The 2D-ARRAY seven29 A new way of dosimetric verification of - PTW

Application note
The 2D-ARRAY seven29
A new way of dosimetric verification of IMRT beams
Jörg Bohsung, Klinik für Strahlentherapie Charité Berlin, Standort Mitte
1. Purpose
The purpose of this application note is to demonstrate the possibilities of the 2D-ARRAY seven29
from PTW in combination with the VeriSoft analysis software as a dosimetric verification tool of
clinical IMRT fields. For this demonstration, the Varian IMRT system is used, which consists of the
IMRT treatment planning system Eclipse Helios V7.2.24 and Varian Clinac accelerators, which are
equipped with dynamic multileaf collimators.
The reader can use this application note in two ways:
• as a demonstration of the possibilities of the PTW 2D-ARRAY for fast and accurate absolute 2D
dose verifications of complex fluence modulated fields.
• as systematic instruction (chapter 5) of the whole verification process for a Varian IMRT system.
Users of non-Varian IMRT solutions should be easily able to transfer all verification steps to their
IMRT system.
2. Introduction
2.1 Intensity modulated radiotherapy
Intensity modulated radiotherapy (IMRT) is a special
form of three-dimensional conformal radiotherapy
(3D-CRT). In contrast to conventional 3D-CRT, where
only the beam apertures are shaped to the irregular
form of the target, IMRT is based on the use of x-ray
beams with individually optimized, non-uniform
photon fluencies across the beam area (see Fig. 1).
The use of IMRT can improve dose distribution within
a patient's body, especially if the target has a
complex three-dimensional shape, e.g., a concave
part that surrounds a critical structure. Fig. 2 shows a
typical dose distribution of an IMRT plan for a patient
suffering from a head and neck tumor. Only the
primary tumor and the elective lymph node area are
treated with high doses, while the spinal cord and the
contra lateral parotid gland receive only uncritical
dose levels. It is impossible to create such a complex
dose distribution with homogeneous photon fields.
Although the advantages of this new treatment technique are obvious, IMRT is used by only a few
centers for a relatively small number of patients. IMRT, still in its infancy, has many different
medical, technical and economic problems that remain to be solved within the next few years.
These problems are discussed in detail in [1].
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Figure 1: An irregularly shaped field with uniform
fluence (left) compared to a fluence-modulated
field (right).
Figure 2: A typical IMRT dose distribution of a head
and neck target volume. The high dose area (red)
follows the target shape, while the spinal cord and
the right parotid gland are spared.

2.2 IMRT delivery techniques
Several IMRT delivery techniques can be found in publications (see, e.g., [1]). Today, most newly
installed IMRT delivery systems use modulated beams with fixed beam directions. The fluence
modulations are created by the multileaf collimator (MLC), which runs in one of two special IMRTmodes:
• The multi-segment method approximates the continuously modulated optimized fluence of each
fixed field direction by a stepwise fluence distribution. This is done by combining a set of small
homogeneous field segments of different weights, which are shaped by the MLC. This method
is often also referred to as step-and-shoot delivery, because the radiation is only turned on
when the leaves have reached their prescribed positions for each segment.
• A technically more advanced method is the dynamic MLC technique (often also called sliding
window method). Here the fluence profile along the moving direction of a leaf pair is created by
sweeping the leaf pair with different openings over the field while the beam is on all the time.
Both methods result in similar final dose distributions, although the dynamic MLC approach can
generate a better approximation of the optimized fluence and allows a faster delivery. On the other
hand, the multi-segment method is easier to implement and needs a slightly lower number of
monitor units.
In contrast to conventional homogeneous fields, the number of monitor units for both methods as
given by the treatment planning system no longer has a simple relation to the delivered dose of
that field. Therefore, many groups feel it necessary to verify each single treatment field
dosimetrically before its clinical use.
2.3 IMRT and quality assurance
IMRT is a very complex treatment modality. Therefore, new quality assurance procedures must be
implemented throughout the complete clinical process [1]. Here we will only discuss some
technical aspects. Typically, the clinical implementation of IMRT passes through at least three
steps:
1. The installation and commissioning phase of the treatment planning and the delivery system
2. The clinical starting phase, where each treatment is tested by an individual verification
procedure, which typically is extremely time-consuming and not very standardized
3. The routine phase, where standardized QA procedures are used to guarantee safe and reliable
IMRT deliveries for a large number of patients
Most groups focus only on the first two phases. However, the third phase is by far the most critical
one for a stable IMRT routine delivery. The routine use of IMRT for large patient numbers is only
possible if standardized QA procedures have been carefully defined and easy-to-use measuring
equipment together with user-friendly analysis software is available.
For both MLC modulation methods, the delivered doses have a complex, non-intuitive relationship
to the number of monitor units. It is also impossible to predict the exact combination of field
segments or the leaf motion patterns. Therefore, all IMRT groups, which are using the MLC for the
creation of fluence modulations, must establish a precise and reliable method for the dosimetric
verification of IMRT plans. Especially during the starting period, each plan must be dosimetrically
verified, but dosimetric verification is also recommended after software upgrades or the expansion
to new tumor entities.
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3. Dosimetric verification of treatment plans
3.1 The phantom substitution method
Because a verification of dose distributions within a real patient is not possible, the phantom
substitution method is often used, which works as follows:
• Within the treatment planning system, the patient plan is transferred - either all together or fieldby-field
- to a special phantom.
• The dose distribution is recalculated within that phantom without changing any dosimetrically
relevant treatment parameter.
• At the treatment machine, the phantom – equipped with appropriate dosimeters – is irradiated
using the IMRT fields of the real patient plan, again without changing any dosimetrically relevant
treatment parameters.
• The measurements are compared against the calculated dose values.
A large variety of phantoms and dosimeters are described in publications and are used clinically
(see for example [2]). The phantom substitution method can be performed in two different ways,
either plan-related or field-related.
3.2 The plan-related approach
For the plan-related approach, the whole plan (i.e., all
fields with their correct beam entry directions) is
transferred within the treatment planning system to a
verification phantom and the dose distribution is
calculated.
A phantom, which is suitable for the plan-related
approach typically has a cylindrical, elliptical or
spherical shape and is often made of slabs, which
allow the filling of the phantom with dosimetric films
(see Fig. 3). Additionally, it is often possible, to equip
the phantom with ion chambers for absolute point dose
measurements. PTW offers several specialized
phantoms for the plan-related approach:
• The cylindrical Head/Neck Verification Phantom T40015 for film dosimetry;
• The Head/Neck Verification Phantom T40014, which accommodates the PTW linear array
LA48;
• The Matrix IMRT Verification Phantom T40026 for the use of up to 25 ion chambers, which can
be individually placed throughout the entire height of the phantom.
The advantages of the plan-related approach are obvious:
• The entire plan can be verified within one run.
• All treatment parameters including the beam entry directions are identical to the real patient
treatment. Therefore, also critical points of the setup such as influence of the treatment couch,
etc., can be detected.
• Because the complete treatment plan is verified, a connection between the measured dose
distribution and the real anatomical situation can be made. It is therefore easy to decide if a
detected local disagreement between calculation and measurement is nevertheless clinically
acceptable or not.
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Dosimetric
film
Phantom
Hole for ion
chamber
Figure 3: A verification phantom for the planrelated
approach. The arrows illustrate the entry
directions of a typical IMRT plan.

However, the plan-related approach also has several practical disadvantages, which make its
routine use difficult:
• Real 3D dose measurements are extremely time consuming and not available under clinical
conditions. A plan-related verification is therefore normally restricted to only a few 1D or 2D cuts
out of the 3D dose cube. Therefore delivery problems (e.g., misaligned leafs) outside the
measuring zone may not be detected.
• Depending on the used measuring system, the plan-related approach may be accompanied by
severe dosimetric problems. A typical example is the film calibration, which is not trivial for films,
which are oriented parallel to the beam entrance direction.
• Because all fields are treated at once, it is often difficult to locate the reason for a detected
dosimetric discrepancy.
• The phantom-preparation is time consuming.
3.3 The field-related approach
For the field-related approach, each single treatment
field is transferred separately to a verification phantom.
All treatment parameters are the same as for the real
patient plan, except the gantry angle, which is normally
set to 0° for all beams. The field-related approach
requires only a very simple rectangular phantom,
which is able to carry a dosimetric film or another
planar dosimeter at a plane perpendicular to the beam
entrance direction (see Fig. 4). Sometimes the
phantom has additional holes for ion chamber
measurements. PTW provides the Universal IMRT
Verification Phantom T40020 for the field-related
approach.
The field-related approach is often criticized, because it does not reflect the real treatment as well
as the plan-related approach. On the other hand, the field-related approach has a number of
advantages, which are especially valuable if IMRT is operated under routine conditions.
• The verification process covers the complete modulated area of each field. All potential delivery
problems are therefore safely detected.
• Dose measurements are always performed in a plane perpendicular to the central axis; hence,
dosimetric problems (e.g., film calibration) are less critical than for the plan-related approach.
• A detected dosimetric discrepancy can easily be traced back to its reason, e.g., an improperly
adjusted leaf.
• The preparation and setup of the phantom is easy and not very time-consuming.
• The field-related approach is perfectly adapted to the use of various electronic 2D measuring
devices such as the 2D-ARRAY seven29.
For groups which have either no access to film dosimetry or want to avoid it because of its
complexity and cost, the latter point is an important argument in favor of the field-related approach.
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Dosimetric
film
Phantom
Hole for ion
chamber
Figure 4: A verification phantom for the planrelated
approach.

4. The 2D-ARRAY seven29 and the analysis software VeriSoft
4.1 2D-ARRAY seven29
The 2D-ARRAY seven29 consists of a plane matrix of 27 x 27 air-filled ion chambers. The vented
plane-parallel ion chambers are 5 mm x 5 mm x 5 mm in size and the center-to-center spacing is
10 mm. The 729 chambers cover a maximum field size of 27 cm x 27 cm. The surrounding
material is acrylic (PMMA). The package includes an interface for fast data acquisition. The display
cycle can be selected between 400 and 1000 ms.
The 2D-ARRAY seven29 allows absolute dose and dose rate measurements of high-energy
photon fields. On-site calibration is not necessary.
The data acquisition software MatrixScan is used to acquire dose or dose rate data; it displays 3D
graphics and transfers the acquired data to the software packages VeriSoft, MultiCheck or
MEPHYSTO.
4.2 VeriSoft
VeriSoft assists physicists in comparing dose distributions measured in an IMRT verification
phantom with dose distributions computed by a radiotherapy treatment planning system. Matrices
of measured and calculated points of an IMRT beam are compared by subtracting the matrices
and visualizing the result. The gamma evaluation method [3] is supported, hot and cold spots can
easily be located, and the maximum and average deviation between treatment plan and measured
beam is determined.
VeriSoft allows to import data from various treatment planning systems.
5. Dosimetric verification of IMRT fields with the 2D-ARRAY -
a step-by-step instruction for the Varian IMRT solution
In this chapter, the field-related dosimetric verification of IMRT fields with the 2D-ARRAY seven29,
as it is performed at the Charité Hospital Berlin, is demonstrated systematically.
5.1 The IMRT system
IMRT treatment planning is done with the Eclipse Helios treatment planning system V7.2.24
(Varian Medical Systems). Within Eclipse, the dynamic leaf motion instructions are calculated,
which are necessary to generate the optimized fluences at the treatment machine.
The modulated photon fields are delivered by Varian Clinac linear accelerators, which are
equipped with different dynamic multi leaf collimators (MLC-120, MLC-80 and MLC-52).
All necessary treatment parameters, i.e., monitor units, field sizes, gantry angles and leaf motion
instructions, are stored in the database of the record-and-verify system VARiS Vision 6.2 and
provided for patient treatment. In Berlin, we perform not only patient treatments but also all IMRT
verifications under the control of VARiS Vision.
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5.2 Phantom setup, CT scanning and preparation within Eclipse
For the field-related verification process, no special phantom is necessary; it is convenient to
arrange the array between plates of water equivalent material. We use a sandwich setup of water
equivalent PTW RW3 plates with a stack of 3 cm below and 5 cm above the array (see Fig. 5).
The phantom arrangement is CT scanned then in exactly the same way as it is later used for the
verification measurements. To achieve an adequate spatial resolution during the following
verification dose calculations, it is essential to scan the phantom with a sufficiently small slice
thickness. We have scanned the phantom with a slice thickness of 2 mm (Fig. 6). The scanned
phantom is imported via DICOM to Eclipse. Directly after import, it is convenient to define a user
origin within Eclipse exactly at the effective measuring point of the central ion chamber of the
array.
2D array
RW3
Figure 5: For the field-related verification the 2D-
ARRAY is simply located between RW3 plates. 3 cm
RW3 material are below, 5 cm above the array.
The effective measuring point is located in the middle of the chamber area and 5 mm below the
surface of the 2D-ARRAY. By that, all fields, which are verified afterwards, are automatically
positioned correctly with their isocenter at the effective measuring point of the central ion chamber.
Note that it is necessary to perform the scanning and import process only once. Within Eclipse, the
phantom with the correctly placed user origin can then be defined as the default phantom, which is
always used for verification.
5.3 Transfer of the patient treatment fields and dose calculation
Eclipse supports both the plan-related and the field-related approach for the verification of IMRT
plans. After an IMRT plan is accepted for treatment, the planner has to use the function “create
verification plan”, to start the verification process. Here, the user can select between several
possibilities (Fig. 7). For the field-related approach with the 2D-ARRAY, the following selections
are necessary (Fig. 7a):
• Each patient field must be placed into separate verification plans
• The couch, gantry and collimator angles must be reset to 0°
Then the verification method has to be defined (Fig. 7b), which is “Phantom” in our case. The user
can then select the correct verification phantom, which then can be set as default.
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Figure 6: The scanned phantom arrangement
after import to Eclipse. The phantom was
scanned with 2 mm slice thickness. The user
origin is marked by a red cross.

a b
Figure 7: Creation of the verification plan (both plan- and field-related) within Eclipse. The correct options for the fieldrelated
approach with the 2D-ARRAY are selected.
For each field of the patient plan, a new verification
plan is now created. The field isocenters are
automatically positioned at the correct place if the
user origin was properly defined (see 5.2). The
created verification fields have exactly the same
dosimetrically relevant treatment parameters as the
real patient fields. Any changes are suspended by
Eclipse. Thus, it is guaranteed that the verification
fields really reflect the behavior of the real patient
treatments.
After the correct transfer of the treatment fields to the
verification phantom, the planner can calculate the
3D dose distribution of each field. In Fig. 8, the dose
distribution of an IMRT field of a complex head and
neck treatment plan within the verification phantom
is depicted. Note that the frontal view already shows
the 2D dose distribution within the effective
measuring plane of the 2D-ARRAY.
5.4 Export of the verification dose plane
The dose comparison in VeriSoft requires a DICOM dose export of the 2D dose distribution within
the effective measuring plane of the ion chambers. To do that, the following steps are necessary:
• The frontal dose view in Eclipse must be adjusted in such a way that it shows the correct
measuring plane. If the user origin was defined as discussed in section 5.2, the y-coordinate of
that plane is y=0.
• The frontal dose view must have the focus. The easiest way to guarantee for that is to enlarge
the frontal dose view (Fig. 9a).
• The plane export is started by a right-click (Fig. 9a).
• Within the dose export wizard, the “absolute” dose export must be selected and the dimensions
of the dose plane must be correctly set (Fig. 9b).
The Eclipse dose export wizard automatically positions the center of the exported dose matrix at
the field isocenter (see Fig. 9b). This function is important for the later dose comparison within
VeriSoft as discussed in section 5.7 and was not available in earlier Eclipse versions.
To avoid confusion with the cryptic DICOM file names, it is convenient to save all DICOM plane
dose files within a special folder, which is given the patient’s name, for example.
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Figure 8: Dose distribution of an IMRT field within the
verification phantom. The dosimetrically relevant
treatment parameters are identical to the patient plan
and cannot be changed by the user.

a b
Figure 9: DICOM export of the correct dose plane with y=0. Note, that the center of the exported dose matrix is
automatically positioned at the isocenter.
5.5 Phantom setup and calibration measurement
The Phantom setup for the measurements is shown in Fig. 10. The gantry and collimator angles
are set to 0°. The 2D-ARRAY is located on top of a package of 3 cm RW3 material and adjusted in
a way that the isocenter is positioned at the effective measuring point of the central ion chamber,
i.e., 5 mm below the front plate of the array (Fig. 10a). Then another stack of 5 cm RW3 material is
added above the array (Fig. 10b). Finally, the SSD of this arrangement should be 94.5 cm.
a
Figure 10: Phantom setup for the verification measurements. The isocenter is adjusted to the effective measuring point
of the central ion chamber of the 2D-ARRAY.
Although the 2D-ARRAY is already calibrated in absorbed dose to water, normally each
measurement must be corrected for different air pressure and temperature, for the used photon
quality and for possible non-water equivalent properties of the phantom. To avoid these
corrections, which are time-consuming and susceptible to errors, we use a simple calibration field
with a known dose, which is delivered before each verification measurement:
• Within the treatment planning system, a simple “calibration plan” with only one homogeneous
field with a field size of 10x10 cm 2 and a gantry angle of 0° is created. Beam quality and
isocenter position are identical to those of the IMRT fields.
• The monitor units of that field are adjusted to an isocenter dose Dcal of e.g., 2.00 Gy.
• After positioning of the phantom (Fig. 10), this calibration field is always treated as the first field
of a verification session and the dose is measured with the 2D-ARRAY using MatrixScan.
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b

• The readout M of the central chamber is noted. This chamber can be selected in the “options”
menu of MatrixScan (chamber coordinates [14,14]). The dose is then visible at the status bar of
MatrixScan.
• A calibration factor f = Dcal/M is calculated, which is used for the correction of all later verification
measurements (see Section 5.7).
This calibration method has the additional advantage that it also corrects for possible deviations in
the output calibration of the linear accelerator.
5.6 Verification measurements
In Berlin, all verification measurements are
performed in the clinical mode under the control
of VARiS Vision. The verification fields are
prepared within VARiS Vision in its own
treatment course, which is linked to the patient
for whom the verification is performed. Unlike the
real treatment fields, however, the verification
fields do not add up any dose to the patient
within VARiS Vision.
Immediately after the delivery of the calibration
field (see section 5.5), the IMRT fields are
irradiated one after the other and measured with
the acquisition software MatrixScan. In Fig. 11, a
screenshot of MatrixScan during the
measurement of an IMRT field is depicted.
To block as little machine time as possible, only
the raw data are saved and no further corrections are performed during the measurements. If the
calculated DICOM dose plane data were already saved in a special patient folder as proposed in
section 5.4, it makes sense to store the measurements within the same folder.
5.7 Analysis of the measurements using VeriSoft and documentation of the
results
To guarantee a smooth and fast data analysis process,
we suggest using the following procedure for each
calculated and measured field pair:
• The calculated data set is read into matrix A. For
small fields, it is worth defining an appropriate region
of interest to avoid too many data points outside the
interesting area.
• The corresponding measured data set is read into
matrix B.
• The measured data set is calibrated by pushing the
“Calibrate” button. Within the calibration window
(Fig. 12), the measured data set is now multiplied by
the determined correction factor f = Dcal/M with
Dcal = 2.00 Gy (see section 5.5).
In the example shown in Fig. 12, the measured central chamber dose M of the calibration field
was M = 2.02 Gy.
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Figure 11: Screen shot of MatrixScan during
a measurement of an IMRT field.
Figure 12: Calibration procedure. In this
example, the measured data set is
corrected by a factor 2.00/2.02 = 0.99.

• By default, VeriSoft normalizes each matrix individually to its maximum dose. This is not
meaningful for absolute dose measurements with the 2D-ARRAY. Therefore, both data sets
have to be normalized to the same meaningful value (e.g., the dose at the isocenter).
• The correct positioning of the two data sets is checked using the “Profiles” option. Normally, no
alignment should be required. However, positional corrections of ±1 mm in both x and y
directions are sometimes necessary due to slight misalignments of the phantom during the
measurements.
Fig. 13a shows the isodose overlay, which already gives a first impression of the concurrence of
the two data sets. This concurrence is quantitatively confirmed by vertical and horizontal line scans
through the isocenter (Fig. 13b).
Line scans have the disadvantage that an investigation of the complete area is quite timeconsuming.
Therefore, VeriSoft provides additional quantitative 2D compare modes.
Figure 13a: VeriSoft isodose overlay of a typical IMRT
head and neck field. The measured isodoses are depicted
as dotted lines.
In Fig. 14, a difference plot is shown, where each measured chamber dose Dm(i,j) is compared to
the averaged calculated dose according to the following formula:
∆D(
i,
j)
D
D
=
m
( i,
j)
D
−
c
D
c
( i,
j)
( i,
j)
* 100%
Note that VeriSoft provides also a percentage difference mode, which is relative to the
normalization dose of matrix A. This mode however is not as sensitive to the low dose.
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Figure 13b: Horizontal (above) and vertical (below)
line scans through the isocenter of the IMRT field of Fig. 13a.
Calculations are shown as blue lines and the measured
chamber values are depicted as green bars.

a
Figure 14: Difference plot of the dose distribution of
Fig. 13a. Most differences are below 3% of the local
measured dose. Higher deviations are only found at highgradient
regions and at very low doses.
While dose difference plots work very good within low-gradient areas of the analyzed dose
distribution, they may show artificially large deviations in high-gradient parts (Fig. 14). To overcome
that problem, which is especially critical for highly modulated IMRT fields, Low et al. developed the
gamma analysis method [3]. The gamma method judges the concurrence of 2D dose distributions
using two criteria: the local dose difference within low-gradient areas and the distance-toagreement
(i.e., the distance of two points, which have the same dose). The user has to define two
acceptance values: The maximum acceptable local dose difference (in %) and the maximum
acceptable distance to concurrence (in mm). In VeriSoft, the gamma method is implemented
according to the method of Depuydt et al. [4], which adapts the Low algorithm to the use of discrete
data sets. For our analyses, we use always 3% dose difference and 2 mm distance to agreement.
Calculated dose value are not analyzed for dose values below 5% of the maximum.
Fig. 15 shows the resulting gamma evaluation. The acceptance criteria are not met for only a few
points below the 10% isodose. This is a typical result. Two factors are responsible for higher
deviations with very low doses:
• The measuring device has a higher relative measuring error.
• For very low doses, the dose of a dynamically collimated IMRT field is mainly determined by leaf
transmission, which is not calculated as precise as the dose within the open part of the field by
the treatment planning system.
We normally accept these dose deviations, if they only show up in low dose areas. In chapter 6,
more verification examples of IMRT fields are discussed
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Figure 15: Gamma index plot of the dose distribution of
Fig. 13a. The red dots indicate points where the
acceptance criteria (3%, 3 mm) are not reached. All
these points are located within low-dose regions.

Fig. 16 finally shows the printout of the IMRT field, as it is provided by VeriSoft and as we use it for
documentation.
5.8 Time requirement
Figure 16: Documentation of the result
The verification process with the 2D-ARRAY seven29 is highly standardized. Therefore, the entire
procedure can be performed within an acceptable time. In Tab. 1, the time requirements for the
different verification steps for a five-field IMRT plan are collected.
Future versions of VeriSoft will allow an even more automated data analysis, so that a further time
reduction can be expected.
Procedure Section Average time
[min]
Transfer of the patient treatment fields and dose calculation 5.3 10
Export of the verification dose plane 5.4 5
Verification measurements including setup 5.5 25
Analysis of the measurements with VeriSoft 5.6 20
Total 60
Table 1: Time requirement for the different steps of the verification procedure.
The time requirement listed above is for the verification process with the 2D-ARRAY of one patient
plan. However, if plans of more than one patient have to be verified, the same hardware setup for
the measurements is used, resulting in time savings. The verification process itself is also more
efficient if all fields of all patients are checked against the plans in one session.
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6. Some clinical examples
In Berlin, we use IMRT as a routine treatment option mainly for treatments of the prostate, head
and neck tumors and for irradiations of the breast if the internal mammary lymph node chain has to
be included into the target volume. In the next sections, some examples of real patient IMRT plans
and their verifications are shown.
6.1 Prostate
For prostate IMRT treatments we use a simultaneous
integrated boost (SIB) technique, where two different doses
are simultaneously delivered to different parts of the target
volume. The SIB technique provides two advantages:
Firstly, it allows an increased dose within a specific target
region (e.g., prostate) and a reduced dose to the peripheral
region (e.g., periprostatic area). Secondly, it keeps the
overall treatment time constant while increasing the target
dose. For our patients, the CTV is irradiated with a single
dose of 2.0 Gy per fraction whereas the PTV is treated
simultaneously with 1.8 Gy. In Fig. 17, a five-field IMRT SIB
treatment plan for a prostate patient is shown. With the SIB
technique, we are able to increase the therapeutic dose
safely up to 82.0 Gy without increasing the doses to the
organs at risk, especially the rectum [5].
In Fig. 18 the isodose overlays and gamma distributions for all five prostate IMRT fields of the
treatment plan of Fig. 17 are put together. Typically, prostate IMRT fields are not modulated very
much. Therefore, no critical areas are normally found during the verifications.
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Figure 17: Dose distribution of an IMRT SIB
prostate treatment plan with five fields.
Field 1 Field 2 Field 3 Field 4 Field 5
Figure 18: Isodose overlays and gamma distributions of all five IMRT fields of a prostate treatment plan (measured
isodose lines are dotted). Nearly all gamma values are < 1 in all fields.

6.2 Head and neck
For postoperative head and neck patients, we also use an SIB IMRT technique with seven
treatment fields. The former tumor volume is irradiated simultaneously with a higher dose level
than the elective lymph nodes. The spinal cord and the contralateral parotid gland are spared. An
example for a head and neck treatment plan was already shown in Fig. 2.
The verification results for all seven fields of that plan are depicted in Fig. 19. As already discussed
in Section 5.7, it is normal for such highly modulated fields that some small areas within the lowdose
part of the dose distributions show gamma values >1.
Field 1 Field 2 Field 3 Field 4
Field 5
Field 6
Figure 19: Isodose overlays and gamma distributions of all seven IMRT fields of a head and neck treatment plan (measured
isodose lines are dotted). Because of the more complex modulation, some gamma values within the low-dose regions are
>1 (see discussion in Section 5.7).
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Field 7

6.3 Breast
The gain of IMRT for breast treatments is not as
clear as for the other discussed tumor sites. If
the breast alone is the target volume,
conventional dose distributions cannot be
improved very much by IMRT. However, if the
internal mammary lymph node chain has to be
included, the target volume is concavely shaped
around the chest wall and all conventional
techniques suffer from a quite high dose burden
to the ipsilateral lung. For this special case,
IMRT can improve the conformity of the dose
distribution. Fig. 20 shows a typical seven field
IMRT plan for a breast treatment with included
internal mammary lymph node chain.
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Figure 20: Axial and frontal dose distribution of a
breast IMRT with included internal mammary lymph
node chain.
Field 1 Field 2 Field 3 Field 4
Field 5
Field 6
Figure 21: Isodose overlays and gamma distributions of all seven IMRT fields of a breast treatment with included internal
mammary chain (measured isodose lines are dotted). Because of the extremely complex modulation, some gamma values
within the low-dose regions are >1 (see discussion in Section 5.7).
Field 7

Fig. 21 shows again the verification results. The typical fields of breast IMRT plans are extremely
modulated with large central low-dose areas to spare the lung. Therefore, some parts of gamma
>1 have to be accepted again. Note that the spatial resolution of the 2D-ARRAY seven29 is good
enough, even for such highly modulated IMRT fields.
7. Conclusion
The 2D-ARRAY seven29 is a very reliable tool for the fast and precise verification of IMRT fields.
Together with the analysis software VeriSoft, even complex IMRT treatment plans can be verified
and documented within approximately one hour. Because air-filled ion chambers are used, no
complicated calibration procedures are necessary; the system is always ready for high-precision
measurements. The effective spatial resolution of the 2D-ARRAY is sufficiently high to obtain
reliable results even for highly modulated IMRT fields.
The dosimetric verification of IMRT fields is an important part of the routine quality assurance
package for IMRT treatments. The 2D-ARRAY and VeriSoft provide a reliable and easy-to-use tool
for that purpose.
Additionally the 2D-ARRAY, in combination with the software MultiCheck, can be used for daily
check of the constancy of main beam parameters of the linear accelerator.
8. Literature
[1] Intensity Modulated Radiation Collaborative Working Group, Intensity-modulated
radiotherapy: current status and issues of interest. Int. J. Radiation Oncology Biol. Phys.
51: 880-914 (2001).
[2] Van Esch A., Bohsung J., Sorvari P., Tenhunen M., Paiusco M., Iori M., Engström P,
Nyström H, Huyskens D. Acceptance tests and quality control (QC) procedures for the
clinical implementation of intensity modulated radiotherapy (IMRT) using inverse
planning and the sliding window technique: experience from five radiotherapy
departments. Radiother. Oncol. 65 (2002) 53-70.
[3] Low, D., Harms, W., Mutic, S., Purdy, J. A technique for the quantitative evaluation of
dose distributions, Med. Phys. 25 (1998) 656-661.
[4] Depuydt, T., Van Esch, A., Huyskens, D. A quantitative evaluation of IMRT dose
distributions: refinement and clinical assessment of the gamma evaluation. Radiother.
Oncol. 62 (2002) 309-319.
[5] Böhmer, D., Bohsung, J., Eichwurzel, I., Moys, A., Budach, V. Clinical and physical
quality assurance for intensity modulated radiotherapy of prostate cancer. Radiother.
Oncol. 71 (2004) 319-325.
Text and pictures by courtesy of
Dr. Jörg Bohsung
Charité Universitätsklinikum
Schumannstr. 20/21
10117 Berlin
D742.208.0/0 2004-06
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