Technical Note: The Uses of I’mRT MatriXX in Electron Beams

doi:10.4236/ijmpcero.2013.21003

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International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2013, 2, 15-18

Published Online February 2013 (http://www.scirp.org/journal/ijmpcero)

http://dx.doi.org/10.4236/ijmpcero.2013.21003

Technical Note: The Uses of I’mRT MatriXX in Electron

Beams

Mutian Zhang, Sicong Li, Hua Deng, Sumin Zhou

Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, USA

Email: [email protected]

Received Ocotber 20, 2012; revised November 22, 2012; accepted November 30, 2012

ABSTRACT

Purpose: The objective of this study is to investigate the properties of I’mRT MatriXX device in electron beams, and to

validate MatriXX in electron dosimetry and quality assurance (QA). Methods: The measurements were conducted us-

ing MatriXX in electron and photon beams from Siemens linacs. The MatriXX was placed horizontally on the linac

tabletop. Solid Water layers were used for buildup. For all the measurements, the linac gantry angle was 0˚, and the

source-to-surface distance was 100 cm from the Solid Water surface. The electron cone factors, cutout factors, and

beam profiles were measured and compared with thimble ionization chamber results. Results: The effective water

equivalent depth of MatriXX measurement point is larger than 4 mm. When measuring at the respective depths of

maximum dose, MatriXX has different responses to different beam energies. The cone factors measured by MatriXX

are nearly identical or close to those derived by ionization chambers. Beam profiles (flatness and symmetry) can be eas-

ily determined using MatriXX and are comparable to water tank results. The planar dose map of electron cutout blocks

can be visually observed, and the cutout factors can be conveniently measured. Conclusions: The MatriXX needs

separate dose calibration factors for electron and photon beams. MatriXX can be used to measure electron cutout factors

and beam profiles, thus has the potentials in electron beam dosimetry and routine linac and patient-specific QA tests.

Keywords: Electron Beam; MatriXX; Dosimetry; Quality Assurance

1. Introduction

The I’mRT MatriXX (IBA Dosimetry GmbH, Germany)

device consists of a two-dimensional (2D) array of ioni-

zation chambers. There are 1,020 vented parallel plate

ion chambers on the array detector, arranged in 32 × 32

grid. The chamber center-to-center distance is 7.62 mm,

and the active area is 24.4 × 24.4 cm2. MatriXX has been

validated for 2D dose measurements [1], and is increas-

ingly used in photon beam dosimetry and patient-specific

quality assurance (QA) [2-4]. The application of Ma-

triXX is also extended to QA checks for proton therapy

[5].

In this work, we report our investigation on the feasi-

bility of using MatriXX in electron beam dosimetry and

routine linac QA or patient-specific treatment QA. This

note is the expansion of an abstract submitted to the 2010

American Association of Physicists in Medicine annual

meeting [6].

2. Methods

The measurements were conducted using electron and

photon beams from Siemens ONCOR and PRIMUSTM

linacs (Siemens Healthcare, Germany). The linacs could

produce 6 MV and 23 MV photons, and electron beams

with multiple energies between 5 MeV and 21 MeV. The

MatriXX was placed horizontally on the linac treatment

couch, supported by 5 or 6 cm Solid Water (Gammex

Inc., USA) blocks (Figure 1). The MatriXX was posi-

tioned using the linac light field. On the MatriXX surface,

30 cm × 30 cm Solid Water layers served as beam

buildup with 1 mm thickness resolution. The linac gantry

angle was 0˚, and the source-to-surface distance was set

at 100 cm from the Solid Water surface.

The MatriXX was previously calibrated for the photon

beams. Before each use, the MatriXX was powered on

for 30 minutes, and irradiated with at least 500 cGy until

stable readings were achieved. For each reading, 100

monitor units were delivered. The measurements of each

data point were repeated three times. When taking the

readings, the calibration factor of 6 MV photon beam

was used, so that the MatriXX responses to different

beam energies could be compared. The chamber array

was placed at the depth of maximum dose (dmax) of the

corresponding beam energy for the measurements of

dose response, cone factor or cutout factor. The MatriXX

measurements were compared with beam data acquired

using calibrated PTW semiflex thimble chambers (PTW

M. T. ZHANG ET AL.

Figure 1. MatriXX setup on a linac treatment tabletop. A

proper thickness of Solid Water would be placed on the sur-

face during measurement.

Freiburg GmbH, Germany).

3. Results and Discussion

3.1. Depth Ionization and Beam Profiles

In order to determine the appropriate Solid Water thick-

ness for the tasks carried out by the MatriXX, we meas-

ured the ionization curves for various electron and pho-

ton energies using MatriXX with Solid Water buildup at

a depth resolution of 1 mm, and normalized the data to

the maximum value of each energy. The ionization cur-

ves acquired by ionization chambers were interpolated to

find the depths of the percentage ionization in water cor-

responding to the thicknesses of Solid Water for the same

percentage ionization of the same beam energies. It is

easy to prove using the method of least-squares that, the

average of the differences between the depths of the per-

centage ionization in water and the corresponding thick

nesses of the Solid Water is equal to the depth of the Ma-

triXX effective measurement point. Our measurements

suggest that the effective measurement point of MatriXX

is deeper than 4 mm water equivalent below the top sur-

face. For instance, the best fit of MatriXX and ionization

chamber data showed that the depth of effective measu-

rement point of a specific MatriXX is 4.2 mm water equi-

valent for the 12 MeV beam of an ONCOR linac (Figure

2). During the remaining measurements, the Solid Water

thickness was determined with an approximate effective

measurement depth of 4 mm.

The MatriXX has the function to analyze beam pro-

files. Our results show that at the specified depths of

photon (10 cm) and electron (dmax) beams, the profiles of

open fields measured with the MatriXX are nearly iden-

tical to those scanned with thimble chambers in a water

tank. Therefore, the MatriXX provides a fast and con-

venient way to detect the changes in beams profiles. For

Figure 2. MatriXX and ionization chamber show close re-

sults in depth ionization measurement. The MatriXX data

points are labeled with Solid Water thickne ss plus 4.2 mm.

this reason, we are increasingly using MatriXX in the

monthly linac QA to check the flatness and symmetry of

electron beams as well as that of photons. We must stress

that MatriXX is useful in checking the beam profile con-

sistency, but it cannot replace scanning water tank in the

measurement of beam profiles.

3.2. MatriXX Response to Electron Beams

We observed that, the reproducibility of MatriXX read-

ing was comparable to that of a thimble chamber. The

ionization of electron energies was measured with Ma-

triXX at dmax using 10 cm × 10 cm cones and compared

with photon beams of the same field size. The readings

of MatriXX central chambers were corrected with the

thimble chamber measurements of the linac output fac-

tors on the same day, and normalized to the 6 MV read-

ing for the same dose at dmax. Our data showed that the

ionization chambers in MatriXX have different dose re-

sponse to electron beams from that to photon beams.

Table 1 shows the relative response of MatriXX to pho-

ton and electron beams of a PRIMUS linac. Our results

suggest that the MatriXX needs a calibration factor for

the electron energy in question, especially when patient-

specific treatment plan QA is conducted [7].

3.3. Electron Beam Dosimetry

The output factors of electron cones were measured at

the depth of the maximum dose. These factors were nor-

malized to the 10-cm cone output of the corresponding

energies, and most matched the water tank ionization

chamber measurements very well (Table 2). For reasons

yet unknown, some data showed larger discrepancies, up

to 0.9%.

The cutout factors of square inserts in 10-cm cone

were measured with MatriXX. For the 3-cm and 2-cm

M. T. ZHANG ET AL.

IJMPCERO

Table 1. The relative response of a MatriXX to clinical photon and electron beams of a PRIMUS linac. The readings were

normalized to 6 MV photon energy using the actual linac output of each modality.

Beam Energy 6 X 23 X 6 MeV 9 MeV 12 MeV 15 MeV 18 MeV

Normalized Reading 1.000 0.993 1.070 1.069 1.068 1.077 1.101

Table 2. The electron cone factors measured with MatriXX (MX) and ionization chamber (IC) on a PRIMUS linac. The 5-cm

cone is circular while the rest cones are square (%diff. = percentage difference).

Energy 6 MeV 9 MeV 12 MeV 15 MeV 18 MeV

Cone size MX IC %diff.MX IC %diff.MX IC %diff.MX IC %diff. MX IC %diff.

5 cm (cir) 0.787 0.789 −0.3%0.879 0.886 −0.8%0.9200.920 0.0% 0.9420.9420.0% 0.968 0.968 0.0%

10 cm 1.000 1.000 NA 1.000 1.000 NA 1.0001.000 NA 1.0001.000 NA 1.000 1.000 NA

15 cm 1.018 1.016 0.2% 0.993 0.992 0.1% 0.9950.993 0.2% 1.0011.001 0.0% 1.001 1.001 0.0%

20 cm 1.029 1.026 0.3% 0.973 0.973 0.0% 0.9640.961 0.3% 0.9690.967 0.2% 0.962 0.966−0.4%

25 cm 1.018 1.009 0.9% 0.965 0.964 0.1% 0.9600.956 0.4% 0.9730.968 0.5% 0.963 0.967−0.4%

cutout, a MatriXX central chamber was placed at the

center of the field for accurate readings. These inserts

were made for electron beam commissioning and their

cutout factors had been measured with a thimble cham-

ber in water phantom. Table 3 compares the cutout fac-

tors for the ONCOR 12 MeV beam from MatriXX and

thimble chamber. The data suggest that the MatriXX may

be a useful tool for the measurement of electron cutout

factor. In most cases, the MatriXX can provide clinically

acceptable cutout factors, and we routinely use the Ma-

triXX to measure electron cutout factors. However, the

MatriXX seems to underestimate the cutout factors of

very small inserts (nearly 1%). This tendency could be

caused by the MatriXX air cavities, which make the dif-

ferences in lateral scattering and attenuation of electrons.

The accuracy of MatriXX for very small electron cutouts

may need further investigation.

With Solid Water buildup of appropriate thickness, the

MatriXX can provide a 2D dose map at the desired depth.

This ability provides a convenient way to visualize elec-

tron dose distribution within a phantom. The 2D dose

profile can help the clinician to judge whether a custom

cutout block can provide enough coverage of the lesion

to be treated (Figure 3). This is a rather useful feature

when the electron treatment plan is based on clinical se-

tup and without a 3D image.

4. Conclusion

The MatriXX responds differently to electron beams and

photon beams, thus separate dose calibration factors

should be established for electron dosimetry. It has been

shown that MatriXX can be used to obtain electron cut-

out factors. The ability of MatriXX to display planar

Figure 3. The MatriXX 2D electron beam profiles of 10 cm

open field (top) and a custom cutout (bottom) at dmax with

Solid Water buildup. The beam energy is 18 MeV, and the

profiles are normalized to the same nominal dose.

M. T. ZHANG ET AL.

Table 3. Electron cutout factors of an ONCOR 12 MeV

beam measured with MatriXX and thimble chamber (%diff.

= percentage difference).

Cutout (cm2) MatriXX Thimble Chamber %diff.

10 × 10 1.000 1.000 NA

8 × 8 1.002 1.006 −0.4%

6 × 6 0.992 0.992 0.0%

4 × 4 0.938 0.943 −0.5%

3 × 3 0.903 0.910 −0.8%

2 × 2 0.870 0.878 −0.9%

dose map provides a useful tool in beam profile measure-

ment. MatriXX has the potentials in electron beam dosi-

metry and routine QA checks.

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