PRECISE DOSIMETRY OF SMALL PHOTON BEAMS COLLIMATED · PDF filePRECISE DOSIMETRY OF SMALL...

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

[email protected]

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 micromultileaf 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


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.


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


(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).


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


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.


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.


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


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.


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

[1] Aspradakis M., Byrne J. P., Palmans H., Conway J., Rosser K., Warrington J., Duane S.,

Small Field MV Photon Dosimetry, IPEM Report Number 103, UK, 2010.

[2] Das, I.J., Downes, M.B., Kassaee, A. and Tochner, Z.: “Choice of radiation detector in

dosimetry of stereotactic radiosurgery-radiotherapy”. Journal of Radiosurgery, Vol. 3, 177–

185, 2000.

[3] W. U. Laub and T. Wong: “The volume effect of detectors in the dosimetry of small fields

used in IMRT”, Med. Phys. 30, 341–347, 2003.

[4] Bouchard H. and Seuntjens J.: “Ionization chamber-based reference dosimetry of intensity

modulated radiation beams”, Med. Phys., Vol. 31(9): 2454–2465, 2004.

[5] Sanchez-Doblado F., Andreo P., Capote R., Leal A., Perucha M., Arrans R., Nunez L.,

Mainegra E., Lagares J.I., Carrasco E.: “Ionization chamber dosimetry of small photon fields:

a Monte Carlo study on stopping-power ratios for radiosurgery and IMRT beams”. Phys. in

Med. and Biol., Vol. 48, Issue 14, 2081–2099, 2003.

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Med. Phys., Vol. 35, Issue 1, 206–215, 2008.

[7] Colección De Informes Técnicos Nº 398, Determinación De La Dosis Absorbida En

Radioterapia Con Haces Externos, IAEA-TRS-398, Viena, 2005.

[8] Almond P., Biggs P., Coursey B. M., Hanson W. F., Saiful Huq M., Nath R., Rogers D.

W. O., AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and

electron beams, Med. Phys. 26(9): 1847-1879, 1999.


[9] Alfonso R., Andreo P., Capote R.,Saiful Huq M., Kilby W., Kjäll P., Mackie T. R.,

Palmans H., Rosser K., Seuntjens J., Ullrich W., Vatnitsky S.: “A new formalism for

reference dosimetry of small and nonstandard fields”, Med. Phys., Vol. 35 (11), 2008.

[10] ImageJ. Image Processing and Analysis in Java, http://rsb.info.nih.gov/ij/ (Acc 2001-06-

26).

[11] D. W. O. Rogers, B. Walters, and I. Kawrakow, “BEAMnrc User’s Manual”, NRC

Report PIRS 509, 2005.

[12] Ascención Y., Lara E., Alfonso R.: “Dosimetric characterization of a tertiary collimator

for Radiosurgery using Monte Carlo simulation, 2013 (This workshop)

[13] B. R. B. Walters, I. Kawrakow, and D. W. O. Rogers, “DOSXYZnrc User’s Manual”,

NRC Report PIRS 794, 2005

[14] Lewis D., Micke A., Yu X., Chan M.: “An efficient protocol for radiochromic film

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http://rsb.info.nih.gov/ij/

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