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    • Abstract: MULTILEAF COLLIMATOR POSITIONALREPRODUCIBILITY EVALUATED WITHA TWO-DIMENSIONAL DIODE ARRAYA ThesisSubmitted to the Graduate Faculty of theLouisiana State University andAgricultural and Mechanical College

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MULTILEAF COLLIMATOR POSITIONAL
REPRODUCIBILITY EVALUATED WITH
A TWO-DIMENSIONAL DIODE ARRAY
A Thesis
Submitted to the Graduate Faculty of the
Louisiana State University and
Agricultural and Mechanical College
in partial fulfillment of the
requirements for the degree of
Master of Science
in
The Department of Physics and Astronomy
by
Kara King Ferachi
B.S., Louisiana Tech University, 1998
May 2003
ACKNOWLEDGMENTS
There are several people without whose support I could not have completed
this project. First, I would like to thank Dr. Thomas Kirby, my thesis committee
chairman. His input and guidance proved invaluable in the development of my
ideas into a successful thesis project. This thesis truly could not have been
completed without him.
I would also like to thank Angela Stam for always listening to my many
complaints and for encouraging me to stick it out when I wanted to give up. I
would like to thank Dr. Bice for his suggestions and for helping me learn along the
way. I am very grateful to all of the staff at Mary Bird Perkins Cancer Center for
sharing their knowledge with me and for always helping me find the answers to my
endless questions.
Finally, I would like to thank my husband Kyle for supporting me throughout
my many, many years as a graduate student. He has been a sounding board for
the past four years and has always encouraged me to do what makes me happy. I
could not have done this without him.
ii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ................................................................................. ii
LIST OF TABLES............................................................................................. iv
LIST OF FIGURES .......................................................................................... v
ABSTRACT...................................................................................................... vi
CHAPTER 1. INTRODUCTION .........................................................................1
CHAPTER 2. REVIEW OF LITERATURE........................................................13
CHAPTER 3. MATERIALS AND METHODS ...................................................20
3.1 Description of the Multileaf Collimator...................................................20
3.2 Description of the Diode Array ..............................................................20
3.3 Diode Array Output Variability...............................................................22
3.4 Description of an Edge Function ...........................................................23
3.5 Determining the Edge Function of a Single Leaf...................................23
3.6 Testing Multileaf Positional Reproducibility on the Central Axis............28
3.6.1 2.0 cm Leaf Extension .................................................................28
3.6.2 7.5 cm Leaf Extension .................................................................31
3.6.3 15.0 cm Leaf Extension ...............................................................31
3.7 Testing Multileaf Positional Reproducibility Off Axis .............................31
CHAPTER 4. RESULTS ..................................................................................34
4.1 Diode Array Output Variability...............................................................34
4.2 Edge Function Comparisons.................................................................34
4.2.1 Film Edge Function vs. Diode Array Edge Function ....................37
4.2.2 Central Axis Functions vs. Off Axis Functions .............................39
4.3 Reproducibility on the Central Axis .......................................................42
4.4 Reproducibility 10 cm Off Axis ..............................................................42
CHAPTER 5. DISCUSSION.............................................................................45
CHAPTER 6. CONCLUSIONS.........................................................................48
REFERENCES .................................................................................................50
VITA..................................................................................................................52
iii
LIST OF TABLES
Table Page
1.1 Examples of tissue-specific side effects of radiation ................................2
3.1 Location of leaves tested relative to the central axis ..............................29
3.2 Approximate fifty percent positions ........................................................29
4.1 Standard function fitting parameters ......................................................39
4.2 Distances corresponding to seventy percent intensity............................40
4.3 Distances corresponding to thirty percent intensity ................................40
4.4 Standard deviations of the diode responses and corresponding
distances on the central axis ..................................................................43
4.5 Standard deviations of the diode responses and corresponding
distances at 10 cm off axis .....................................................................44
iv
LIST OF FIGURES
Figure Page
1.1 A Varian linear accelerator head .............................................................5
1.2 A single leaf from a multileaf collimator .................................................10
1.3 Comparison of IMRT and conventional 3-D radiation therapy ...............11
3.1 The Sun Nuclear Corporation prototype diode array ..............................21
3.2 A typical edge function from a collimator edge.......................................24
3.3 Effect of leaf displacement on diode readings........................................27
3.4 An illustration of the locations of the leaves tested within the multileaf
collimator................................................................................................30
3.5 Multileaf collimator leaf extensions.........................................................32
4.1 Fluctuations in the diode response along the central axis column
of the diode array ...................................................................................35
4.2 Fluctuations in the diode response along the central axis row of
the diode array .......................................................................................36
4.3 Comparison of edge functions produced by film and by the
diode array .............................................................................................38
4.4 Comparison of edge functions fitted to central axis and 10 cm off
axis data.................................................................................................41
v
ABSTRACT
When delivering the total dose via a sequence of small fields shaped by a
multileaf collimator, it is important to consider leaf positional reproducibility. A
small error in the leaf position can result in large dose errors to the entire field.
This is true for both dynamic multileaf collimation and step and shoot delivery.
The goal of this research project is to design a method of quality assurance that is
easily reproducible, sensitive to small changes in leaf position, and requires
minimal time on the part of the medical physicist to carry out. This paper
describes a system of measurements performed with a two-dimensional diode
array that can be used in conjunction with a leaf edge function determined from
radiographic film to quickly and easily test the reproducibility of the multileaf
collimator position with acceptable sensitivity.
vi
CHAPTER 1
INTRODUCTION
Radiation was first used therapeutically in the late 1800s. Radiation
therapy treatment machines have evolved from very low energy x-ray machines
into a variety of treatment options that include high-energy electrons, high-energy
photons, and even heavier particles such as neutrons or protons. The goal of
radiation therapy is to kill tumor cells while at the same time limiting the dose to
normal tissues. Accomplishing this goal will enhance tumor control probability and
limit the adverse side effects that result from irradiating healthy tissue. The target
volume must be selected to maximize the chance of controlling the tumor.
Diagnostic data from computed tomography (CT), positron emission tomography
(PET), and magnetic resonance imaging (MRI) scans can be used to localize the
tumor. In addition, clinical experience must be used to choose margins so that
microscopic cancer cells surrounding the observable tumor volume are included.
Large doses of radiation to healthy tissues will not only make the patient
more uncomfortable through the course of treatment but can also leave lasting
complications, depending upon the type of tissue that is irradiated and the dose
the healthy tissue receives. For example, radiation exposure to the skin will cause
temporary reddening, or erythema, of the skin. More serious side effects, such as
necrosis, can occur in the skin if sufficient dose is delivered. Some examples of
adverse effects associated with irradiation of specific healthy tissues are shown in
Table 1.1. These and other side effects associated with different types of tissues
must be considered in the course of treatment planning. The radiation oncologist
1
must therefore choose a treatment volume that will encompass the tissues of
possible tumor cell involvement while minimizing the volume of normal tissue that
is irradiated.
Table 1.1. Examples of tissue-specific side effects
of radiation.
Tissue Associated Adverse Effect
Neurologic deficit
Brain
Loss of cognitive function
Spinal Cord Paralysis
Radiation pneumonitis
Lung
Fibrosis
Kidney Radiation nephropathy
Stomach Ulcer
Radiation is conformed to the tumor volume in several different ways. The
most straightforward method is blocking non-involved areas from the radiation
beam. The physician can outline the radiation area desired on a two-dimensional
image such as a conventional x-ray or a computer generated view. The radiation
beam is conformed to this area by collimators and custom blocks, which will be
discussed later in this chapter. To further minimize the amount of radiation
passing through healthy tissue on the way to the tumor volume, several beams
may be utilized from different directions to deliver the total dose, rather than a
single beam. In this way, doses from the different beams add together to
maximize the dose to the tumor volume without delivering the maximum dose to
the healthy tissues. These beams may be assigned different weights to further
2
limit the amount of radiation passing through especially sensitive structures such
as the spinal cord.
With the advent of computer controlled treatment machines and increased
treatment planning computer capability, it has become possible to further shape
the radiation distribution in three dimensions. Intensity modulated radiation
therapy, or IMRT, is an example of three- dimensional beam shaping and will be
discussed later in this chapter. The development of these advanced treatment
techniques has helped to improve tumor control and cure rates.
The first machines used in radiation therapy produced low energy x-rays
that were only capable of penetrating a few millimeters into tissue. These units
are therefore useful to treat only superficial lesions. An orthovoltage therapy x-ray
unit has a slightly higher energy in the 200 to 300 kilovolt range. Many institutions
still use the lower energy units regularly, especially for the treatment of skin
lesions (Hendee and Ibbott 1996). However, due to the low penetrating ability of
the beams, it is not useful to superimpose different beams, so single beams are
generally used. Beam conforming for these machines is limited to shaping
(blocking) in two dimensions.
Higher energy machines capable of producing photon beams in the
megavolt range were developed to overcome the limitations of the orthovoltage
units. One of the early megavoltage units used in radiation therapy is the Cobalt
unit. These units contain a radioactive Cobalt-60 source located inside of two
stainless steel containers that are welded shut to prevent radioactive material from
escaping. The source is shielded with materials such as lead or tungsten so that
3
the beam travels in a nearly single direction out of the container when desired.
The Cobalt units also contain collimators that restrict radiation not traveling in the
desired direction. When the Cobalt unit is not in use the source is retracted to a
fully shielded position to prevent any unintended radiation exposure. Due to the
higher energy radiation compared to the orthovoltage units, beams can be
combined as well as shaped to conform to tumor volumes. Cobalt units may still
be found in some radiation departments but have mostly been replaced by linear
accelerators.
Even higher energy radiation beams are provided by medical linear
accelerators, the machine most commonly used today to deliver high doses to the
tumor volume (Figure 1.1). Patients are treated by beams of electrons or x-rays
that are produced by a linear accelerator. To produce these beams, electrons are
accelerated to very high energies using microwaves. If an x-ray beam is desired,
a target material is moved into the path of the electron beam. The electrons
interact with this material and a photon beam is produced from the target. The
photon beam passes through a stationary primary collimator that attenuates
photons not directed in the desired cone of directions. The beam is next directed
through an ionization chamber and a flattening filter. The current produced in the
ionization chamber is proportional to the intensity of the radiation beam. This
current is converted into monitor units. The dose to the patient is delivered by
programming the linear accelerator to produce a certain number of monitor units.
The purpose of the flattening filter is to differentially attenuate the beam so that a
4
more uniform dose distribution is achieved perpendicular to the beam’s central
axis at ten centimeters depth.
Secondary
Collimators
Leaf Motion
Multileaf
Collimators
Figure 1.1. A Varian linear accelerator head. The location of the secondary
collimators, the multileaf collimators and their direction of motion are all
illustrated.
Shaping of the radiation beam before it reaches the patient is accomplished
using several devices. The first of these is the secondary collimators. These
collimators consist of jaws that are under motor control and can be moved to
5
create a rectangular field of any size up to 40 cm by 40 cm at isocenter. The
secondary collimator jaws move along an arched path to follow beam divergence
and shape the beam in two dimensions. As previously mentioned, in order to
minimize complications in healthy tissue, areas outside of the tumor volume
should be shielded from the radiation beam. Secondary collimators are used to
shape a rectangular field, but because no tumor volume is rectangular in shape,
additional methods are needed to further contour the beam to the tumor volume.
Additional field shaping can be done in either two or three dimensions with a
tertiary collimator.
Traditional tertiary blocking or beam shaping is most commonly
accomplished by custom-made cerrobend blocks. Cerrobend, a high-density
material with a low melting point, may be melted and poured into a custom-made
mold. The shape of the mold is specific to each beam used to treat each patient.
Once the Cerrobend hardens, the custom block is used as a tertiary collimator
mounted outside the head of the accelerator in the path of the beam to shape the
field before it reaches the patient.
Cerrobend blocks are an effective way to shape the radiation beam, but
require a time consuming manufacturing process. Another tertiary collimator that
almost completely eliminates the need for custom blocks and therefore reduces
production time is the multileaf collimator (MLC). A MLC is made up of many
opposed pairs of small leaves mounted into two carriages on either side of the
field. The leaves may be extended under motor control to shape a field that is
conformal to the tumor volume. The location of the MLC on the accelerator head
6
depends upon the accelerator manufacturer. Some manufacturers replace one
pair of secondary collimator jaws with leaves, in which case the MLC is not a
tertiary collimator. “The disadvantage of having the MLC leaves so far from the
accelerator isocenter is that the leaf width must be somewhat smaller and the
tolerances on the dimensions of the leaves as well as the leaf travel must be
tighter than for other configurations” (Boyer et al 2001). Tighter tolerances for leaf
travel are required closer to the target because the further away the leaf is from
isocenter, a small error in leaf positioning will translate to a larger distance at
isocenter. The smaller leaf size in multileaf collimators that replace the secondary
jaws can also be an advantage because the leaves do not need to be as long and
therefore decrease the size and bulk of the treatment head. Manufacturers that
design the MLC as a tertiary collimator mount the MLC leaf banks just below the
secondary collimator jaws, as shown in Figure 1.1. A disadvantage of this method
is that it increases the bulk of the treatment head. Also, the amount of clearance
between the treatment head and the patient may be decreased if custom blocks or
wedges are used along with multileaf collimation (Boyer et al. 2001).
In addition to varying positions of the MLC, multileaf collimators also vary in
their direction of leaf motion and leaf shape according to the MLC manufacturer.
Some manufacturers use a “focused” MLC in which the leaf carriages travel in an
arch to follow beam divergence. In other machines, the leaves move only in a
plane perpendicular to the treatment beam. This is a “nonfocused” multileaf
collimator that does not follow beam divergence. The leaf edges for this type of
MLC must be rounded in order to produce an acceptable penumbra. Sun and Zhu
7
define the penumbra as “the region at the edge of a radiation beam over which the
dose changes rapidly as a function of distance from the beam axis” (1995). The
penumbra is described mathematically by an edge function. Edge functions and
penumbra will be discussed in greater detail in chapter three.
There are two ways to describe the edge of the leaf. First is the physical
edge of a leaf which is measured by the shadow of the light field. Second is the
radiographic edge of the leaf, which is considered to be the point in the radiation
field that is fifty percent of the intensity of an open field. In this project, all
references to the leaf edge correspond to the radiographic edge, not the physical
edge. It is possible with a rounded leaf edge that the penumbra is wider than that
of a focused leaf. There is also some concern that the penumbra of a rounded
leaf edge can change with distance off axis (Boyer et al. 2001). In addition, the
edge of the light field does necessarily agree with the radiation beam edge for a
rounded leaf end at off-axis locations. The sides of each leaf have a tongue and
groove design in order to minimize inter-leaf leakage. Figure 1.2 shows a single
leaf from a multileaf collimator, illustrating the rounded leaf edge and the tongue
and groove design of the sides of the leaf.
Another application of the tertiary collimator is three-dimensional beam
shaping. Three-dimensional shaping of the radiation beam results in a modulated
intensity throughout the treatment field. This treatment technique is called
intensity modulated radiation therapy (IMRT). IMRT treatments provide a better fit
to the tumor volume than previous conformal radiation therapy. Figure 1.3 shows
a comparison of the fit of the dose delivered with an IMRT plan to that delivered
8
with a conventional conformal therapy plan. IMRT treatment plans are created
with an inverse planning technique. In inverse planning, the physician is able to
specify the dose to be delivered to the tumor volume and may also enter the dose
limits to the healthy structures surrounding the tumor volume. These values are
entered into a treatment-planning computer, which then creates a plan that
matches the specified parameters as closely as possible. The IMRT plan is
delivered as a combination of fields entering the patient from different angles.
Either compensators or multileaf collimators are used to modulate the intensity of
each beam.
Compensators are made by varying the thickness of a particular material to
partially attenuate the beam before it reaches the tumor volume. The material
used in the compensator varies depending on the amount of attenuation required.
The compensator material covers the entire field and is mounted to the outside of
the treatment head. Compensators are usually used in conjunction with custom
blocks for additional beam shaping.
To create intensity modulated fields with a MLC the leaves are moved to
shape very small field sizes. Varying amounts of monitor units determined by an
inverse planning algorithm are delivered to these small fields to closely fit the dose
only to the tumor volume. Two different methods are used to deliver intensity-
modulated fields with a multileaf collimator: dynamic multileaf collimation and step
and shoot delivery. In dynamic multileaf collimation, the leaves are in constant
motion throughout the beam delivery. In step and shoot delivery, the beam
delivery is paused while the leaves are repositioned.
9
Direction of Direction of
beam travel Leaf Motion
Direction of
beam travel
Figure 1.2. A single leaf from a multileaf collimator. This picture shows in detail
(a) the rounded leaf edge and (b) the tongue and groove design of the sides of the
leaves, as well as indicating the direction of beam travel with respect to the leaf.
10
(a)
(b)
Figure 1.3. Comparison of IMRT and conventional 3-D radiation therapy. The
plan shown in (a) is a conventional three-dimensional radiation therapy prostate
treatment plan and (b) is an IMRT prostate treatment plan.
11
When delivering the total dose via a sequence of small fields shaped by a
multileaf collimator, it is important to consider leaf positional reproducibility. As will
be discussed in detail in the next chapter, a small error in the leaf position can
result in large dose errors to the entire field. This is true for both dynamic multileaf
collimation and step and shoot delivery. This research project will attempt to
design a method of quality assurance that is easily reproducible, sensitive to small
changes in leaf position, and requires minimal time on the part of the medical
physicist to carry out. This project describes a system of measurements
performed with a two-dimensional diode array that can be used in conjunction with
a leaf edge function to quickly and easily test the reproducibility of the multileaf
collimator position with acceptable sensitivity.
12
CHAPTER 2
REVIEW OF LITERATURE
The introduction of multileaf collimators into radiation oncology has
provided many advantages, while at the same time introducing new challenges.
Multileaf collimators, when used as a replacement for conventional blocks, reduce
the time required by eliminating the block production process, as well as reducing
the time required for the radiation therapist to set up between sequential fields
(Jordan and Williams 1994). However, with advances in technology such as
intensity modulated radiation therapy (IMRT), the leaves of a multileaf collimator
may be utilized in a manner beyond what was originally intended (LoSasso, Chui,
and Ling 2001). Additional quality assurance methods are therefore needed to
ensure normal leaf function and accurate leaf position.
There are several possible causes of error in leaf position addressed in the
literature. LoSasso et al. found that leaf positional inaccuracy “appears to be
related to the amount of usage of individual leaf motors”. LoSasso et al. found that
after IMRT was initiated at their facility, leaf motors had to be replaced more often.
The leaves at the center of the multileaf collimator were found to be the most
susceptible to motor failure. These center leaves are used for prostate IMRT
treatments at their institution (2001). LoSasso, Chui, and Ling also cite loss of
counts in the primary leaf position encoders as a source of leaf position error.
They state, “On occasion that the chronic loss of counts of a primary encoder
becomes excessive, leaf position errors could exceed 0.5 mm at isocenter.
Reinitializing the MLC will temporarily alleviate the problem, but position errors
13
may go unnoticed because an interlock will not be activated until the error reaches
2 mm”. LoSasso, Chui, and Ling recommend a semi-weekly test to check for
these types of encoder errors (2001). Two other sources of uncertainty according
to Budgell et al. are “the precision of the MLC control system” and “the absolute
accuracy of calibration of the MLC leaf positions”. Budgell et al. assert, “If leaves
are calibrated within ±1 mm, an MLC controller precision of 0.1 mm can only
guarantee an absolute positional accuracy of ±1.1 mm” (2000). According to
LoSasso, Chui, and Ling, a “1 mm error in the calibrations of the jaws and leaves
can be tolerated” when the multileaf collimator is being used only to shape a static
field. In IMRT treatments “leaf movements need to be executed much more
precisely. Therefore, a much tighter tolerance of ~0.2 mm” is necessary (2001).
The American Association of Physicists in Medicine Task Group No. 50
describe calibration of the leaf position for a Varian multileaf collimator in the
following way:
The Varian MLC calibrates the leaf positions using narrow infrared
beams built into the collimator assembly that transect the paths of the leaves.
The calibration procedure is carried out automatically each time the MLC
operating system is initialized. Each leaf is driven through its range of travel.
As a given leaf intersects the infrared beam, the values returned by its
position encoders are acquired. These values are used … to calibrate the
leaf position. The calibration values are saved in a table for use by the
control system. (Boyer et al. 2001)
This calibration procedure is a very important step to ensure leaf positional
accuracy. According to Boyer et al., “periodic checking and recalibration are also
needed to ensure the integrity of the controlling system” (2001).
Several researchers have documented the importance of quality assurance
of multileaf collimator leaf stability. Budgell et al. state that in dynamic multileaf
14
collimation used for IMRT, a 1 cm slit traveling across a distance of 10 cm can
have up to a 10% change in dose for a 1mm change in the width of the slit due to
leaf position inaccuracy in dynamic multileaf collimation. According to Budgell et
al., “leaf position errors will normally not cause dosimetric errors in step and shoot
deliveries except for very thin fields for which output factor is strongly dependent
on field width” (2000). Low et al. state that accuracy in leaf positioning for step
and shoot deliveries is very important in regions where there are multiple sub field
abutments. Low et al. also state, “errors in leaf positioning will cause
corresponding errors in the delivered dose in the abutment region, with the dose
errors proportional to the penumbra slope at the edge of each sub field.” Low et
al. concluded that errors in the abutment regions of sub fields could cause 16.7%
dose errors for 6 MV photons for each millimeter of error (2001).
According to Chui, Spirou, and LoSasso, “positional inaccuracy of the
leaves may affect the dose distribution everywhere within the field” (1996). This is
a major difference from conventional blocking with multileaf collimators. In
traditional blocking, the leaves are set to a single position throughout the entire
treatment, so an error in leaf position will affect only the area just inside the block.
With dynamic multileaf collimation, an error in a single leaf’s position can be
carried across the entire field, leading to hot spots if the leaf is lagging behind or
cold spots if the leaf is in front of its intended position (Chui, Spirou, and LoSasso
1996). Hot and cold spots can also result in step and shoot deliveries in the areas
of multiple sub-field abutments. These findings emphasize the importance of
15
assuring leaf positional accuracy when the multileaf collimator is being used
clinically for IMRT in either the dynamic or step and shoot mode.
Many different quality assurance procedures have been proposed in the
literature. LoSasso, Chui, and Ling recommend a semi-weekly test using Kodak
V2 Ready Pack film to verify leaf positions at the end of the treatment day. This
test exposes the film to “a DMLC field that produces a matrix of high intensity
regions, 1 mm wide and spaced 2 cm apart”. If any leaf position is off by more
than 0.2 mm, that leaf’s motor must be replaced. The physicist evaluates the film
visually without having to digitize it. This decreases the time required for
evaluation, an important point for any QA process (2001). LoSasso, Chui, and
Ling also introduced a quality assurance method that uses a cylindrical ion
chamber to measure the dose “for a uniform field delivered dynamically with a
small, 0.5-cm-wide, sweeping gap”, which is normalized to a static 10 x 10 cm2
field created by collimator jaws. The authors state that this method is “capable of
detecting less than 0.1 mm deviation” in the leaf position (1998).
Budgell et al. created a test to measure daily variation in leaf position using
a 1 cm slit formed only with the leaves. This slit moves across a distance of 10
cm. Budgell et al. found that “a 1 mm change in slit width leads to a 10% change
in dose” making this test very sensitive to inaccuracies in position. The doses
were measured with an ionization chamber and normalized to a 10 cm x 10 cm
static field (2000). Another test was devised by Chui, Spirou, and LoSasso to be
used as a routine quality assurance check. In this test, “the paths of the left and
right leaves were intentionally offset by 1 mm to produce hot spots on the resultant
16
intensity profile”. If the leaves are traveling to their intended positions, lines of
increased intensity will be shown on film at equally spaced intervals. The widths of
these hot spots are easily determined by visual inspection. This provides a quick
and easy way to identify leaf positional inaccuracies on a daily basis (1996).
LoSasso, Chui, and Ling have also tested accuracy of leaf position with
gantry angle. LoSasso, Chui, and Ling performed the previously mentioned 0.5
cm wide sweeping gap test at gantry angles of 0, 90, 180, and 270 degrees “to
assess the effect of gravity on the performance of the multileaf collimator in
dynamic mode.” The dose delivered was measured with a cylindrical ion chamber
at isocenter and was found to be independent of gantry angle (1998). In a
separate quality assurance test, LoSasso, Chui, and Ling used the Sun Nuclear
Corporation Profiler, a diode array, to measure relative dynamic multileaf
collimator output at gantry angles at 90 and 270 degrees. They found that “small
variations were observed on the central axis, consistent with the ion chamber
measurements, but larger variations exist at off-axis points” (2001). Chui, Spirou,
and LoSasso state that their previously mentioned quality assurance test that
produces high intensity lines at equally spaced intervals can be performed with the
gantry angle at ninety degrees with the collimator turned so that “the leaves move
perpendicular to the floor to maximize the gravity effect” to test the effect of gravity
on leaf positional accuracy (1996).
Most of the quality assurance procedures discussed in the literature involve
using radiographic film and/or ion chambers. Other research has been carried out
to determine the usefulness of diode arrays compared to film and ion chamber
17
measurements. According to Paul Jursinic, the biggest advantage of using diodes
is the immediate availability of the results. Jursinic also states, “other advantages
of diodes include high sensitivity, good spatial resolution, small size, simple
instrumentation…ruggedness, and independence from changes in air pressure”
(2001). According to Essers and Munheer, another advantage of diodes is “the
sensitivity per unit volume of a diode is about 18,000 times higher than for an air-
filled ionization chamber” (1999). To evaluate the intensity profile produced using
dynamic multileaf collimation, Papatheodorou et al. used radiographic film, point
measurements from a cylindrical ion chamber, and the SNC Profiler 1170 linear
diode array. Papatheodorou et al. found very good agreement between the
profiles measured by the Profiler and the ion chamber. They concluded that the
diode array is “a very useful tool for measuring intensity profiles with the condition
that the relative sensitivity of diodes is carefully corrected” (2000). Zhu et al. also
evaluated the SNC Profiler as a tool for measuring the profiles of enhanced
dynamic wedge dosimetry. Zhu et al. found the diode array to be about twenty
times faster than an ion chamber for quality assurance purposes, with very good
agreement between the measured profiles. Zhu et al. concluded that a diode
array, specifically the Profiler, “provides good spatial resolution and is useful for
commissioning a dynamic wedge” (1997). Hansen et al. used the Schuster BMS-
96 diode array to measure beam profiles of small radiation segments. The diode
array was used because the dose was delivered by 10 MU and that is not enough
time for a scan across the beam that is used during the standard calibration
procedure. According to Hansen et al., “This device is ideal for profile
18
measurements of any segment size, and can measure profiles for field sizes up to
40 cm” (1998).
Although much research has been done on multileaf positioning, most of
the work was done with cylindrical ion chambers, film, or a linear diode array. The
diode arrays were shown to be less time consuming than film or point
measurements from an ion chamber and the profiles measured with the diode
array were in good agreement with the film and ion chamber measurements. A
two-dimensional diode array should be able to provide more information over an
entire field, in a more efficient manner than film or an ion chamber, about the
multileaf performance at a variety of off-axis points. By mounting the diode array
to the gantry, the diode array can also be used to test multileaf positional
reproducibility at any gantry angle.
19
CHAPTER 3
MATERIALS AND METHODS
3.1 Description of the Multileaf Collimator
The multileaf collimator used for this research was a Varian Medical
Systems Millennium MLC-120, mounted below the secondary collimators on a
Varian Clinac 21 EX linear accelerator. The bottom of the multileaf collimator is
53.5 cm from the linear accelerator target, which corresponds to a distance of 46.5
cm from isocenter. The isocenter of the machine “is the point of intersection of the
collimator axis and the gantry axis of rotation” (Khan 1994). The isocenter for the
21 EX is at a distance of 100 cm from the source. The total field size of the
multileaf collimator is 40 cm x 40 cm, shaped by 120 individual leaves. This
multileaf collimator has two different leaf widths. The central 20 cm of the field is
shaped by leaves with a 0.5 cm projected width at isocenter and the leaves that
shape the outer 20 cm of the field project a 1.0 cm width at isocenter. The leaves
are mounted within two carriages and each leaf can move to a maximum
extension of 15 cm at isocenter from the carriage. The individual leaves have a
rounded edge and the sides of the leaves have a tongue and groove arrangement
to minimize leakage as illustrated in chapter one.
3.2 Description of the Diode Array
The diode array used in this research was a prototype version of the
MapCheckTM two-dimensional therapy beam measurement system made by Sun
Nuclear Corporation of Melbourne, Florida. The prototype contains a 10 cm x 10
cm array of 221 diodes. The active detector area of the diodes is 0.8 mm x 0.8
20
mm. The diodes are spaced 1 cm apart on each row and each row is spaced 5
mm apart, as shown in Figure 3.1. The inherent buildup on top of the diodes is
about 1.32 g/cm2. The physical dista


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