The Radiological Physics Center (RPC) anthropomorphic quality assurance (QA) phantom program is one tool the RPC uses to remotely audit institutions participating in clinical trials. The phantoms contain thermoluminescence dosimeters (TLDs) as the

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Development and Implementation of the use of
Optically Stimulated Luminescent Detectors in the
Radiological Physics Center Anthropomorphic
Quality Assurance Phantoms 1Jennelle Bergene,
1Stephen Kry, 1Andrea Molineu, 2David Bellezza,
1Laurence Court, 1Valen Johnson, 1David
Followill 1Department of Radiation Physics, The
University of Texas M.D. Anderson Cancer Center,
Houston, TX 2St. Lukes Episcopal Hospital,
Houston, TX
Introduction The Radiological Physics Center
(RPC) anthropomorphic quality assurance (QA)
phantom program is one tool the RPC uses to
remotely audit institutions participating in
clinical trials. The phantoms contain
thermoluminescence dosimeters (TLDs) as the
absolute dosimeter in the phantoms, and a switch
to optically stimulated luminescent dosimeters
(OSLDs) is desired. OSLD have been well studied
by the RPC under reference conditions, and have
been shown to agree well with TLD and ion chamber
measurements 1, however, the use of the OSLD
within the anthropomorphic phantoms has not been
studied. The problem with implementing the OSLD
in the anthropomorphic phantoms lies in the
angular dependence exhibited by the
dosimeters. This study aims to characterize the
angular dependence of the OSLD in the RPC pelvic
phantom for effectively utilizing the dosimeters
as a replacement for TLD in the RPCs
anthropomorphic QA phantoms.
The color scale distribution maps of the pixels
passing the 7/4 mm gamma criteria were compared
for the coronal film normalized to TLD doses
(Figure 5) and corrected OSLD doses (Figure 6)
from an IMRT irradiation. The areas of pixels
passing the criteria are essentially the same for
both the TLD normalized film and corrected OSLD
normalized film, and confirm that the corrected
OSLD dose can be used to normalize film for the
purpose of credentialing with the RPCs
anthropomorphic QA phantoms.
The angular dependence correction factors for the
6 MV and 18 MV coplanar, and 6 MV non-coplanar
irradiations of the spherical phantom were
determined and are summarized in Table 2.
  Angular Correction Stdev
Coplanar 6 MV 1.040 0.007
Coplanar 18 MV 1.019 0.004
Non-coplanar 6 MV 1.039 0.010
Table 2. Angular correction factors for OSLD from
spherical phantom irradiations
Figure 3. Positioning of OSLD and TLD (red
circles) in dosimetry insert at the center of the
target
The spherical phantom results reveal an
under-response of the OSLD of approximately 4 at
6 MV and 2 at 18 MV. These results are in
agreement with the data published by Kerns et al.
2, which demonstrated an under-response of the
OSLD of 4 and 3 for 6 MV and 18 MV photon
beams, respectively, for beams incident parallel
to the surface of the dosimeter. The doses
measured by the OSLD from the pelvic phantom
irradiations were corrected with the correction
factors in Table 2. The ratios of the measured
TLD doses to the corrected OSLD doses can be seen
in Table 3. The angular correction factors
effectively corrected the OSLD dose to within 1
of the TLD dose for both the coplanar and
institution trial irradiations, but not for the
three CyberKnife irradiations.
Treatment plans of increasing angular beam
delivery were developed for the pelvic phantom.
Three coplanar plans were developed in Pinnacle,
and one non-coplanar plan was developed in
Accurays MultiPlan. Each plan was delivered
three times to the phantom loaded with TLD-100
capsules and nanoDot OSLD. The IMRT treatments
included the same dosimeters, in addition to
radiochromic film in the coronal and sagittal
planes. The pelvic phantom was also sent to two
institutions to be irradiated, one delivering
IMRT and the other CyberKnife. The doses
measured from the TLD and OSLD were calculated
for each irradiation, applying the correction
factor to the OSLD dose. The ratio of TLD
measured dose to angular corrected OSLD dose was
determined for each irradiation. The films from
the IMRT deliveries and institution trials were
normalized to the TLD and corrected OSLD doses.
Dose profiles were taken and gamma analysis was
performed using a 7/4 mm criteria, for both TLD
and corrected OSLD normalized films, and the
results were compared.
Figure 5. IMRT color scale gamma results for
coronal film normalized to TLD dose
Methods and Materials A 10-cm diameter,
high-impact polystyrene spherical phantom (Figure
1 left) was constructed to hold a nanoDot OSLD
(Figure 1 right) from Landauer, to study the
angular response of the dosimeter under the
simplest of circumstances. The OSLD were
irradiated to 100 cGy in a coplanar geometry for
6 MV and 18 MV photon beams, and in a
non-coplanar geometry for a 6 MV photon beam.
The responses of the dosimeters were normalized
to the response when the beam was incident
normally on the face of the dosimeter
(face-on). The inverse of the average
normalized responses, not including the face-on
response, was calculated and used as the angular
dependence correction factor.
  TLD/OSLD Dose StDev
Coplanar 0.995 0.006
Non-Coplanar 0.965 0.003
Institution Trials 1.001 0.007
Figure 6. IMRT color scale gamma results for
coronal film normalized to corrected OSLD dose
Table 3. Ratios of TLD to corrected OSLD doses
from pelvic phantom irradiations
Conclusion For all irradiations, with the
exception of the three CyberKnife irradiations
performed, the angular dependence correction
factors established from the spherical phantom
irradiations effectively corrected the OSLD
measured dose to within 1 of the TLD measured
dose. Based on the results of the study, OSLD
can effectively be used as the absolute dosimeter
in the RPCs anthropomorphic QA phantoms for
coplanar treatment deliveries when a correction
factor is applied for the angular dependence
exhibited by the dosimeters. The angular
correction factor determined for non-coplanar
treatment deliveries is not recommended for use,
due to considerable differences in the resulting
TLD to OSLD dose ratios from the CyberKnife
irradiations. References 1. Aguirre et al.
"WE-D-BRB-08 Validation of the commissioning of
an optically stimulated luminescence (OSL) system
for remote dosimetry audits," Med Phys 37 (6),
3428 (2010). 2. Kerns et al. "Characteristics of
optically stimulated luminescence dosimeters in
the spread-out Bragg peak region of clinical
proton beams," Med Phys 39 (4), 1854-1863
(2012). Support This investigation was supported
by PHS grant CA10953 awarded by the NCI, DHHS.
The films included in the pelvic phantom
irradiations were normalized to the TLD measured
doses and the OSLD doses, to validate that the
dosimeters can provide equivalent dose profile
and gamma analysis results when the angular
dependence of the OSLD have been corrected. The
lateral dose profiles from the coronal films,
normalized to TLD doses and angular corrected
OSLD doses were compared. The two dose profiles
appear almost identical, and confirm that the
corrected OSLD dose can be used to normalize film
for the purpose of credentialing with the RPCs
anthropomorphic QA phantoms. The percentage of
pixels passing the 7/4 mm gamma criteria are
shown in Table 4 for the coronal and sagittal
films for the two institution trials, as well as
the average IMRT pixel passing rate.
Figure 4. High-impact polystyrene slab phantom
Figure 1. Spherical phantom showing base and
insert holding OSLD (left), nanoDot OSLD from
Landauer (right)
Energy correction factors for OSLD in full
phantom conditions were determined for 6 MV and
18 MV using a high-impact polystyrene slab
phantom (Figure 4), with an OSLD placed at a
depth of 10-cm. The response of the dosimeter at
the investigated energy was compared to the
response of a dosimeter irradiated in a cobalt-60
beam, and this was used to calculate energy
correction factors. Results The OSLD full
phantom energy correction factors for 6 MV and 18
MV photon beams are shown in Table 1. These
energy correction factors were used to calculate
the dose to OSLD for the pelvic phantom
irradiations.
The RPCs pelvic phantom (Figure 2) was used for
this study to investigate the angular response of
the OSLD in the anthropomorphic QA phantoms. The
dosimetry insert of the pelvic phantom was
modified to contain two OSLD in the axial plane,
in addition to the two TLD within the target
volume (Figure 3).
  Film TLD Gamma Pass Rate OSLD Gamma Pass Rate
Avg IMRT Coronal 84 84
  Sagittal 80 80
IMRT Trial Coronal 100 100
  Sagittal 99 99
CK Trial Coronal 93 93
  Sagittal 94 94
Beam Energy KE Stdev
6 MV 1.02 0.010
18 MV 1.08 0.011
Figure 2. RPC pelvic phantom shell (left),
dosimetry insert (middle), and imaging insert
(right)
Table 4. Percent of pixels passing gamma criteria
of 7/4 mm for TLD and corrected OSLD normalized
films
Table 1. Full phantom OSLD energy correction
factors