Technical Note: Response measurement for select radiation detectors
in magnetic fields
M. Reynoldsa)
Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue,
Edmonton, Alberta T6G 1Z2, Canada
B. G. Fallone
Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton,
Alberta T6G 1Z2, Canada and Departments of Oncology and Physics, University of Alberta,
11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
S. Rathee
Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton,
Alberta T6G 1Z2, Canada and Department of Oncology, Medical Physics Division,
University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
(Received 20 December 2014; revised 20 April 2015; accepted for publication 21 April 2015;
published 15 May 2015)
Purpose: Dose response to applied magnetic fields for ion chambers and solid state detectors has been
investigated previously for the anticipated use in linear accelerator–magnetic resonance devices. In
this investigation, the authors present the measured response of selected radiation detectors when the
magnetic field is applied in the same direction as the radiation beam, i.e., a longitudinal magnetic
field, to verify previous simulation only data.
Methods: The dose response of a PR06C ion chamber, PTW60003 diamond detector, and IBA PFD
diode detector is measured in a longitudinal magnetic field. The detectors are irradiated with buildup
caps and their long axes either parallel or perpendicular to the incident photon beam. In each case,
the magnetic field dose response is reported as the ratio of detector signals with to that without an
applied longitudinal magnetic field. The magnetic field dose response for each unique orientation as
a function of magnetic field strength was then compared to the previous simulation only studies.
Results: The measured dose response of each detector in longitudinal magnetic fields shows no
discernable response up to near 0.21 T. This result was expected and matches the previously published
simulation only results, showing no appreciable dose response with magnetic field.
Conclusions: Low field longitudinal magnetic fields have been shown to have little or no
effect on the dose response of the detectors investigated and further lend credibility to
previous simulation only studies. C 2015 American Association of Physicists in Medicine.
[http://dx.doi.org/10.1118/1.4919681]
Key words: linac-MR, solid state detectors, ion chambers, longitudinal magnetic field, dosimetry
1. INTRODUCTION
The ultimate goal of the advanced real time adaptive
radiotherapy (ART2) team at the Cross Cancer Institute (CCI)
in Edmonton, Canada, is to develop a hybrid linear accelerator
(Linac) and magnetic resonance imager (MRI) to allow for real
time imaging and tumour tracking during radiation treatment.1
The ART2 team has developed a rotating biplanar magnet
capable of irradiating either parallel (longitudinal magnetic
field) or perpendicular (transverse magnetic field) to the static
magnetic field of the imager.2 A Linac–MR integrating a
solenoidal magnet with a static field perpendicular to a rotation
axis of a Linac is also being developed at UMC Utrecht in the
Netherlands.3 Both of these prototypes are capable of real time
irradiation and tumour tracking via MR imaging.
There are previous studies which characterize the response
of various radiation detection devices as a function of magnetic field strength, and the relative orientations of magnetic
field, photon beam, and detector4–12 for the expressed use in a
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Med. Phys. 42 (6), June 2015
Linac–MR environment. These studies focus primarily on ion
chambers and solid state detectors and included measurements
made exclusively in transverse magnetic fields. Ion chambers
are used for reference dosimetry and for the calibration
of various radiotherapy machines. Solid state detectors are
typically used for relative dosimetry, such as beam profiles,
or percent depth doses (PDDs). Solid state detectors also have
the advantage of typically being small enough to accurately
measure dose in small fields.13,14 Ion chambers and various
solid state detectors can be used in conjunction with one
another to offer a full range of dosimetry options.
This work will attempt to experimentally validate the previously simulated longitudinal magnetic field dose responses
of the PR06C ion chamber, PTW60003 diamond detector,
and IBA PFD diode detector.4,5 Experimental verification
of these results is of importance in ascertaining both the
accuracy and correctness of implementation of the Monte
Carlo model employing longitudinal magnetic fields. The
ratio of the measured detector signals with and without a
0094-2405/2015/42(6)/2837/4/$30.00
© 2015 Am. Assoc. Phys. Med.
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Reynolds, Fallone, and Rathee: Response measurement in magnetic fields
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static magnetic field under otherwise identical setup and
irradiation is compared to the same ratio simulated previously.
Close agreement between measured and previously simulated
longitudinal magnetic field dose responses would be indicative
of correctly implemented modeling within the Monte Carlo
code system. We would also gain confidence in the use of
these commercially available detectors as is in a Linac–MR
environment.
2. MATERIALS AND METHODS
2.A. Description of geometry
It is important to note the different relative orientations
of photon beam, magnetic field, and radiation detector when
discussing magnetic field dose response of radiation detectors.
Simulation and measurement work on the magnetic field dose
response has been done in situations where the magnetic
field is perpendicular to the photon beam, henceforth called
transverse magnetic fields.4,5,7,9 The case where the magnetic
field is parallel to the photon beam is henceforth referred to as
the longitudinal field case. Within transverse and longitudinal
magnetic fields, geometrically cylindrical detectors can be
independently oriented either parallel or perpendicular to
the photon beam direction. The longitudinal field case has
been studied previously in simulations for various detectors;4,5 however, measurements have previously not been
performed.
This work deals with detectors having cylindrically shaped
packaging (PR06C, PTW60003, IBA PFD) in longitudinal
field orientations only. To adhere to previous work, the
orientation in which the detector long axis, magnetic field,
and photon beam are all aligned in parallel will be referred to
as orientation III.4,5 The case where the detector long axis is
perpendicular to the photon beam and magnetic field (photon
beam and magnetic field are parallel) will be referred to as
orientation IV.4,5
2.B. Measurement setup
2.B.1. Longitudinal field verification measurements
The longitudinal field dose response of the PR06C farmer
chamber,4 PTW60003 diamond detector,5 and IBA PFD diode
detector5 is verified experimentally in this work, details on the
physical properties of these three detectors can be found in
Table I. Each detector was irradiated in air with a tight fit
buildup cap to ensure electronic equilibrium; the buildup caps
used are noted in Table I and were either a 1.3 cm PMMA
F. 1. Longitudinal electromagnet cross section. The magnetic field is directed upwards, in the same direction as the blue incident photon beam. The
green cylinder represents the long axis of the radiation detector (pictured is
orientation III).
buildup cap or a 0.2 cm brass buildup cap. The previous
simulations of these detectors in orientations III and IV
also employed buildup caps to ensure electronic equilibrium.
The detectors with buildup were irradiated by the 6 MV
photon beam of a Varian 23EX accelerator (Varian Medical
Systems, Palo Alto, CA). This is the same photon beam
used in the measurement of transverse magnetic field dose
response.4,5 The simulations employed a previously published
6 MV beam from a Varian accelerator.15 Spectral differences
between the measurement and simulation beams are expected
to be comparable to those differences from accelerator to
accelerator, and of little practical significance.
The longitudinal magnetic field was produced by a
pair of individual solenoidal electromagnet coils (2 × GMW
11801364 coil, San Carlos, CA) stacked on top of one another
(see Fig. 1). The central bore created in this configuration
is 26.5 cm in depth, and 17.7 cm in diameter; the outer
diameter of the coils is 39.3 cm. This configuration allows
for a relatively homogeneous longitudinal magnetic field in
the central bore region; this is the region where the detectors
will be placed in either orientation III or IV and irradiated.
The magnetic field in the bore was measured with a 3D hall
probe (Metro Lab THM1176) as a function of applied current
and was 0.003 T/A in the central region. The weight of the
electromagnet configuration is too substantial to be placed in
the accelerator couch; as a result, the magnets reside on a small
stand on the floor beneath the accelerator source. The center of
the bore of the electromagnet resultantly sits at 182 cm from
the accelerator photon source. This distance differs from the
T I. Physical properties of radiation detectors studied previously.
Detector
PR06C
PTW60003
IBA PFD
Detector packaging
diameter (mm)
Detector packaging
length (mm)
Detection
volume
Wall material
Buildup
material
Buildup
thickness (cm)
7
7.3
7.2
24
20
17
0.65 cm3
1.7 mm3
2.45 mm3
C-552
Polystyrene
ABS/epoxy/tungsten
PMMA
Brass
Brass
1.3
0.2
0.2
Medical Physics, Vol. 42, No. 6, June 2015
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Reynolds, Fallone, and Rathee: Response measurement in magnetic fields
F. 2. Measured and simulated relative dose response of the PR06C ion
chamber as a function of longitudinal magnetic field strength.
F. 4. Measured and simulated relative dose response of the IBA PFD diode
detector as a function of longitudinal magnetic field strength.
100 cm distance employed in the simulation studies; however,
this change in distance will only have a significant effect on
the dose rate seen by the detector. The relative nature of the
dose response measurements precludes any correction for this
effect. Each of the three detectors was placed in the center of
the bore of the electromagnet (see Fig. 1) and measurements
were taken in orientations III and IV as the magnetic field
strength was varied from 0 to ∼0.2 T. The field sizes at the
plane of the detectors’ active volume centroid were 4 × 7 cm
for the PR06C, and 2×4 cm for the PTW60003 and IBA PFD
detectors. The longitudinal field measurements were in part
completed to verify previously simulated results. Thus, the
field sizes used for the various detectors are identical to those
that were simulated in the previous studies.4,5
The measurements are presented as the ratio of charge
collected with an applied longitudinal magnetic field to that
collected without a magnetic field. This ratio will allow
for the determination of a correction factor—the inverse of
the observed response (0.95 for a response of 1.052)—for
the associated radiation detector when used in a particular
orientation at a specified magnetic field strength. Three
100 MU measurements for each magnetic field strength were
taken, and the average of these three measurements was
taken as the data point for the associated magnetic field. The
stability of the measurements was verified by remeasuring
three field strengths (including zero magnetic field) to
ensure they were unchanged. Before use, the detectors were
preirradiated according to manufacturer recommendations
where applicable. The PR06C chamber was used with a 300 V
bias, the PTW60003 detector was used with a 100 V bias,
and the IBA PFD detector was used without bias as per
manufacturer recommendations.
3. RESULTS AND DISCUSSION
F. 3. Measured and simulated relative dose response of the PTW60003
diamond detector as a function of longitudinal magnetic field strength.
a)Author
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3.A. Longitudinal field measurements
The results of the longitudinal field measurements of the
PR06C farmer chamber, PTW60003 diamond detector, and
IBA PFD diode detector are presented in Figs. 2, 3, and 4,
respectively. Each figure contains both the orientation III and
IV measurements at low field longitudinal magnetic fields
and previously published simulation data of these detectors in
orientations III and IV.4,5
The standard deviation in the simulation data sets was
0.25%, 0.7%, and 0.9% for the PR06C, PTW60003, and
IBA PFD, respectively, while the estimated error in the
measurements was ∼0.1%–∼0.2% for all three detectors.
Both the measurements as well as the simulated ratio of the
detectors’ response with and without magnetic field applied
during irradiation are close to 1.0. Moreover, it is likely
that any observed deviances in the measurements are merely
statistical anomalies, as the difference between measurement
and simulation is, in all cases, smaller than the combined
standard deviation. The measurements for all three detectors
suggest that there is no appreciable magnetic field dose
response at field strengths up to near 0.21 T in longitudinal
magnetic field configurations. Further, these measurements
corroborate the previously published low field orientation III
and IV simulations, giving credence to the validity of the
higher magnetic field simulation results.
4. CONCLUSION
Measurements made at low field longitudinal magnetic
fields with the PR06C ion chamber, PTW60003 diamond
detector, and IBA PFD diode detector all vary very little
with magnetic field strength up to the maximum field strength
obtainable (about 0.21 T). These measurements are in the near
vicinity of the previously simulated results at the same field
strengths. This closeness of measurement and simulation lends
further confirmation to the accuracy of the previous simulation
only studies. It is clear that at low magnetic fields, a correction
factor for these investigated detectors is not required, so long
as the magnetic field is longitudinal in nature.
to whom correspondence should be addressed. Electronic mail:
michaelreynolds@ualberta.net
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