Dose verification of helical tomotherapy intensity modulated radiation therapy planning using 2D-array ion chambers
1 Department of Radiation Oncology, PLA General Hospital, Beijing, China
2 Department of Radiation Oncology, Monmouth Medical Center, New Jersey, United States
Purpose: To investigate the clinical usage of dose
verification of Helical Tomotherapy plans by using 2D-array ion chambers, and
to develop an efficient way to validate the dose delivered for the patients
during treatments. Materials and Methods: A pixel-segmented ionisation
chamber device, IMRT MatriXX� and Multicube� phantom from IBA were used on ten
selected Tomotherapy IMRT/IGRT head and neck plans in this study. The combined
phantom was set up to measure the dose distribution from coronal and sagittal
planes. The setup of phantom was guided for verifying the correction position
by pre-treatment Tomotherapy MVCT images. After the irradiation, the measured
dose distributions of coronal and sagittal planes were compared with those from
calculation by the planning system for cross verification. The results were
evaluated by the absolute and relative doses as well as Gamma (γ)
function. The feasibility of the different measuring methods was studied for
this rotational treatment technique. Results: The dose distributions
measured by the MatriXX 2D array were in good agreements with plans calculated
by Tomotherapy planning system. The discrepancy between the measured dose and
predicted dose in the selected points was within �3%. In the comparison of the
pixel-segmented ionisation chamber versus treatment planning system using the 3
mm/3% γ-function criteria, the mean passing rates of 2 mm dose grid with
γ-parameter ≤1 were 97.37% and 96.91%, in two orthogonal planes
(coronal and sagittal directions), respectively. Conclusion: MatriXX
with Multicube is a new system created for rotational delivery quality
assurance (QA) and found to be reliable to measure both absolute dose and
relative dose distributions, simultaneously. It achieves the goal of an
efficient and accurate dosimetry validation method of the helical delivery
pattern for the Helical Tomotherapy IMRT planning. � 2010 Biomedical
Imaging and Intervention Journal. All rights reserved.
Keywords: Tomotherapy, dose verification, IMRT, radiation
With the development and combined utilisation of Intensity
Modulated Radiation Therapy (IMRT) and Image-Guided Radiation Therapy (IGRT)
for the past few years, forms of Helical Tomotherapy and advanced Intensity
Modulated Arc Therapy (IMAT) have been evolving into new kinds of rotating
radiation treatment techniques after step-and-shoot and sliding window IMRT
deliveries. With the improvement of the beam entry angles and modulation
parameters, it is obvious that rotational therapy will become an important
delivery methodology in modern radiation oncology departments for better dose
conformality and critical organ sparing. One of the important characteristics
of rotating radiation is its dynamic nature and dosimetry variability in
radiation delivery. Therefore, this is a great challenge for clinical
physicists currently seeking tools to achieve quality assurance (QA) goals of
this dynamic treatment planning and its associated delivery issues.
Radiochromic film has been proven as a viable tool for
quantitative 2D dosimetry for conventional and IMRT modalities for quite some
time [1-13]. The greatest advantage of film dosimetry is its fine spatial
resolution. However, film dosimetry is somewhat influenced by the limiting
factors which are time-consuming, uncertainties of developing-chemicals, and
tedious analysing process. Pre-treatment patient plan verification is a part of
standard procedures for IMRT treatment. No adequate solution has been available
for the new dynamic techniques such as IMAT and Tomotherapy. Point-measurement
with ionisation chambers is usually insufficient and film dosimetry requires a
lot of work if clinical physicists try to achieve excellent agreements. Vendors
who have provided the 2D array detectors are all engaged in the new development
for the verification of rotational dosimetry. Sun Nuclear has produced the
MapPHAN� along with its MapCHECK� diode array to serve this purpose in
acquiring the dose verification. Initial results are promising from
multiple-angle testing. However, for lateral-entry beam angles, the phantom
material buildup with detector location creates certain levels of dosimetric
uncertainties. If a proper phantom material calibration is applied, then the
dosimetry data are quite comparable to the EBT film dosimetry provided by
International Specialty Products (Wayne, NJ, U.S.A.). With the new design
specification, the rotational-treatment delivery verification using a 2D matrix
detector to acquire rotational dosimetry data was described in detail . PTW
has also introduced the OCTAVIUS� phantom for the QA of rotational treatment,
along with their 2D-array ion chamber matrix which is currently being used in
the planar dose analysis. They also claimed that the signal acquired is
independent of the beam angles . A novel 3D device Delta-4 (ScandiDos, Uppsaka, Sweden) has also been reported in acquiring the 3D dose maps for IMRT treatments,
which claimed to be useful in the rotational dosimetry measurements.
Herzen J et al. reported that the response of a MatriXX QA
device is linear with dose and energy independent. The authors concluded that
the detector is a suitable device for QA and 2D dose verifications for IMRT QA
. The same QA device has also been investigated for proton therapy QA by
the group from MD Anderson Cancer Center . Their MatriXX measurement
results on different energies of proton therapy beams were with the accuracy
comparable to those of ion chamber measurements and film dosimetry. More
recently, Li JG et al. compared the two commercially available detector
arrays (MapCHECKTM and MatriXX) for their use in the QA of
patient-specific IMRT treatment plans . Both detectors showed negligible
errors (< 1%) when measuring doses of more than ~8 cGy, but exhibited errors
of ~3% when measuring doses on the order of 1 cGy. The authors obtained
excellent passing rates for both detector arrays when compared with the planar
dose distributions from the treatment planning system for three 6 MV IMRT
fields and three 18 MV IMRT fields. For the verification of rotational
delivery, such as Tomotherapy or volumetric modulated arc therapy, the MapPHAN�
was developed for new or existing MapCHECK users.
In this article, the authors utilised the newest
development, MatriXX phantom with the 2D ion chamber arrays, from IBA (Louvain-Ia-Neuve, Belgium) to evaluate the most difficult head and neck treatment cases at
the authors' center. The methodology mentioned in this article was different
from the conventional QA verification procedure which only achieved the fixed
beam entry angle of LINAC with planar dose distribution . This particular
verification system is created to assess 2D array suitability for rotational
dosimetry. This QA study was performed for ten-patient plans from Helical
Tomotherapy, with proper buildup and geometry for side scattered compensation.
This 2D MatriXX device then transformed into a pseudo 3D tool for various plan
verification in order to conquer the complex helical nature of radiation
Materials and methods
This QA phantom composed of MatriXX and Multicube was
positioned on the treatment couch, verified with isocenter alignment tool
provided by Tomotherapy planning and the laser systems. The irradiation source
used for verification process is a single energy photon beam of 6 MV on a
linear waveguide. MatriXX device consists of 1020 (32x32) vented pixel ion
chambers. Each chamber occupies 0.08 cm3 volume and holds the
resolution of 7.62 mm. If multiple readings are generated, then 1 mm resolution
with interpolation is applicable. Measurement dose rate ranges from 0.1 to 20 Gy/min
with the dose resolution of 0.5 mGy/min. All verification plans in this study
were created for this phantom on Tomotherapy planning station, with version
2.2.4. Measurement software provided by IBA dosimetry is OmniPro-IMRT version
Acquisition of the phantom images for verification
The pixel ionisation chamber MatriXX was inserted into the
Multicube phantom in order to form a complete assembly with a proper buildup,
similar to the setup layout for dose verification picture on the couch of
Tomotherapy system as shown in Figures 1a and 1b. The assembly images for
coronal and sagittal orientations of the combined phantom were previously
scanned by a Philips Brilliance Big Bore CT simulator (Philips Medical Systems)
and stored as a phantom data set. A 2 mm slice thickness was used to
guarantee the precision of 2D dose distribution calculated by Tomotherapy
planning system, and also to maintain the exported intensity map iso-tropically
for alignment purpose. The suitable locations to place the three fiducial marks
were the pre-designed crosshairs on the MatriXX�s enclosures. This helps easily
identify the center of the device on the phantom images acquired by the CT
scanner. While generating the CT DICOM data set, the user has to make sure the
phantom images are named accordingly for proper patient-specific
identification. After the proper configuration and entry of basic information,
images of combined phantom along coronal and sagittal orientations were then
imported into Tomotherapy planning station for calculation of all patients in
Tomotherapy Delivery QA (DQA) setup
First, after importing the pre-scanned CT images of the
combined phantom for the corresponding setup, the image value-to-density table
(IVDT) for the dose converting parameters is imported simultaneously for dose
calculation. Secondly, the CT couch was removed from the phantom images and was
replaced by the Tomotherapy delivery couch so that the different geometry
effect of the couch was accurately modeled during the dose calculation process.
According to the patient image volume, register the phantom images and save the
laser positions to the DQA plan for setup correlation. Isocenter location is
then properly determined. At the end, the dose distribution was calculated for
the phantom image volume for each specific patient based on the final
calculated and deliverable plan. The dose intent of the phantom planning should
be identical as the prescribed dose to the patient per fraction from the final
Systematic measurement preparation
Warm-up time of MatriXX 2D-Array ion chamber usually takes
at least 15 minutes, as it is required before true delivery starts. Similar to
other 2D-Array ion chamber devices available for clinical usage, a total dose
of 100 cGy was pre-irradiated on the MatriXX array in order to eliminate noises
and to keep the uniform dose response characteristic with better
signal-to-noise range. This pre-irradiation procedure of IBA MatriXX array
specially created for Tomotherapy system was automatically moving the couch
longitudinally into the bore while maintaining 5x40 cm MLC opening at the
static position at 0 degree. This method will keep the 2D‑Array ion
chamber irradiated uniformly with minimum noise differentiation of the delivery
dose responses. For the absolute dose calibration, there are two possible ways
to perform this task. A typical calibration field was delivered at the dmax
point to cross calibrate the central ion chamber, which represents 1 cGy/MU
with the C‑arm LINAC. This method can be performed with a conventional
LINAC on site and cross comparison has to be executed in measuring the
Tomotherapy calibrated dose. Another method is to use the Tomotherapy static QA
procedures to deliver known doses to the associated ion chamber locations, thus
obtaining the corresponding calibration factors to the specific ion chamber (at
least the center ion chamber needs to be cross checked).
According to the experiments performed by Tim Holmes,
Ph.D. , the directional dependence of this MatriXX system is quite
acceptable for rotational delivery. The 1020 ion chambers are configured in a
way that very little high Z metal is present in the radiation field and the
calculation is very accurate (convolution/superposition dose model). Rotational
delivery can tolerate relatively larger errors over small angles. With 51 beam
entries, even a single lateral beam contributes 10% dose error, which will only
translate into 0.2% total dose error. Geometrical blurring is also inherited in
the rotational delivery, hence errors occurring over small distances are
blurring out. An anterior avoidance plan has been delivered to verify the
lateral beam angle dependence by Tomotherapy and MatriXX - results indicated
that the passing rate of a lateral beam entrance plan is 93.54%, which has
proven this device has minimum lateral beam entrance effect in Tomotherapy
delivery. (Figure 3)
Verification measurement for Tomotherapy IMRT plan
The phantom plans for ten randomly selected patients with
the coronal and sagittal directions were designed and saved into the data
server for validation. These selected patients are all Nasopharyngeal Carcinoma
(NPC) patients with patient 9 and 10 being the most challenging cases with a
sharp dose gradient through the cord, brainstem and parotid glands while the
tumor location is critically located in between. The reason for selecting head
and neck cases is because such cases consist of the best testing modality for
the rigidity of MatriXX for QA. Also, head and neck cancers are often very
difficult to deal with. Another reason is that head and neck cases are the most
common cancers in the authors� areas which need IMRT treatments, and also
present a tough choice for QA methodology. Before the delivery of the DQA
plans, the set-up positioning as well as the laser alignment configuration are
demonstrated as Figures 1a and 1b. Set-up accuracy of combined phantom location
was corrected and confirmed by using planning CT/MVCT registration process,
which is a standard procedure at the Tomotherapy operational console. During
the pre-treatment IGRT process, Multicube and BBs are the areas that need to be
covered by the MVCT scan, typically we use fine scan for the alignment
purposes. If the laser and couch sag are properly managed, then the manual
shift is not necessary because the auto-registration of Tomotherapy program
functions well with MVCT images of MatriXX and its associated CT scans. After
finishing the MVCT image registration, MatriXX within the combined phantom was
irradiated according to verification plans generated from each selected patient
in Tomotherapy planning console.
Data analysis and processing after image and dose acquisition
Two methods were implemented separately in order to
analyze and compare the measurement data after Tomotherapy delivery. The dose
distributions measured by the 2D-Array ion chamber were compared with those
calculated by Tomotherapy DQA plans using vendor provided software tools.
OmniPro-IMRT analysis software (from IBA)
After data is exported from the Tomotherapy planning
station, the intensity map file of dose calculation results was imported into
OmniPro-IMRT software and co-registered with the measured dose intensity map.
The proper file to send from Tomotherapy for export into OmniPro-IMRT software
is the Text DQA Header and Image Files. These are files created by Tomotherapy
for export. Those two files usually can be transferred via network or even a
simple USB drive to the MatriXX computer with installed analysis program. After
completion of file transfers, these two files were compared by using 2 mm and 1
mm calculation grid points. 3 mm/3% DTA and absolute dosimetry errors, as Gamma
(γ) criteria [21, 22] are also used for evaluating the dose distribution
on both the coronal and sagittal directional planes. The results of the
quantitative and qualitative analysis were compared simultaneously for the
specific phantom setup.
Tomotherapy DQA software (from Tomotherapy)
Tomotherapy planning tool also provides the evaluation
software for dosimetry analysis. While the TIF image with registration points
designed specifically for Tomotherapy was exported from MatriXX data, the
linear CAL file for film calibration would be given automatically. After the
acquisition process of MatriXX data, one can export the TIF file with registration
points designed specifically for Tomotherapy. This will deliver the TIF image
and a linear CAL file for film calibration. This particular functionality in
OmniPro-IMRT version 1.6 software is located under the �Export files� tab. The
procedure for registering the TIF image in the DQA workspace was the same as
using for a regular film-based dosimetry. The comparison between measured and
calculated dose distributions was also done by the 3 mm/3%
γ-criteria, which calculated the gamma histogram to display those
associated parameters in a real-time mode.
Dose verifications for ten pre-selected Tomotherapy NPC
IMRT plans were performed in both coronal and sagittal orientations using the
2D MatriXX ion chamber device. Dose distribution between measured and
calculated dose distribution showed good agreements from the data analysis.
Figure 2 shows the percentage deviation of absolute dose distribution of the
ten previously treated patients, which is shown within �3% error margin.
Selection criteria of points for absolute dose measurement was situated in the
high dose and low gradient region of MatriXX measurement plane, according to
the IMRT measurement guidelines in the AAPM Task Group report #119.
OmniPro-IMRT software process
Table 1 lists the results of Gamma analysis (3%/3 mm)
with 2 mm and 1 mm calculation grids, respectively, for the selected 10 NPC
patients. The average passing rates for the DTA of 3%/3 mm for the coronal
plane at 2 mm and 1 mm grids are 97.37% and 98.71%, respectively. And those for
the sagittal plane are 96.91% and 97.86%, respectively. Figures 4(a)-4(e) and
5(a)-5(e) show the results of calculated and measured dose distributions and
Gamma analysis (3%/3 mm) for a typical NPC plan along the coronal and sagittal
direction. In Figure 5, the same NPC patient shown in Figure 4 is selected for
analysis with the different plane (sagittal). The isodose matches and Gamma
plot were displayed, as well as the statistical analysis. The passing rate is
more than 90% with great confidence level of clinical implementation.
Tomotherapy DQA analysis
Although the Tomotherapy dosimetry analysis software does
not provide passing rates for the dose distribution, in the Tomotherapy DQA
process, dose fluence (profile) and histogram of the combined 3% and 3 mm gamma
criteria could still be analysed. Figures 6(a)-6(c) and 7(a)-7(c) show the dose
distribution and the Gamma results for the plan along the coronal and sagittal
setup direction. And 6(d) and 7(d) show the Gamma histograms, with two
Clinical example in Figure 4 shows a typical NPC patient
with bilateral PTV coverage and the critical component for IMRT is cord
sparing. Coronal planned/measured dose comparison using MatriXX and Multicube
has shown excellent agreement, with passing rate 92.93% of 2 mm calculation
grid. Gamma index plot also indicates only few areas are larger than 1.0.
Figure 5 shows the sagittal view of the same patient MatriXX evaluation; however,
the passing rate has increased to 94.82% with same dose grid calculation. The
cord sparing of the measurement is very obvious and Gamma plot shows only few
areas with the index larger than 1.0. Figures 6 and 7 are the reports generated
from Tomotherapy evaluation tools with similar passing rates and the Gamma
index plot, which has confirmed the QA device of MatriXX can be an excellent
substitute of the film dosimetry due to the similar experimental results.
The grid for dose calculation within Tomotherapy phantom
plan was set at 2 mm and it was the same resolution as in the phantom planning
and fan-beam CT scans. The 2 mm calculation grid was set in such a way that the
longitudinal direction can match the CT scanning slices. Regarding the
selection for the find grid for the dose calculation on the axial slice, the
field of view is set at 512x512 to reach the maximum image resolution for
calculation. In order to achieve the patient dose verification before the
patient� delivery, 2 mm on the calculation grid should be also used for
registration purpose. Even though the passing rate with 1 mm grid was slightly
higher than that with 2 mm, it would be better to select the 2 mm precision of
dose calculation in these plans.
Improvement of the MatriXX efficiency, compared to the
regular Tomotherapy DQA procedures, the QA time has been cut down from 90
minutes film dosimetry to about 43 minutes for each patient DQA using MatriXX.
Major time saving is from no marking and processing the film for measurement.
Also, ion chamber of MatriXX reports the dose value immediately if properly
calibrated, no separate ion chamber measurement setup is needed for point dose
measurement. Tomotherapy represents a new approach of dynamic rotational IMRT
with excellent image guided capability. Dose verification has been achieved by
using the MatriXX 2-D array phantom along the coronal and sagittal directions
for ten specific Tomotherapy patients. The analysis results for the absolute
and relative doses between measured and calculated were found in excellent
agreements. Therefore, the MatriXX is a 2D QA device, not only for dose
verification of dynamic IMRT on conventional LINAC [23, 24], but also for
helical Tomotherapy IMRT/IGRT treatment validation. Results of the measurement
for two different setups have shown that the MatriXX on both directions are
capable of generating superior clinical dosimetry results and extremely
effective for measurements in helical Tomotherapy. The number of passing points
with γ‑parameter ≤1 in coronal and sagittal direction also
show excellent agreements with the final approved plans. Even with the tight
margin and very complicated NPC cases, the authors still observed more than 90%
passing rate. For instance, if the combined 5%/3 mm criterion was used for
analysis, the number of points with γ≤1 would increase to 94.37%. In
addition, these results from two different analysis software (IBA and
Tomotherapy) were very consistent. But if Tomotherapy DQA software is being
utilised for the analysis, the counter clockwise setup along sagittal direction
of the combined MatriXX phantom is recommended strongly for a better passing
rate (Figure 1(b)).
Chan et al. also reported that the diode array data showed
a trend of angular dependence, decreasing from 100% at 0� down to 90% at 80�
gantry angle ; however, the overall measurement results from rotational
delivery are within clinically acceptable accuracy due to the averaging effect
from all the gantry angles . Rotational delivery usually can tolerate
larger errors in smaller angles, and the authors have observed the directional
sensitivity of MatriXX ion chambers with water equivalent Multicube to less
effect in the Tomotherapy delivery scheme. The MatriXX in Multicube phantom is
ion chamber array system and it is well known that the angular dependence of
ion chambers is not as severe clinically for rotational delivery. Planar dose
validation device with buildup certainly presents the great challenge and that
is the reason for SunNuclear to create a device called ArcCHECK� to truly serve
the arc therapy dosimetry QA purpose. This is one of the first reports on
patient-specific rotational dosimetry verification using MatriXX and Multicube.
However, compared to the traditional film dosimetry,
MatriXX is still a much easier device in clinics than the film-based QA
program. In this study, the authors selected 10 NPC patients due to their sharp
dose gradient and complexity, which bear the challenges to acquire superior
agreement compared with simple prostate IMRT cases. This patient pool
represents a challenging situation and the authors also observed good agreement
with high passing rates in both coronal and sagittal planes.
Similar to other commercial 2D-array, the effective
field-size of 24.4�24.4 cm2 of the MatriXX system presents some
limitations (i.e., QA on large IMRT fields). But it should be a key issue for
dose verification on most Tomotherapy IMRT plans. However, it's important to
avoid irradiation of the electronic parts outside ion chamber so that it will not
affect the lifetime of the QA device. On the other hand, the spatial resolution
of MatriXX due to its structure is less than those of film for dose
verification as obviously noticed. The area of high-dose gradient should be
further considered for data analysis. From the final study results, threading
effect was inevitable in the helical Tomotherapy delivery process. With the
development of hardware and software of commercial 2D-Array, it�s possible that
dose verification using 2D-Array ion chamber could provide the cubic rotational
dose reconstruction accurately. Weight of the combination of MatriXX and
Multicube is another concern, the total weight is about 33 kg, which could also
be cumbersome in transporting the phantom to and from the couch top.
MatriXX 2D-Array has provided a simple and effective dose
verification tool for rotational dynamic IMRT such as Tomotherapy technique.
The measurement for the final plans of Tomotherapy shows that MatriXX is
capable of both absolute and relative dose measurements within good agreements.
Its geometrical-machine buildup for the 2D array reduces uncertainties during
the helical delivery in comparison with the Tomotherapy DQA dosimetry modules.
MatriXX with Multicube phantom creates a smoother and efficient operation with
reasonable QA analysis time and results for helical Tomotherapy at the authors�
busy cancer center.
Figure 1 Pictures of IMRT MatriXX 2-D Array and Multicube phantom clinical setting used to measure the Tomotherapy IMRT plan from two orthogonal orientations � (a) coronal, and (b) sagittal � respectively.
Figure 2 Percentage errors for the absolute dose of pre-selected Tomotherapy IMRT plans.
Figure 3 MatriXX directional sensitivity planning and measurement validation. (a) Target contours, (b) Planned, (c) Planned and measured isodose lines, (d) Statistics .
Figure 4 Analysis results between the measured and calculated dose at the 2 mm grid along the coronal direction. (a) Measured dose, (b) Calculated dose, (c) Dose comparison, (d) Gamma plot, (e) Gamma analysis.
Figure 5 Analysis results between the measured and calculated dose at the 2 mm grid along the sagittal direction. (a) Measured dose, (b) Calculated dose, (c) Dose comparison, (d) Gamma plot, (e) Gamma analysis.
Figure 6 Comparison of measured and calculated results on the coronal measurement of MatriXX. (a), (b), (c) show the profile distribution and the Gamma results for the plan along the coronal setup (Isodose curve comparison, dose profile of the arbitrary surface, and gamma plot respectively). 6(d) is the Gamma histogram.
Figure 7 Comparison of measured and calculated results on the sagittal measurement of Tomotherapy provided software. (a), (b), (c) show the profile distribution and the Gamma results for the plan along the saggital setup (Isodose curve comparison, dose profile of the arbitrary surface, and gamma plot respectively). 6(d) is the Gamma histogram.
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|Received 1 June 2009; received in revised form 7 October
2009; accepted 16 November 2009
Correspondence: Department of Radiation Oncology, Monmouth Medical Center, 300 Second Ave, Long Branch, NJ 07740, United States. Tel.: +732-923-6894; E-mail: email@example.com (Jack Yang).
Please cite as: Xu S, Xie C, Ju Z, Dai X, Gong H, Wang L, Yang J,
Dose verification of helical tomotherapy intensity modulated radiation therapy planning using 2D-array ion chambers, Biomed Imaging Interv J 2010; 6(2):e24