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IX Latin American IRPA Regional Congress on Radiation Protection and Safety - IRPA 2013
Rio de Janeiro, RJ, Brazil, April 15-19, 2013 SOCIEDADE BRASILEIRA DE PROTEÇÃO RADIOLÓGICA - SBPR
PRECISE DOSIMETRY OF SMALL PHOTON BEAMS COLLIMATED
BY AN ADD-ON DYNAMIC MICRO-MULTILEAVES COLLIMATOR.
De La Fuente-Rosales L. 1, Alfonso-Laguardia R.
2, García-Yip F.
1, Ascencion Y.
1
1Department of Radiotherapy, Instituto de Oncología y Radiobiología (INOR), Havana, Cuba
2 Instituto Superior de Tecnologías y Ciencia Aplicadas (InSTEC), Havana, Cuba
ABSTRACT
Purpose: To improve the accuracy in the measurement of relative output factors (OFs) for small photon beams
used in stereotactic techniques at INOR.
Methods and Materials: The OFs of small photon beams defined by a dynamic micromultileaf collimator
(DMLC) 3D-Line® were measured with different detectors: (1) semiflex 0.125cc ionization chamber (IC), (2)
pinpoint micro-ionization chamber, (3) shielded diode type P, (4) unshielded diode type E, (5) gafchromic EBT2
and (6) EBT3 films; in water or water equivalent plastic phantom, as required.
Isocentric measuring setup was selected, at a depth of 5 cm with a source–surface distance of 95 cm; the
detector orientation was set parallel to the central axis of the beam (CAX) for chambers and diodes, and
perpendicular for films. Small square fields ranging from 0.29 cm2 to 5.8 cm2 were studied; the intermediate
5.8 cm2 reference field was used for OF calculation. Relative dose factors were also computed by a Monte
Carlo simulation of the same irradiation geometry, in order to compare measurement results with calculated
OFs. Additionally, correction factors for all detectors relative to Monte Carlo calculated, EBT2 and EBT3 film
measurements were determined.
Results: The six detector types used in this work are accurate enough to determine the output factors of field
sizes bigger than 2x2 cm2, i.e. the OFs showed practically no dependence with the detector type. For smaller
field sizes, the 0.125 cc (semiflex) chamber underestimates the OF by more than 55% while, as expected, the
rest of the detectors (with smaller active volumes) agreed in higher OF values in this region of interest.
Conclusions: Significant deviations were encountered when non appropriate detectors are used for small field
dosimetry. The correction factors obtained can be used for eventual reduction of errors. The use of several types
of detectors is strongly recommended for OF measurements in very small fields.
1. INTRODUCTION
The wide availability of treatment techniques like Stereotactic Radiotherapy (SRT) or
Radiosurgery (SRS) involves an increasing use of small photon beams in the clinic. These
beams can be produced in a conventional linear accelerator (linac) with the addition of a
mini- or micro- multileaf collimator (mMLC), or in specialized treatment units specifically
designed for stereotaxy.
Care must be taken during measurements of stereotactic fields and small segments due to (a)
the presence of lateral electron disequilibrium, (b) the smaller collimator aperture results in
partial occlusion of the source and (c) a large detector compared to the field dimensions
perturbs particle fluence in the medium and (d) the signal is affected by volume averaging
effect [1]. Due to these conditions the availability of suitable detector is critical. Additionally,
a change in photon and secondary electron energy spectra and as a consequence a change in
beam quality may occur when decreasing field size.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
Several authors described problems related to the dosimetry of small beams (Das et al.[2],
Laub et al [3], Bouchard and Seuntjens [4], Sanchez Doblado et al [5], Ding et al.[6]) which
are very far from reference conditions recommended in existing Codes of Practice (CoPs) or
dosimetry protocols such as IAEA TRS-398 [7] and AAPM TG-51 [8].
In order to develop a standard methodology that complements the existing CoPs an
international working group on reference dosimetry of small and nonstandard fields has been
established by the International Atomic Energy Agency (IAEA) in cooperation with the
American Association of Physicists in Medicine (AAPM) Therapy Physics Committee. This
group published in 2008 a proposal of a formalism for the determination of absorbed dose to
water using ionization chambers in situations different from the conventional reference
conditions.
The aim of this paper is to improve the accuracy in relative output factors (OFs) in small
static photon beams used in stereotactic techniques at INOR based on the formalism
described by Alfonso et al [9].
2. METHODS AND MATERIALS
2.1. Measurements
Measurements were performed in an Elekta Precise® (Elekta Oncology Systems) dual energy
linac coupled with a dynamic micro-multileaf collimator (DMLC) model “L´ARANCIO”,
manufactured by 3D-Line® (now Elekta, Stockholm). The DMLC consists of 24 double-
focused leaf-pairs made of tungsten and allows dynamic arcs. Material and geometrical data
of DMLC may be seen on Table 1.
Six type of detectors available at INOR´s were irradiated at 6MV photon beam energy, using
isocentric configuration SAD= 100cm, at a depth of 5cm in a PTW/MP3 water tank or RW3
water equivalent plastic phantom. The orientation relative to the beam central axis (CAX) for
each setup was chosen according the detector type.
For this study square fields determined by the leaf width at isocenter from 5.8 cm2 down to
0.29 cm2 (which is the smallest field size that can be reached with the DMLC) were selected.
Table 1. DMLC General Specifications
Weight Leaf Material Tungsten (93%)
⁄
Leaf Number 48 (24 pairs)
Leaf Height 8 cm
Leaf Width at
isocenter
2.901 mm
Leaf Overtravel 31.25mm
Minimum field
size at isocenter
2.901mm
Maximum field
size at isocenter
69.62x68.12mm
Focalization Double
IRPA 2013, Rio de Janeiro, RJ, Brazil.
Leaf Maximum
velocity ⁄
Transmission 0.5%
2.2. Detectors
Among the common requirements of external radiation therapy detectors, those used for
narrow fields need to satisfy additional features like the small active volume and the tissue
equivalence. The ionization chambers, which are the detector of excellence in broad beam
dosimetry, are not suitable due to the presence of high dose gradients, time dose variance and
non-uniform beam distributions.
There is a wide variety of small field detectors in the market, but generally only few are
available in the daily clinic, this justify the study of its behavior when using for absolute and
relative small beam dosimetry.
The detectors used in this work are summarized in Table 2.
Table 2. List of detector used in this study
ID. Type Manufactur
er
Model
PFD
shielded Si
diode
PTW-
Frieburg
60016
EFD unshielded Si
diode
PTW-
Frieburg
60017
0.125c
c IC
cylindrical
ion chamber
PTW-
Frieburg
Semiflex
31010
0.015c
c IC
cylindrical
ion chamber
PTW-
Frieburg
PinPoint
31006
Film radiochromic
film
ISP
EBT2 and
EBT3
2.3. EBT2 and EBT3 Dose Calibration.
Gafchromic® film pieces of 2x3,5 cm2 were exposed to 6MV photon beam at a measurement
depth of 5cm, using isocentric set-up and 10x10 cm2 field size. A Farmer type ionization
chamber PTW 30013 was used to monitor the linac output and the dose delivered to the film
at the same time of film irradiation.
The film pieces were exposed perpendicularly to the radiation beam axis in a 30x30x21.3cm3
solid water RW3 phantom.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
Figure 1: Gafchromic Films EBT2 and EBT3 Dose
Calibration at the linac(left). Film Scanner with
the cardboard template (right).
The calibration films were scanned 21 hours after exposure in an A3 color bed scanner of 48
bits Microtek ScanMaker 9800XL. Prior to any scanning 5 blank readings were taken since
this model can not warm up the lamp in preview mode. The scanner was operated in
transmission mode, in RGB channels, with a resolution of 72 ppi and all the enhancements
options turned off. The film pieces were scanned in landscape orientation and saved as
uncompressed TIFF format. To ensure the same film position along the longitudinal axis of
the scanner a cardboard frame was constructed and removed before scanning. The use of the
cardboard ensures reading reproducibility and also avoids the non uniform response of the
scan field, placing the ROI in the center of the scanner bed.
During EBT2 and EBT3 reading, special care must be taken with the selection of ROI size
and position relative to the scanner bed center. It was observed that a small change in its size
leads to a different average pixel value, determining a critical variation in relative output
factor estimation. Additionally, is necessary to check that there are not bad pixels inside the
ROI perimeter. The ROI size for each field was selected according field dimensions.
ImageJ [10] software was used for image processing. The region of interest (ROI) selected
was 2x2 mm2, and only the red component was read.
2.4. Formalism for relative dosimetry
The formalism proposed by the IAEA working group for relative dosimetry in machines that
are not capable of reproduce the reference conditions stated by CoPs, introduced a field factor
as:
or
(1)
where:
is the absorbed dose to water at a reference point in a phantom for a clinical field fclin of
quality Qclin and in the absence of the chamber,
the absorbed dose to water for a reference field fref or the machine-specific reference
field fmsr of beam quality Qref or Qmsr, respectively in the absence of the chamber.
In relative dosimetry of single static fields this field factor is usually called “output factor”.
There are two ways of calculate this
or :
(a) directly as a ratio of absorbed doses to water using Monte Carlo simulations, or
(b) as a ratio of detectors readings multiplied by correction factors
and :
or
IRPA 2013, Rio de Janeiro, RJ, Brazil.
(2)
which can be measured using a suitable detector with a very small sensitive volume and an
energy independent response, like radiochromic film or a liquid ion chamber, or calculated by
Monte Carlo according to next equation:
[
⁄
⁄
] or
[
⁄
⁄] (3)
When correction factors are close to unity, the ratio of detector measurements is a good
estimation of field factors.
As in our case we used an estereotactic system attached to a linac (linac based system) we
can apply the so called “daisy chaining method”, introducing a step between the reference
field and the clinic field, an intermediate field fint, ensuring a correct measuring with an IC
for field sizes larger than the intermediate field and limiting the effect of the energy
dependence in small beam detectors like diodes.
( )
( )
( )
( )
(4)
det is the suitable detector for small field dosimetry, and;
IC the ionization chamber.
The correction factor
can be expressed as:
( ) ( ) (5)
Figure 2: Representation of the dosimetry of small
static fields with reference to the so called
“machine-specific reference field”( from Alfonso et
al formalism).
IRPA 2013, Rio de Janeiro, RJ, Brazil.
2.5. Output factor measurements
In order to ensure the correct detector alignment respect to the beam central axis, before each
measurement with the DMLC, beam profiles were acquired in the inplane and crossplane
directions.
Table 3. shows the orientation relative to the beam CAX for the five type of detectors used.
In our specific case we choose 5,8x5,8 cm2 as an intermediate field (fint) which is large
enough to avoid small beams conditions, and at the same time with limited energy
dependence. Another advantage is that this fint can be adopted by the DMLC coupled to the
linac head.
Table 3. Detector orientation as respect to beam central axis.
Detector type Detector
reference
Output
factors
Cylindrical
ion chamber
Axial
axis
Parallel
Cylindrical
micro ion
chamber
Axial
axis
Parallel
Silicon
shielded diode
Axial
axis
parallel
Silicon
unshielded
diode
Axial
axis
parallel
Radiochromic
Films
Film
surface
perpendicular
2.6. Correction Factors relative to gafchromic EBT2 and EBT3, and Monte Carlo.
For all fields sizes and detectors used it was determined the ratio of the clinical and
intermediate field signals ( )
( )⁄ . A similar ratio, but in terms of dose
⁄ was determined with EBT2 and EBT3 readings and with Monte Carlo
results. Using the previous data, correction factors were determined following the
methodology explained in section 2.3.
2.7. Monte Carlo Simulations
Using the BEAMnrc code system [11] Ascención et al [12] generated a space phase of a 7x7
field from the Elekta Precise head coupled with the DMLC at 95 cm from the target. Using
this data file, the particle transport in a 30x30x30 cm3 water phantom was simulated with
DOSXYZ [13] code. During numerical simulation it was found that voxel size is key in the
computed OFs, a voxel size of 0.25x0.25x0.2 cm3 was chosen.
3. RESULTS
IRPA 2013, Rio de Janeiro, RJ, Brazil.
3.1. Output factor measurements
For field sizes between 2×2 cm2 and 5,8×5,8 cm2 the OFs measured with all detectors show
an agreement within 3% relative to those measured by EBT2 and calculated by Monte Carlo.
A same deviation was obtained with EBT3 films except for the 2×2 cm2 field using the
semiflex IC, here the difference reached 5%.
As expected, an unacceptable deviation (larger than 55%) for the smaller field size
(0.29x0.29 cm2) was encountered with the semiflex IC. This difference is due to the lack of
lateral electron equilibrium, the partial occlusion of the source causing overlapping
penumbra. As a consequence, since the chamber is placed in a region of high dose gradient,
the volume averaged reading is unreliable.
Table 4. Summary of the measurement characteristics.
Accelerator Elekta Precise
Nominal energy 6 MV
Beam quality TPR20,10 = 0.68
Collimator
system
Conventional MLCi (fix
7x7 cm field), add-on
mMLC 3Dline Orange
(2.9 mm with leaves)
Fields Square 3, 6, 9, 12, 15, 18,
21, 27, 36, 45, 60 mm (ref)
Measurement
depth
5 cm in water
Source Surface
Distance (SSD)
95 cm (SAD setup)
Detectors SemiFlex 0.125 cc IC
(PTW-31002), PinPoint IC
(PTW-31016), Shielded
diode (PTW-60008),
Unshielded diode (PTW-
60012), Gafchromic
EBT2, EBT3
Calculations Monte Carlo,
0.25x0.25x0.2 cm3 voxels
size in water
Source of data Measurements
Another important result is that in most of the small fields measured with diode detectors we
found that the OF in general are larger than those obtained with the EBT2, this is mainly
because the material surrounding the diode detector tend to decrease lateral electronic
disequilibrium. The second reason that causes an overestimation of OFs measured with
diodes is that when secondary electron energy increases the electron mass collision stopping
power ratio between water and silicon slightly decreases. The same tendency was also
observed when comparing diodes with EBT3.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
Figure 3: OFs measured with different detectors: (1) semiflex 0.125cc ionization
chamber, (2) pinpoint micro-ionization chamber, (3) shielded diode type P, (4)
unshielded diode type E, (5) gafchromic EBT2 and (6) EBT3 films and calculated by
MC (7).
When comparing the OF determined by films and diodes against MC calculation for the
0,58×0,58 cm2 field the discrepancy is bigger in the case of diodes. The reason of this can be
related to water equivalence of the film material in contrast with the diode detector
composition.
3.2. Correction Factors relative to gafchromic EBT2 and EBT3, and Monte Carlo.
As discussed above, correction factors can be calculated by Monte Carlo or
measured with a suitable detector with small sensitive volume and an energy independent
response, like gafchromic films.
Dose ratios using EBT2 (Fig.4, Table.5) and EBT3 (Fig.5, Table.6) films were determined
for the field sizes listed in Table 4. that range from 0.29 cm2 to 5.8 cm2. Additionally, five
spot cases were calculated by Monte Carlo for a redundant validation of the dose ratios (see
Fig.6 and Table.7)
Figure 4: Correction factors 𝐤𝐐𝐜𝐥𝐢𝐧 𝐐𝐢𝐧𝐭𝐟𝐜𝐥𝐢𝐧 𝐟𝐢𝐧𝐭 relative to Gafchromic EBT2 film.
Table 5. Correction factors 𝐤𝐐𝐜𝐥𝐢𝐧 𝐐𝐢𝐧𝐭𝐟𝐜𝐥𝐢𝐧 𝐟𝐢𝐧𝐭 relative to Gafchromic EBT2 film.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
We found that the absolute absorbed dose determination is very dependent of the time delay
between exposure and reading process, the handling of the film and finally the calibration
procedure followed. It is also extremely important to check the reproducibility of the whole
system.
The function used for calibration data fitting was a rational one recommended by Lewis et al
[14].
Figure 5: Correction factors 𝐤𝐐𝐜𝐥𝐢𝐧 𝐐𝐢𝐧𝐭𝐟𝐜𝐥𝐢𝐧 𝐟𝐢𝐧𝐭 relative to Gafchromic EBT3 film.
Table 6. Correctios factors 𝐤𝐐𝐜𝐥𝐢𝐧 𝐐𝐢𝐧𝐭𝐟𝐜𝐥𝐢𝐧 𝐟𝐢𝐧𝐭 relative to gafchromic EBT3 film.
Field Size(cm) Flex 0.125 PP Diodo P Diodo E EBT2
0.29 3.74 1.62 1.09 1.08 1.18
0.58 1.51 1.11 0.93 0.98 0.99
0.87 1.15 1.04 0.96 1.00 1.00
1.16 1.05 1.01 0.95 0.98 0.98
1.45 1.00 0.99 0.96 0.97 0.98
1.74 1.00 0.99 0.97 0.98 0.99
2.03 1.03 1.03 1.01 1.02 1.02
2.61 1.01 1.01 1.01 1.01 1.01
2.90 1.01 1.01 1.01 1.01 1.02
3.77 1.01 1.01 1.01 1.01 1.03
4.64 1.00 1.01 1.00 1.00 1.00
5.80 1.00 1.00 1.00 1.00 1.00
Field Size(cm) Flex 0.125 Pin
P
Diodo P Diodo E EBT3
0.29 3.18 1.37 0.92 0.91 0.85
0.58 1.52 1.12 0.94 0.99 1.01
0.87 1.14 1.04 0.96 0.99 1.00
1.16 1.06 1.03 0.97 1.00 1.02
1.45 1.03 1.01 0.98 0.99 1.02
1.74 1.01 1.01 0.98 0.99 1.01
2.03 1.00 1.00 0.99 1.00 0.98
2.61 1.00 1.00 1.00 1.00 0.99
2.9 1.00 1.00 0.99 1.00 0.98
3.77 0.98 0.98 0.98 0.98 0.97
4.64 1.00 1.01 1.00 1.00 1.00
5.8 1.00 1.00 1.00 1.00 1.00
IRPA 2013, Rio de Janeiro, RJ, Brazil.
Figure 6: Correction factors 𝐤𝐐𝐜𝐥𝐢𝐧 𝐐𝐢𝐧𝐭𝐟𝐜𝐥𝐢𝐧 𝐟𝐢𝐧𝐭 relative to Monte Carlo.
Table 7. Correctios factors 𝐤𝐐𝐜𝐥𝐢𝐧 𝐐𝐢𝐧𝐭𝐟𝐜𝐥𝐢𝐧 𝐟𝐢𝐧𝐭 relative to Monte Carlo calculations.
Field
Size,cm
Flex
0.125
PP Diodo
P
Diodo
E
EBT
2
EBT
3
0.58 1.52 1.12 0.94 0.99 1.00 1.01
1.16 1.07 1.04 0.98 1.00 1.01 1.02
1.74 1.02 1.02 0.99 1.01 1.01 1.02
2.9 1.00 1.00 1.00 1.00 1.00 0.99
5.8 1.00 1.00 1.00 1.00 1.00 1.00
Differences between Monte Carlo values and data measured with ion chambers in Table 7 for
such small fields can be explained by the larger volume of these detectors. Other deviations
may be due to geometric misalignments.
4. DISCUSSION
Dosimetric data for small photon beams should be measured using detectors with small active
volume, in case of using others just take into account that corrections to detector reading need
to be applied.
The obtained differences between MC calculated OFs and those measured with semiflex and
pinpoint chambers may be related with their larger volume and small geometric
misalignments, it is quite important that whichever detector we use it must be correctly
aligned with the beam axis. Any slight detector offset from the CAX reduces the output factor
values.
Discrepancies obtained between diode measurements and MC calculations are a result of
detector composition, the non-water equivalence of diode material leads to an overestimation
of the output factors.
5. CONCLUSIONS
This investigation contributes to the data being collected for each machine characteristics and
available detectors presented in the clinic, following the methodology proposed by the IAEA
working group.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
The result of OF measurements for field sizes larger than 2×2 cm2 does not depend on the
detector type used. When reducing the field size there is an increased in OF estimation using
a wrong detector.
To account for any deviation due to the handling and scanning process of the Gafchromic
films Lewis et al suggested the “simple two point rescaling”[14], taking into account that for
the same lot of EBT2 or EBT3 films the dose response can be represented by a generic
function which can be adapted using one exposed reference film plus an unexposed reference
film. This rescaling can improve the absolute dose estimated by the Gafchromic films and
consequently the dose ratio.
As expected, corrections factors obtained for EBT2, EBT3 and MC are very close to unity for
field sizes larger than 2x2 cm2.
As a recommendation, we consider that instead of MC simulations in water, a more realistic
material composition can be used to obtain the detector response factors; this applies to
diodes, EBT2 and EBT3 films.
6. REFERENCES
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