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CHAPTER 3
ABSOLUTE DOSE VERIFICATION USING
IONIZATION CHAMBERS OF DIFFERENT
VOLUMES IN RAPIDARC TREATMENTS
3.1 INTRODUCTION
The delivery of intensity modulated fields has been proposed by
means of a number of techniques. These include static compensators, dynamic
electrons, step and shoot or dynamic multileaf collimators (DMLC),
tomotherapy and the volumetric modulated arc delivery (RapidArc). The
latter two methods of delivery are novel. In external radiotherapy highly
accurate calculation of dose distribution is necessary for the successful
treatment of cancer. Dose discrepancies between the TPS and measurements
can arise due to inaccurate beam and component modeling, the dose
calculation algorithm, and beam delivery. The verification of the dose
delivered at a certain reference point within the patient (The International
Commission on Radiologic Units and Measurements (ICRU) report 50, 1985)
is an important step. Point dose verification should be done to correct
potential errors prior to start of treatment. This is especially true in intensity
modulated radiation therapy (IMRT) delivery, where dose delivery can be
quite complicated, with many beam angles and intricate intensity maps. To
deliver the planned dose distributions, intensity profiles are most commonly
translated into various multileaf collimated segments. High resolution
30
absolute and relative dosimetry is of great importance to evaluate the dose
distributions and also monitor unit verifications for such small segments.
Due to the complexity of RapidArc treatment planning and delivery
techniques the patient specific pre-treatment quality assurance plays a vital
role in RapidArc. Mostly IMRT plans are verified by phantom measurements
where the doses in the phantom are calculated by transferring the fluence
distribution from the patient treatment plan and the pre-treatment verification
will be done by film and ionization chambers respectively. RapidArc
generated dose distributions often have complex shapes with high gradient
regions surrounding critical patient structures. Analysis of discrepancies
between measured and calculated doses by single point measurement in high
gradient regions is a complicated task. RapidArc treatment fields consist of
small and large irregular multileaf collimator openings. So the traditional
process of Monitor unit (MU) verification is almost unfeasible, because of
large number of irregular field openings. But the independent MU checks
cannot predict the uncertainties during the actual delivery as the true delivery
depends on the condition of accelerator, which may vary with time and the
independent MU check algorithm is subjected to limitations and
approximations in their dose calculation models. Also the accuracy of these
types of measurements should be verified with other measurement techniques
before it should be widely used. Hence a point dose measurement is
commonly used. Ionization chamber based point dose measurements in a
phantom found to be the most reliable and practical technique presently used
for intensity modulated type delivery.
RapidArc has evolved towards the use of many small radiation
fields which will be defined by number of control points to increase the
resolution of the intensity map. So the pre-treatment verification requires
ionization chambers that can accurately measure the dose with millimeter
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spatial resolution in order to minimize lack of lateral equilibrium and volume
averaging effects. One major advantage with the cylindrical ionization
chambers is the relative independence of response as a function of incident
beam angle when the beam direction is orthogonal to the chamber
longitudinal axis. This response independence makes cylindrical ionization
chambers very convenient for verifying doses delivered using coplanar beam
geometries. The characterization of the detector such as energy dependence,
the size of the collecting volume, charge leakage, and stem materials before
measurements are important as the RapidArc dosimetry conditions are
radically different from the open field chamber calibration. Different studies
were published comparing ionization chambers of various volumes for IMRT
absolute dose verifications. In this study absolute point dose were measured
for volumetric modulated arc therapy (RapidArc) treatment delivery using
five different chamber phantom combinations.
3.2 MATERIALS AND METHODS
Thirty five different RapidArc plans conforming to the clinical
standards were selected for the study. Verification plan was subsequently
created for each treatment plan with different chamber-phantom combinations
CT scanned. This includes Medtec IMRT phantom with Exradin micro
ionization chamber (0.007cm3), Medtec IMRT phantom with PTW pinpoint
chamber (0.015cm3), PTW Octavius with PTW semiflex chamber (0.125cm
3),
PTW Octavius with 2D array and indigenously made cylindrical wax
phantom with 0.6 cm3 chamber. Table 3.1 shows the chamber-phantom
combinations and electrometers/software used in the study.
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Table 3.1 Chamber- phantom combinations and electrometer used in
this study
Chamber- Phantom
combinations Electrometer
1.
Micro ionization chamber (0.007
cm3) combined with MEDTEC
IMRT phantom
Farmer electrometer 2570/1
2.
Pinpoint chamber (0.015 cm3)
combined with MEDTEC IMRT
phantom
PTW TanSoft V 1.2
3.
Semi flex chamber (0.125 cm3)
combined with PTW Octavius CT
phantom
PTW TanSoft V 1.2
4.
2D array 729 ion chamber array
combined with PTW Octavius
LINAC phantom
PTW- Matrix Scan (S080050)
5.
Farmer chamber (0.61 cm3)
combined with Indigenously made
cylindrical wax phantom
Farmer electrometer 2570/1
CT images were taken at 1mm slice thickness by means of a
devoted CT scanner for IMRT Medtec phantom, PTW Octavius phantom and
indigenous made circular wax phantoms along with the chamber inserted in
its position. Exact couch parameters used in the treatment machine were
included while planning to reduce the errors, with panel surface having -300
HU, panel interior having -1000 HU and movable structural rails having 200
HU. Table 3.2 shows the technical specifications of the ionization chambers
used in the study.
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Table 3.2 Technical specifications of the ionization chambers used in
the study
Ionization
Chamber
Exradin
A16
PTW
TM31014-
0193
PTW
TM31010-
1571
NE 2571
PTW 2D-
Array 729
(T10024)
Type Micro
chamber
Pin Point
Chamber
Semiflex
Chamber
Farmer
Chamber
vented
cubic ion
chamber
Array
Active
volume 0.007 cm
3 0.015 cm
3 0.125 cm
3 0.61 cm
3 0.125 cm
3
Polarizing
voltage 300V 400V 400V 300V 400V
Wall material PMMA +
Graphte
PMMA +
Graphite
PMMA +
Graphite Graphite Graphite
Calibration
Factor (ND,W)
(cGy/nC)
366.7867 228.1515 29.9138 5.37 -
Calculations were done in Eclipse treatment planning system (TPS)
version 8.6 using the Analytical Anisotropic algorithm (AAA). All the
verification plans were done without presetting any planning parameters.
Verification plans were executed on a Varian Clinac 2100 linear accelerator
equipped with multileaf collimators having 120 leaves, with centrally forty
pairs of leaves having a projection of 0.5cm at isocentre and twenty pairs
having a projection of 1cm at isocentre. The measured isocentre absolute dose
was compared with the TPS planned.
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The ionization chambers were chosen for the study by means of
analyzing the minimum desirable chamber characteristics for exposure
measurements. The output of the linac was calibrated before measurements to
ensure the stability of output using Technical Report Series (TRS) 398
protocol. Also all the chambers were cross calibrated with a secondary
standard dosimeter at our hospital for the photon beam. The in-house
fabricated cylindrical phantom developed for this study was made of tissue
equivalent wax. Also custom water equivalent inserts were fabricated for
0.6 cm3 farmer ionization chamber that accurately matches the external
chamber dimensions. The cylindrical phantom geometry was chosen because
it is simple and very reproducible. During the measurements, all the phantoms
were positioned with its axis perpendicular to the radiation axis and its
“reference point” located at accelerator isocenter. Figure 3.1 and Figure 3.2
shows the photographs of different phantoms and ion chambers respectively
used in the study.
Figure 3.1 Photographs of the different phantoms used for the study
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Figure 3.2 Photographs of the different chambers used for the study
3.2.1 Verification Plan Creation
The verification plans were created for each phantom-chamber
combinations with the isocenter placed exactly at the center of respective
chambers. Figure 3.3 shows the computed tomographic image of the different
phantom-chamber combinations with respective chamber inserts. The
verification plans were recomputed with unmodified fluence patterns and
transferred to the respective phantom-chamber combinations with isocenter at
the centre of the chamber volume.
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Figure 3.3 Computed tomographic image of the different phantom-
chamber combinations with respective chamber inserts
The calculated doses from TPS at isocenter were tabulated for all
chamber- phantom combinations. For farmer ionization chamber (0.6 cm3),
the sensitive volume was delineated in the treatment planning system and the
mean dose was measured. The charge measured was converted into absolute
dose by applying suitable correction and calibration factor and the values
were tabulated. The errors that may occur during phantom delivery relate to
the effects of the MLC such as the penumbra, rounded leaf ends, intra and
interleaf leakages and the tongue and groove design, which were not taken
into account while creating verification plan.
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3.2.2 Phantom Setup for Verification Plan
All the chambers were positioned in the phantom at the isocentre.
Also the chambers were positioned along the coronal plane passing through
the target center. A special insert were provided in the indigenously made
cylindrical wax phantom for the insertion of 0.6 cm3 chamber. Figure 3.4
shows the verification plan window in Eclipse treatment planning system for
indigenously made cylindrical wax phantom with 0.6cm3 chamber.
Figure 3.4 Verification plan window in Eclipse treatment planning
system (version 8.6) for indigenously made cylindrical wax
phantom with 0.6cm3 chamber
As different phantom chamber combinations were used, the dose
delivered to the isocentre varies. Individual correction and calibration factors
were applied for the each measurement. The measured absolute point dose
was compared with the verification plans done in the treatment planning
system. The phantoms used in this study have different dimensions; so the
source to surface distance (SSD) also varies. The SSD was kept at 85 cm for
Medtec IMRT phantom, 86 cm for PTW Octavius phantom, and 87.5 cm for
indigenously made cylindrical wax phantom with a constant source to chamber
effective point of measurement (SAD) of 100 cms for all the phantoms.
38
3.3 RESULTS
Performance of all the ionization chambers for measuring absolute
point doses was investigated and the results were compared with the fluence
measurements done using 2D array seven29 detector. Discrepancies between
calculated and measured dose distribution were found to be within the range
of experimental uncertainty for all the chamber-phantom combinations used
in this study. Absolute dose deviations with respect to TPS dose analyzed
were grouped into 3 categories. (a) micro ionization chamber compared with
semiflex and farmer ionization chamber (b) pinpoint compared with micro
ionization chamber (c) semiflex compared with farmer ionization chamber.
The percentage deviation between TPS planned and phantom measured for
different chamber-phantom combinations were shown in Figure 3.5, Figure
3.6 and Figure 3.7. Table 3.3 shows the percentage variations, mean, and
standard deviation between chambers for all the patients studied.
Figure 3.5 Percentage variation for TPS planned and measured
isocenter dose comparing 0.6cm3, microchamber and semiflex
chamber
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Figure 3.6 Percentage variation for TPS planned and measured
isocenter dose for pinpoint and microchamber
Figure 3.7 Percentage variation for TPS planned and measured
isocentre dose for semiflex and 0.6cm3 chamber
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Table 3.3 Results showing % variations, mean, and standard deviation
between chambers for all patients
Sl
No.
Ionization
Chamber
Maximum
negative
deviation (%)
Maximum
positive
deviation
(%)
Mean
deviation
(%)
Standard
deviation
1 Micro chamber - 4.75 1.35 - 0.92 1.7402
2 Pinpoint chamber - 4.83 1.38 - 2.02 1.5604
3 Semiflex chamber - 1.49 1.29 -0.16 0.8669
4 0.6cm3 Chamber - 1.57 2.23 0.57 0.8878
Dosimetric errors were found in all the five chambers used. The
errors may be due to steep gradient across the point of measurements.
Nevertheless, all the chambers measured the absolute dose within 5% for all
the RapidArc plans. When comparing micro ionization chamber semiflex and
farmer chambers, used in our study, micro ionization chamber shows more
deviations as compared to semiflex and farmer chambers with a maximum
variation of -4.76%, -1.49% and 2.23% respectively. When compared with
micro ionchamber, semiflex and farmer ionization chambers, percentage
variation of results varies from negative to positive, which indicates that, the
chamber with higher volume over estimates. Also farmer chamber shows
higher deviation when compared to semiflex. The deviation was found to be
less than 1% with semiflex and farmer chamber almost all the cases. Also
positive percentage deviations were observed in most of the cases with farmer
chamber. The variation in absolute dose with micro ionization chamber (0.007
cm3) was found to be less than 2% for twenty seven cases. Farmer chamber
underestimates the measured absolute dose by a maximum of 1.57%, whereas
pinpoint chamber underestimates the calculated isocentre dose by maximum
41
of 4.8%. All the results were compared with the independent fluence
measurements done with 2D seven29 ionization chamber array (Figure 3.8).
Measured fluence agrees well with that of the calculated by the TPS for 3 mm
DTA, 3% DD, for 95% of the evaluated dose points for all the cases, in the
treated volume region.
Figure3.8 Gamma analysis results using 2D array for pre treatment
quality assurance of 35 RapidArc cases
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The degree of underestimation was found to be higher for the
smaller volume chambers. The effect of leakage on the measured charge is
relatively greater for smaller volume chambers, as the chamber sensitivity is
proportional to volume. Also the charge contribution from the small field
located at significant distances from the point of measurement may be below
the small chambers threshold and hence not detected. On the other hand, large
volume chambers used for dosimetry of conventional external fields are quite
sensitive, since higher volume chambers are long, and the electron fluence
through them may not be uniform. Under the condition of spatial fluence
uniformity, the charge collected by the large chamber may accurately
represent the absolute dose delivered by RapidArc to the point of
measurement.
3.4 DISCUSSION
The complexity of the RapidArc technique compared to IMRT
required a re-evaluation of current methodology of treatment verification.
This study investigates the effect of detector volume in the dosimetry of
RapidArc pre-treatment quality assurance. Ionization chambers have a
volumetric effect when used in radiation measurements particularly with
farmer chamber because of its large volume. The volumetric effect can be
clearly explained with respect to its position in the radiation fields. Within the
penumbra region, the chamber may behave differently inside the field or
outside the field. At these positions, the lack of charged particle equilibrium
acts in opposed ways, with more electrons coming into the chamber than
leaving it or vice versa. However, the effect is not very likely to hold for a
complete plan because adding up all the contributions from the different
segments in RapidArc tends to compensate each other as the MLC leaves are
continuously moving over the sensitive volume of the ionization chamber.
Larger volume chambers are mostly not suited to any type of intensity
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modulated radiotherapy dosimetry because of the greater uncertainties, unless
and until dose should be averaged and mean dose should be measured instead
of point dose.
The larger chambers exhibited severe under response at the small
field's center. This is due to partial irradiation of active volume of the
chamber in transverse position. Pinpoint chamber type 31006 over - respond
for large field sizes as suggested by Martens (2000) and Stasi (2004) et al.
This was due to the photoelectric interactions in the steel electrode, which
made the chamber over-sensitive to low energy compton scattered photons.
Therefore, pinpoint chamber is primarily suitable for relative dose
measurements in small field dosimetry. Most of the chambers used in this
study have aluminium central electrode. This should significantly reduce the
chamber over-response to large field sizes and can be used for absolute
dosimetry. The response of the pinpoint chamber increases with depth and
field size as proposed by C Martens et al (2000). Also for the smaller field
openings, the volume effect of the pinpoint chamber becomes important. At
the nominal operating voltage of 400V the pinpoint type 31014 chamber
demonstrate a strong field size dependency of the polarity correction factor
and an excess of the charge collected, which can lead to underestimation of
the collection efficiency, as specified by S. Agostinelli et al (2008). An
advantage of using the 2D array is its independent energy and dose-rate
response, which is an important characteristic, especially for small IMRT
segments.
When the ionization chamber is positioned in smaller fields
(i.e. 1x1 cm2, 2x2 cm
2),the pinpoint chamber (0.015cm
3) can give accurate
readings when compared to larger volume chambers. This is because the
smaller beams will partially irradiate the bigger volume chambers. Also the
dose contribution from the remote fields may not be detected by the smaller
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volume chamber. But the contribution can be detected by the larger volume
chambers due to high sensitivity. In RapidArc treatment delivery, the above
two cases may be possible, so there is a possibility to compensate the
deviations between the smaller and bigger volume chambers. For clinical
intensity modulated radiotherapy fields, the smaller volume chambers
sometimes show relatively greater variation when compared to larger volume
chambers. This might be because of variation in positional accuracy or stem
effect. Any small variation in positional accuracy of small volume chambers
in IMRT fields having high gradient can show large variations in the dose
measured in IMRT fields. Also Smaller volume chambers usually are more
sensitive to radiation induced leakages and charge multiplications, which are
usually negligible for large volume chambers .Since in intensity modulated
radiotherapy delivery uses small fields, there is a tendency to employ smaller
volume ionization chambers of active volume of approximately 0.1cm3 or less
for absolute dose verification. Recently it was experimentally verified that
pinpoint ion chamber with an active volume equal to 0.009 cm3 may be used
for absolute dose verification provided the area of uniform target dose
dimension greater than or equal to 1 cm and leakage corrections are taken into
account. However the use of ionization chambers for small field dosimetry
remains debatable due to the lack of electronic equilibrium across the field.
In low gradient regions the semiflex ionization chamber provides
accurate dose values for field sizes in the range 2 to 40 cm for photon beams.
At small field sizes, volume averaging, which is due to finite size of the
detector sensitive area, and modifications to electron transport, can occur.
This may be because of the non-water equivalence of the detector which can
over or under estimate the real dose values. Also Semiflex ionization with
0.125cm3 volume and type 3642 is a rigid stem chambers have a uniform
spatial resolution along all the three axes and designed for absolute dose
measurements. As the RapidArc delivery involves small subfield deliveries,
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the ionization chamber collecting volume may be partially irradiated at a
given instant. Even if the entire volume is irradiated; the fluence distribution
along the cavity may not be uniform due to the non-uniform beam profile.
However as the entire arc is delivered, a spatial fluence uniformity would
exist in the chamber volume. So the dose measured by the chamber may not
be affected by its dimensions. However chamber sensitivity potentially drops
as the chamber collecting volume decreases. Also the chamber leakage plays
a considerable role as the sensitivity is less. Since dose is delivered in a short
time in RapidArc, the dosimetric system leakage will not affect more on the
charge collecting efficiency.
The farmer (0.6cm3) ionization chamber can give a better result
when used in large monitor unit delivery segments or in high scoring regions.
Also the errors in using a large volume chambers due to volume averaging
effect is not predominant under such conditions. So it is suggested that dose
can be averaged over the volume while using 0.6cm3 ionization chamber for
measuring RapidArc absolute dose measurements. Also farmer chamber used
with the indigenously made cylindrical wax phantom might have improved
the results and almost comparable with other chamber-phantom combinations.
The average dose calculated by the TPS for farmer chamber is dependent
upon the contoured volume which was delineated manually and can be larger
than the manufacture’s nominal chamber volume due to slice averaging
effects present in CT images. So it is suggested that least possible slice
thickness of the order of less than or equal to 1 mm is ideal for delineating the
chamber volume. It was demonstrated that different chambers respond
individually to DMLC field conditions. The discrepancy in measurements
among chambers may be mainly due to detector design and construction
material and also the different phantoms used. Also fields with moving leaves
contain a larger proportion of the scattered photons because of less sharp
penumbras due to rounded leaf edges that travel across the entire field. The
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deviations may also be due to a high sensitivity to positioning accuracy due to
the cross-section of the beamlet. Also the field size and depth of measurement
will alter the scatter fluence reaching the detector. Correction factors can be
applied to each ionization chamber to account for fluence perturbation effects
in RapidArc delivery.
3.5 CONCLUSION
Absolute dose measurements using semiflex ionization chamber
with intermediate volume (0.125cm3) shows good agreement with the TPS
calculated among the detectors used in this study. Positioning is very
important when using smaller volume chambers as they are more sensitive to
geometrical errors within the treatment fields. Also it is suggested to average
the dose over the sensitive volume for larger volume chambers if used for
RapidArc absolute dose measurements. All the ionization chamber-phantom
combinations used in this study can be used interchangeably for routine
RapidArc patient specific quality assurance with a satisfactory accuracy for
clinical practices.
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