Moreover, the
development process
affects the film response. Radiographic films are also notorious for over-
responding to low-
energy photons and they are not water-equivalent. Finally, the precision of a
radiographic film
used for dose measurement in the clinic is often limited to 5%. Radiochromic
films, which do
not require development and are closer to water-equivalence in the megavoltage
energy range,
can also be used to evaluate dose distributions. However, radiochromic films
are temperature
dependent and sensitive to ultraviolet light. Achieving better than 5%
reproducibility in routine
fashion with radiochromic film is also challenging.
Detector arrays have been implemented in the clinic to achieve a faster and
more precise
dose reading than films. To date these arrays have been made of either
semiconductor dose
detectors or ion chambers. These arrays allow online evaluation of a dose
pattern with the
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precision of a single dosimeter. The spacing between the detectors determines
the resolution of an
array: the closer the detectors are to each other, the more continuous the
dose information will be.
Because the materials used with current detector arrays are not water-
equivalent (typically made
of silicon or air), the use of such an array creates a perturbation in the
particle fluence. Moreover,
the non-water equivalence of these detectors prevents the use of three-
dimensional arrays with
closely packed detectors. The current detector arrays also suffer from other
limitations depending
on the type of detector in use. For semiconductor detectors, there may be
significant angular
dependence and poor reproducibility for low dose fractions. For ion chambers
there may be some
dose averaging because their detecting volume is usually larger than other
dose detectors.
Thermoluminescent detectors (TLD) have also been used in groups to measure
dose at different
location simultaneously, but the need to read each one individually limits
frequent use of a large
number of TLDs.
All of the above-mentioned detectors allow, at most, measurements in two-
dimensional
planes. The only detectors that can be used for three-dimensional measurements
are dosimetric
gels. Dosimetric gels are either based on the behavior of ferrous ions or on
the polymerization of
a monomer. They can be produced using a large variety of chemical formulas and
each has its
own set of advantages and disadvantages. However, most gels share a delicate
fabrication process
and require a time-consuming development process that makes them unsuitable
for online
measurements.
One of the key properties that is desired in the next-generation dosimetry
systems is
water-equivalence. Water-equivalence guarantees that the measurement
instrument, when
immersed in a water tank, does not perturb the beam fluence and allows
stacking and arraying of
multiple dosimeters in the treatment field for two- or three-dimensional dose
measurements. In
order to facilitate the quality assurance of complex treatment modalities,
there exists a need for
small, water-equivalent dosimeters.
SUMMARY OF THE INVENTION
In accordance with the present invention, a radiation dosimeter is provided
for measuring
a relative dose of a predetermined radiation type within a detection region by
using a plurality of
scintillating optical fibers. The scintillating optical fibers are located in
a predetermined spatial
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arrangement in the detection region, and generate optical energy in response
to irradiation with
the predetermined radiation type. Optically coupled to the scintillation
optical fibers are a
plurality of collection optical fibers that receive the optical energy
generated by the scintillation
optical fibers. The collection optical fibers transmit the optical energy to a
photo-detector that, in
turn, generates electrical signals indicative of the optical energy received.
In an exemplary embodiment, a phantom material, such as a water-equivalent
material, is
located within the detection space, and the scintillation optical fibers are
embedded in the
phantom material. The phantom material may take the form of a plurality of
modular slabs, at
least some of which have the scintillation optical fibers embedded within.
Slabs with no
scintillation optical fibers may also be used as desired for equilibrium
material.
The scintillation optical fibers are relatively short in length, and function
as small local
radiation detectors. These detectors may be distributed as desired within a
three-dimensional
detection space so as to gather data regarding dose distribution within the
space. The scintillation
optical fibers may be arranged parallel to each other or perpendicular, or in
any other desired
arrangement. In one embodiment, a longitudinal axis of the scintillation
optical fibers is
perpendicular to a transmission axis of a radiation beam being used while, in
another, the
longitudinal axis is parallel to the beam axis. Similarly, a plurality of the
scintillating fibers may
all reside in a common plane perpendicular to the beam axis, or there may be
scintillating fibers
located in multiple such planes.
The photo-detector of the present invention has an imager that converts the
optical energy
transmitted by the collection optical fibers to an electric signal that may
thereafter be converted to
a dose. It may also include a set of optical filters that allow discrimination
between the light
produced through scintillation and light produced through other means. In an
exemplary
embodiment, a housing surrounds the photo-detector components and shields them
from stray
light and radiation.
An optical connector may also be used for providing repeatable optical
coupling between
the collection optical fibers and the photo-detector. The photo-detector
imager takes a two-
dimensional image of the input optical signals. The connector may be such that
it maintains the
output ends of the collection optical fibers in a predetermined spatial
relationship relative to one
another, and connects to the photo-detector in such a way that the collection
optical fibers have a
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precise location relative to the imager. In one embodiment, the connector
maintains the
collection optical fibers in an equally-spaced arrangement that minimizes
optical crosstalk
between the optical signals that are output from the fibers. Various
arrangements of the
collection optical fibers relative to the imager may be used, such that a
correlation between the
image taken by the photo-detector and the spatial distribution of the
scintillating optical fiber
detectors may be selected as desired by the user.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic side view of a scintillating fiber dosimeter according
to the present
invention.
Figure 2 is a schematic side view of a scintillating fiber dosimeter similar
to that of Figure
1, but for which a single slab is used for a phantom component.
Figure 3A is a schematic view of a layout of scintillating fiber detectors
embedded in a
water-equivalent slab, where the fiber detectors are parallel and aligned at a
mid-line of the slab.
Figure 3B is a schematic view of an alternative layout of scintillating fiber
detectors
embedded in a water-equivalent slab, where the fiber detectors are distributed
in an "X" pattern.
Figure 3C is a cross sectional view of a water-equivalent slab according to
the present
invention having scintillating fiber detectors fully embedded inside.
Figure 4 is a schematic view of an embodiment of the invention in which the
scintillating
fiber detectors are embedded in a vertical direction in a water-equivalent
slab.
Figure 5A is a detailed schematic view of a scintillating fiber detector that
may be used
with the present invention.
Figure 5B is a detailed schematic view of a scintillating fiber detector
similar to that of
Figure 5A, but for which the detector is coated with a reflective material.
Figure 6A is a schematic view of a photo-detector component that may be used
with the
present invention.
Figure 68 is a schematic view of a photo-detector component similar to that of
Figure 6A,
and which includes a set of optical filters and mirrors.
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Figure 7 is a cross sectional view of a connector system that may be used with
a
scintillating fiber dosimeter array according to the present invention.
Figure 8A is a graphical depiction of a relative radiation dose measurement
taken using a
scintillating fiber dosimeter array according to the present invention, for
which the dose
distribution of a radiotherapy beam is in a lateral direction.
Figure 8B is a graphical depiction of a relative radiation dose measurement
taken using a
scintillating fiber dosimeter array for which a dose distribution of a
radiotherapy beam is in an
axial direction.
DETAILED DESCRIPTION
The present invention provides a dosimeter that makes use of scintillating
optical fiber
detectors. In an exemplary embodiment of the invention, miniature plastic
scintillating fiber
detectors are used that have a unique combination of properties. Their water
equivalence is
maintained over a broad energy range (e.g., 0.2 to 25 MeV). Furthermore, they
provide a highly
sensitive medium, which enables small sensitive volumes (e.g., less than 2
mm3) and high
resolution, linearity to dose, dose rate independence, energy-independent
response, and real-time
readout.
Shown in Figure 1 is a scintillating fiber dosimeter array according to the
present
invention. The dosimeter is located on a radiotherapy treatment table 13, a
phantom component
being composed of a series of water-equivalent modular slabs. Slabs 12a, 12b
and 12c have
scintillating fiber detectors embedded in them, while slabs 11 a and 11 b do
not. While five slabs
are shown in the embodiment of Figure 1, those skilled in the art will
recognize that it is possible
to have more or fewer slabs depending on the particular application. The slabs
may be made of a
common plastic material with properties similar to water, such as polystyrene,
acrylic or Lucite,
or may be some other special chemical compound designed to be similar to water
or human
tissue. With the scintillating optical fibers being embedded in a water-
equivalent phantom, beam
perturbation is minimized.
During operation, the slabs are irradiated by a radiation beam 14. The impact
of the
radiation on the scintillating fiber detectors embedded in the slabs 12a, 12b
and 12c results in the
generation of scintillation light in the detector fibers in an amount
proportional to the radiation
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dose detected. The scintillation light is transported by optical fiber cable
15 to photo-detector
component 17, to which it is connected by a connector system 16 that provides
reliable,
reproducible coupling.
The phantom material and the scintillating fiber detectors may be arranged in
any desired
configuration. Since each of the detectors is small, and may act as a point
detector, detection
points may be selected as desired within a three-dimensional detection space.
The embodiment
shown in Figure 1, for example, has three slabs within which detectors are
embedded, as well as
slabs on the top and on the bottom of the phantom in which there are no
embedded detectors.
These top and bottom slabs therefore serve only to provide equilibrium
material for the phantom,
while the detectors provide detection points within each of three different
vertical planes. It may
also be desired to use the detectors in an application that has no phantom.
For example, the
scintillating fiber detectors may be used during a radiotherapy treatment of a
patient.
Figure 2 shows another example of the invention in which only a single slab 12
is used
which has embedded scintillating fiber detectors. Two slabs ha, 11 b, above
and below the slab
12, have no detectors, and serve only as equilibrium material. While Figures 1
and 2 show two
possible configurations of the present invention, those skilled in the art
will recognize that
numerous other combinations of scintillating fiber detectors, with or without
different
combinations of equilibrium material, may be used without departing from the
intended scope of
the invention. Moreover, the slabs shown in the figures are modular, allowing
them to be mixed
and matched as desired to form a phantom component of a particular
configuration. However,
other arrangements of phantom material may also be used.
Different layouts of scintillating fibers may be used within a slab. Figure 3A
depicts one
possible configuration, in which small, equal-sized pieces of the
scintillating fibers are arranged
in a parallel configuration, equally spaced along a midline 18 of the slab 23.
The scintillating
fiber segments are coupled to non-scintillating collection fibers 19, which
extend from the side of
the slab 23. As indicated in the figure, the scintillating fiber segments 28
occupy only a small
space relative to the overall fiber combination, and the detection that they
provide is therefore
only along the midline 18. The lengths of combined scintillating and
collection fibers may be
embedded in grooves machined in the slab which, for example, may be made of
water-equivalent
plastic. The grooves may be of the same depth as the fiber diameter, so that
the fibers are
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completely contained within the grooves. For example, one-millimeter diameter
fibers may be
embedded in grooves having a depth of one millimeter. The grooves are machined
to provide a
tight fit to the optical fibers, and to leave as small of an air gap as
possible. As shown in the
configuration of Figure 3A, the grooves are equally spaced and extend from a
first edge of the
slab to the mid-line 18. The portions of the collection optical fibers that
protrude from the slab
23 together form optical cable 15 that is connected to the photo-detector
component 17 (as shown
in Figures 1 and 2).
Figure 3B shows another possible configuration for fibers embedded in a
detector slab 23.
In this example, the detector fibers are arranged in the shape of the letter
"X," which allows them
to provide more spatial information. As in the arrangement of Figure 3A, the
scintillating fiber
segments of Figure 3B are located only at the very end of the fibers shown in
the figure, e.g., in
the last one millimeter. In this configuration, half of the detectors are
located at the ends of the
collection fibers of fiber group 22, which are embedded in one side of the
slab 23, one quarter of
the detectors are located at the ends of the collection fibers of fiber group
20, which are
embedded in a direction perpendicular to the group 22, and the remaining one
quarter of the
detectors are located at the ends of collection fibers of fiber group 21,
which extend from the side
of the slab 23 opposite that of group 20. Thus, the scintillating fiber
detectors, being very short,
together form an "X" shape across the slab 23, and thereby represent a variety
of detection points
spanning a two-dimensional space.
For a slab within which fibers are embedded in grooves made in the slab, it
may be
desirable to fix a thinner slab of water-equivalent material with shallow
grooves atop the slab
holding the fiber detectors. Figure 3C is a schematic view showing a cross
section of two such
slabs 23, 24 that are fixed together, for example, by glue to form what is
essentially a single slab.
This configuration allows the fiber detectors to be tightly surrounded by
equilibrium material,
with little or no air space in between. It may also serve to keep a light-
tight enclosure around the
scintillating fiber detectors, so that no external light can reach them.
Another embodiment of the invention is shown in Figure 4. In this embodiment,
the fiber
detectors are oriented in a vertical configuration, such that the terminal
ends of the fibers face
toward the radiation source. That is, rather than the fibers being oriented in
a direction
perpendicular to a transmission direction of the radiation source, they are
parallel thereto. This
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configuration allows the scintillating fiber detector to have a smaller cross
section relative to the
beam direction and therefore better spatial resolution. As in the other
orientations, the
scintillating fiber segments are relatively short, and are coupled to
collection fibers 25 that
protrude from the bottom of the slab and together form the optical cable 15
that is connected to
coupler 16. The scintillating fiber segments may each be located as desired
within the three-
dimensional space of the phantom material 10, thus allowing a user to tailor
the detection
arrangement to a specific application.
The configurations shown in Figures 3A-3C and 4 show only a small number of
detectors
shown to provide for better clarity. However, those skilled in the art will
understand that many
more detectors may be used to provide good resolution within the detection
space.
Figure 5A is detailed schematic view of a scintillating fiber as it might be
embedded in
the slabs of a phantom component of the present invention. A piece of
scintillating fiber 26 is
coupled to a light carrying optical collection fiber 27. The scintillating
fiber 26 has a core 28 that
produces scintillation if stimulated by radiation, and a non-scintillating
cladding 29 that improves
the guidance of scintillation light produced in the core 28. The collection
fiber 27 also possesses a
core 30 and cladding 31 but does not produce scintillation. A black jacket 32
surrounds the
portion of the collection fiber 27 that is external to the water-equivalent
slab to shield it from
ambient light. A coupling 33 between the scintillating fiber 26 and the light
carrying optical fiber
27 is made with a bonding agent that will minimize the differences in
refractive index between
the scintillating fiber 26 and the light carrying optical fiber 27. A lens, a
fiber optic taper or other
device may also be inserted in the coupling region to increase coupling
efficiency.
The embodiment shown in Figure 5B is similar to that of Figure 5A, but a
reflective
coating is added to the scintillating fiber. The reflective coating surrounds
the piece of
scintillating fiber, except for the portion that is coupled to the non-
scintillating fiber 27. The use
of this reflective coating tends to increase the collection of a usable amount
of light by the
scintillating fibers in that it minimizes loss from the fiber by reflecting
inward scintillation light
generated therein. The reflective coating may also act as a shield by
preventing external light
from entering the fiber segment. In this way, the entry of ambient light into
the fiber is
minimized.
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A photo-detector system that may be used with the scintillating fiber detector
of the
present invention is shown in Figure 6A. An external casing 35 anchors the
photo-detector
components, and shields them from ambient light. An imager 36 converts the
light coming from
the input fiber optic cables to a quantitative electronic signal in the form
of an image. The imager
can be a CCD camera, a CMOS sensor, or any other device capable of producing a
reproducible
image either in monochrome or color with the level of light produced by the
scintillating fiber
detectors. Also located within the external casing are an objective lens 37,
an intermediate space
38 and a connector system 39, which connects the non-scintillating optical
fiber to the photo-
detector system. The outer casing may be made of thick metal to act as a
radiation shield that
will protect the imager 36 from stray radiation. In case where the thickness
is not sufficient to
completely protect the imager, a post-processing algorithm may be used to
remove transient noise
produced by scatter radiation.
A variation of the photo-detector system of Figure 6A is shown in Figure 6B.
In this
embodiment, a set 42 of optical filters and mirrors is added to the
intermediate space 38 to allow
for filtration or separation of undesired light from another source such as
Cerenkov radiation.
Such an undesired light source can contribute to the noise of the system, and
it is desirable to
remove it. Filtration can be achieved by different techniques including, but
not limited to, the use
of band-pass filters to isolate the spectral component of the scintillation
emission spectrum that
overlaps only minimally with that of the undesired light source. One other
possible way of
filtering the undesired light source is by measuring the signal that contains
both the scintillation
light and the overlapping undesired light with different color filters,
thereby applying spectral
decoupling.
The objective lens 37 of the photo-detector system collects the light from the
optical fiber
coupled to the connector system 39 and projects an image onto the
photosensitive surface of the
imager 36. This allows for simultaneous reading of the output of all the
scintillation detectors
coupled to the connector system. For maximal collection efficiency, this
objective lens should
have a small F-number. The collection fibers are coupled to the connector
system in a known
arrangement, such that a correlation is made between the signal detected from
each fiber and the
corresponding scintillating fiber detector within the detection space. Thus,
the image projected
onto the imager is indicative of the distribution of radiation for the
detectors that are coupled to
the connector system 39. Those skilled in the art will recognize that the
specific correlation
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between the image of the photo-detector system and the spatial location of the
detectors is
completely customizable, and may be selected as desired by the user.
Mechanical reproducibility is an important feature of an absolute dosimeter
array. Shown
in Figure 7 is a schematic view of a connector system that may be used with
the present
invention. The connector attaches to the photo-detector system and is
configured so as to
accommodate a plurality of non-scintillating collection fibers that transport
light from the fiber
detectors. The connector shown in Figure 7 has a plate with a row of grooves
40 within which
the collection fibers reside. The grooves are spaced at an equal pitch (e.g.,
two millimeters) along
the length of the connector, and may thereby serve to fix numerous fibers in
an equally-spaced
configuration. The grooves may be sized so that each provides just enough
space for one of the
collection fibers. Thus, if a flat surface is placed atop the plate portion
containing the grooves, the
fibers are snugly contained within the groove space, with very little air
space surrounding them.
This helps to minimize noise from being introduced due to stray ambient light.
It also provides
for the ability to stack several plates together to form a connector with
multiple rows of collection
fibers. The plate having the grooves 40 in Figure 7 also has a second set of
grooves 41 on the
opposite side. The set of grooves 41 can accommodate another row of collection
fibers in the
same manner as the set of grooves 40. As this plate is stacked on another
plate with a flat top
surface, the fibers are snugly contained within the grooves 41.
Those skilled in the art will understand that the connector may have multiple
plates with
different number of grooves as may be desired by the user. Moreover, for
clarity the drawing
shows a relatively small number of grooves per row, and each row may have many
more grooves.
For example, in one embodiment, a connector may hold sixty fibers per row. In
addition, several
of the connectors can be mounted to the external casing of the photo-detector
system thus
allowing a very large number of optical fibers to be read simultaneously by
the imager. By virtue
of the predictable groove configuration, the connector enables repeatable
positioning of the fibers
relative to the imager across multiple connector insertion and extraction
cycles. It is also
desirable that all of the fibers being imaged simultaneously have their ends
(from which optical
energy is emitted) in a common plane. This allows for proper imaging of the
optical outputs onto
the image. Also shown in Figure 7 are mounting holes 45, by which the
connector may be
mounted to the photo-detector system. These mounting holes aid in repeatable
positioning of the
connector relative to the photo-detector.
CA 02611834 2014-11-20
Figures 8A and 8B are graphical depictions of dosimetry measurements obtained
with a
scintillating fiber dosimeter array such as that shown in Figure 2, using a
slab containing
scintillating fiber detectors 12 in a configuration like that depicted in
Figure 3A. For this specific
application, twenty-nine detectors were embedded in the slab 12. Figure 8A
shows the results of
measurement of three different beam profiles. The respective field sizes
(i.e., the collimated
beam sizes at a distance of 100 cm from the source) for the three results
shown in the figure are
4x4, 10 x10 and 20x20, all in square centimeters. The corresponding results in
Figure 8A (having
a position axis in millimeter units) can be seen to be quite accurate.
Figure 8B shows a depth-dose curve acquired with the same apparatus as is used
for the
results shown in Figure 8A. In each of Figure 8A and Figure 8B, the symbols
making up the
points on the curves represent measurements performed with the present
invention, while the
lines represent measurements taken with a single ion chamber scanned across a
volume of water.
Thus, as can be seen, the scintillating fiber detector array provided a
correct relative output factor
(within 0.3%) for each of the different field sizes. The standard deviation on
a series of ten
consecutive measurements was less than 1% within the field. The maximum
relative difference
between the scintillation detector and ionization chamber was equal to 0.9% in-
field for the
profiles (Figure 8A) and 1.6% for the depth dose (Figure 8B).
The present invention enables rapid and accurate measurements of the most
complex
radiation therapy treatment modalities such as IMRT, tomotherapy and
radiosurgery. Due to the
unique properties of the plastic scintillating fiber, the total time required
for mandatory quality
assurance tests of these complex treatment modalities is minimized. The
dosimeter described
herein possesses the accuracy and precision of a single point detector without
requiring time-
consuming processing such as the development or scanning of films. The
scintillating fiber
detectors of the invention are also fully water equivalent so as to not cause
perturbation of the
beam even when the detectors are closely distributed in three-dimensional
space.
While the invention has been shown and described with reference to a preferred
embodiment thereof, the scope of the claims should not be limited by the
preferred embodiments
set forth in the examples, but should be given the broadest interpretation
consistent with the
description as a whole.
What is claimed is:
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