JC on OSLDs
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Transcript of JC on OSLDs
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PRESENTED BY :Dr LALIT SINGH NEGIPG 2nd Yr
GUIDED BY :Dr NAGESH BINJOO
Thursday, September 12, 2013 1JOURNAL CLUB Dept of OMR
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Characterization of an opticallystimulated dosimeter fordentomaxillofacial dosimetryOral Surg Oral Med Oral Pathol Oral Radiol Endo2011;112:793-797AUTHORSJayasanker V. ValiyaparambilUNIVERSITY OF CONNECTICUT Farmington, CT
Sanjay M. MallyaUNIVERSITY OF CALIFORNIA Los Angeles, CA
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CONTENTSINTRODUCTION
MATERIALS & METHODS
DOSIMETERS & READERSDetermination Of The Dose Response For OSLD Nanodots
Determination of Signal Feeding
Determination of Angular Dependence
Statistical AnalysisRESULTS
Dose response of OSLD Nanodots
Angular Dependence of OSLD Nanodots
DISCUSSION
CRITICAL EVALUATION
PROS & CONS
INFERENCE
REFERENCES
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Reasons for Article selection
The innovation in the field of dosimetry withvarious types of dosimeters
To know about the make & working of opticallystimulated luminescent dosimeters(OSLD)
To compare the efficacy of TLDs & OSLDs
To analyze differences in their mechanism of action
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Thursday, September 12, 2013 5JOURNAL CLUB Dept of OMR
knowledge of dose delivered by a diagnostic radiographicexamination is key for its risk-benefit analysis
doses determined using dosimeters placed at several sites
in a tissue-equivalent anthropomorphic phantom to measureabsorbed doses at specific organ sites
TLDs most widely used dosimeters for point dosimetry
estimates
most commonly used thermoluminescent material is lithium
fluoride doped with magnesium and titanium
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Thursday, September 12, 2013 6JOURNAL CLUB Dept of OMR
on exposer to ionizing radiation, dosimetercrystals absorb energy, produce freeelectrons
Electrons get trapped in a metastable state atsites of imperfections in crystal latticestructure
upon heating trapped electrons return totheir stable, ground state & energydifferential is released as visible light photons
intensity of emitted light is proportional toabsorbed energy & serves as a measure ofthe absorbed radiation dose
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Advantag
es ofTLDs
convenientto place on
body
resilient toenvironmental
changes
reusable
dose
response islinear over awide range
small insize
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Disadv
of TLDs
Expensiveequipment also
requires nitrogengas
Calibrationof the
measurement system
Cumbersomemeasurements
in low doserange
requireannealing to
removetrappedcharges
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Optically stimulated luminescence dosimeters(OSLDs) are composed of carbon-doped aluminumoxide,
basic principles of dose measurement are similar tothat of TLDs
instead of heating, dose readout is performed by
controlled illumination of dosimeter with 540-nmlight photons
doses can be read repeatedly with only a 0.05%decrease in signal intensity
OSL dosimeter for single-point radiationmeasurements contain a disk of aluminum oxideencased in a small plastic sleeve
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OSLD nanoDot dosimeter- An opened OSLD nanoDotdemonstrates the disk of dosimeter material
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dynamic range of OSLDs is 10 Gy to 10 Gy,5 which is broader
than TLDs.
do not require an annealing process to prepare dosimeters for use
Dose readouts are quick and non-destructive, allowing for
dose verification
analysis of total dose accumulation
immediate reuse of dosimeters
ADVANTAGES of OSLDs
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instruments used to measure OSLD absorbed dose are small, and
portable
OSL nanoDot dosimeters have an engraved bar code that encodes
dosimeter sensitivity
a unique identification to allow for efficient and accurate tracking
of dose
use in dosimetry of diagnostic and therapeutic radiation
techniques will increase
ADVANTAGES of OSLDs
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The aim of this study, was to
examine the suitability of OSL
nanoDot dosimeters for dosimetry
of diagnostic maxillofacialradiographic examinations and
compare their performance
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TLD Captures radiation dose
information in a crystal matrix
Releases light when heated, lightintensity proportional to radiation
dose absorbed Durable
Can be expensive (reusable chips)
Information destroyed when
processed
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OSL Captures information in an Aluminum
Oxide matrix
Releases information by laser stimulation
Can be reread after processing
Durable Landauer Only
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Pocket Ionization Electro-statically charged leaf
discharges as it is exposed to ionizingradiation
Not considered a legal record
Low accuracy (+/- 20%) Physical impacts can affect radiation
dose readings
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Dosimetry Reports
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Thursday, September 12, 2013 18JOURNAL CLUB Dept of OMR
Dosimeters and readers
OSL nanoDot dosimeters were purchased
After exposure, OSLD-absorbed doses were measured on a light photon counter
The readout was determined approximately 20 minutes after radiation exposureOSLD readers were calibrated for 80 kVp
To account for energy dependence of dosimeter, correction factors were applied
to measured doses
Correction factors were 1.0 for a 70-kVp beam and 1.2 for a 120-kVp beam
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Thursday, September 12, 2013 19JOURNAL CLUB Dept of OMR
Purchased TLDs also were analyzed within 36 hours after radiation
exposure
OSLDs were exposed within 24 hours after radiation exposure
Radiation exposures measured with a calibrated ionization chamber
served as control
Ion chamber doses measured in mR were converted to mrad using a
conversion factor of 0.95 and were subsequently converted to Gy
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Determination of the dose response for
OSLD nanoDots
relationship between radiation exposure & measured doses was
examined
OSLD nanoDots and an ionization chamber were exposed to radiationfrom a dental x-ray unit
Exposure parameters were 70 kVp and 7 mA
source film distance and exposure times were varied to provide a
dose range of 10 Gy to approximately 4900 Gy, as determined by
ionization chamber.
Separate OSLD nanoDots used for each exposure setting
Two independent exposures done for each setting
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Determination of signal fading
Two OSLD nanoDots were exposed to a 70-kVp x-ray beam
absorbed doses from OSLD nanoDots were determined 20
minutes after exposure
Then Dosimeters were stored at room temperature for 1 month
the absorbed dose was re-measured
Three sequential readings were made for each dosimeter
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Determination of angular dependence
OSLD nanoDots, TLDs, and an ionization chamber were used
source-dosimeter distance was 30 cm
Dosimeters were exposed at beam incident angles of : 0, 15,
30, 45, 60, 75, and 90
90 represents angle at which central ray of x-ray beam is
perpendicular to largest surface area of dosimeter
Separate OSLD nanoDots were used for each exposure setting
and 3 independent exposures were made for each dosimeter angle
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Thursday, September 12, 2013 23JOURNAL CLUB Dept of OMR
Determination of angular dependence
CBCT exposures were also done & TLDs and OSLD
nanoDots were placed at selected anatomic sites
Phantom was custom modified to accommodate OSL
nanoDot dosimeter & was exposed at 3 different fields of
view to yield a range of radiation doses
Exposure parameters were 120 kVp and 150 mA.
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Statistical analysis
Statistical analyses were done using GraphPad Prizm 5.0d and
InStat 3.1a software programs
relationships among doses measured was analyzed using
linear regression analysis and Spearmans correlation
For studies examining angular dependence of OSLD nanoDots
and TLDs, 2-way analysis of variance to analyze effects of 2
nominal variables (dosimeter system and angle of incidence) on
measurement variable (measured dose) was done
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Dose response of the OSLD nanoDotsTo simulate low doses absorbed by tissues during intraoral or
panoramic radiographic examinations, the response was
examined over range of 10 to 4900 Gy
OSLD-measured doses closely matched actual radiation dose,
as determined by a calibrated ionization chamber
There was a linear relationship and a strong correlationbetween doses measured with OSLD and ionization chamber (r2
0.997).
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Loss of signal on storage
dose was read at 20 minutes and 4 weeks after exposure
no significant difference in measured dose between 2 timeperiods
minimal loss of latent signal over time.
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Angular dependence of OSLD nanoDots
Angular or directional dependence is variation inresponse of a dosimeter with angle of incident radiation
Depending on magnitude of this variation, a dosimeter
could potentially over- or underestimate radiation dose
directional dependence of OSLD nanoDots was
examined and compared these variations with TLDs
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Both dosimeter systems demonstrated a significant angulardependence
This variation ranged from 88% to 109% for OSLD nanoDots
and from 91% to 105% for TLD
Overall, there was no significant difference in magnitude of
angular dependence between these 2 dosimeters
Notably, doses measured with TLDs were slightly higher thandoses measured with OSLD nanoDots
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To examine clinical significance of difference between OSLDnanoDots and TLDs both these dosimeters were used to measure
absorbed dose from CBCT examinations
Given rotation of x-ray beam in CBCT, measured doses will
reflect both sensitivity of dosimeter system & inherent angular
dependence
Dose was evaluated at 3 anatomical sites using 3 different CBCT
examinations
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Doses measured by 2 dosimeters differed: OSLD nanoDotmeasured doses ranged from 15% lower to 20% higher than
corresponding TLD dose
there was a linear relationship and a strong correlation
between doses measured with OSLD and TLD systems (r2 0.99)
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Previously, TLD dosimeters used to estimate effective
dose
Landauer, indicated that OSLD systems will replace
TLD systems for point dosimetry
Manufacturer have introduced adapters to allow use
of OSLD nanoDots
Use of OSLD is justified & outcome will make
significant changes to our current dose estimates
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important when new studies using OSLD systems
compare dose estimates to existing data that are based
on TLD measurement
Authors have report on suitability of OSLD nanoDots ;
dose response of OSLD nanoDots evaluated over the
dose range of 10 to 4900 Gy
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Thursday, September 12, 2013 33JOURNAL CLUB Dept of OMR
OSLD dose response is linear over dose range and
correlated with dose using an ionization chamber
OSLD can accurately measure doses, even over low
dose range
With TLDs, dosimeters is exposed 2 to 3 times to
yield a dose that is high enough to be reliably measured
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Even doses as low as 10 Gy were reliably measured
from a single exposure using OSLD nanoDots
linear relationship and strong correlation between
doses measured by OSLD nanoDots & TLDs observed
Discrepancy observed between systems, with OSLD
measured dose ranging from 15% lower to 20% higher
than corresponding TLD dose
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Magnitude of this variation is not clinically significantSuch effective dose measurements serve as an
estimate, and not as a precise indicator of the dose that
will be received by individual patients
They evaluated angular dependence of the OSLdosimeters
The magnitude of this angular dependence was not
significantly different between these 2 dosimeter
systems
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OSL nanoDot dosimetry system is well suited for
radiation dose measurements from maxillofacial
radiographic examinations
They anticipate that these dosimeters will perform as
well as currently used TLDs
Effective dose estimates using this new system
should not differ significantly from current TLD-based
dose estimates
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PROS & CONSThe authors have chosen a very important issue & havecompared & comprehend the analysis very clearlyBut they have failed to derive a conclusionThey have successfully derived the results regardingthe superior efficacy of the OSLDsbut they are unable to interpret the resultsThe presentation is upto the mark but in the discussionpart
the mechanism of action has not been clearly explainedeither pictorially or theoretically
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In my view a good study was taken up by theauthors, conducted properly, analyzed categoricallybut the interpretation of results was misguiding & thereaders were not presented with a definitive choice
of a better dosimeter based on the studyDespite several references presented in scientificliterature the better efficacy of OSLDs in comparisonto TLDs was not documentedFew OSLD benefits are documented in the followingslide.
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OSL technology uses light to measure dose,significantly faster process in comparison withheating detector during thermoluminescencereadout
Light exposure can be controlled with very highprecision hence accuracy of dosimetricmeasurements is greatly improved.
OSL allows for multiple readouts and is used
routinely to re-confirm reported radiation doses.Only a fraction of radiation exposure signalcontained in the Al2O3 material is depleted uponstimulation with the green light
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Current TLD dosimeters can be analyzed only onceand film cannot be re-developed to correct forprocessing errors
OSL has a high degree of environmental stability.Heat, humidity, or chemical solvents do not affectdetector
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The enhanced features of this dosimeter include highersensitivity, wider range of exposure measurement, greaterprecision, increased environmental stability, and an extendedwear period (2 months)
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1. Horowitz YS. The theoretical and microdosimetric basis of thermoluminescence
and applications to dosimetry. Phys Med Biol 1981;26:765-824.2. DeWerd LA, Wagner LK. Characteristics of radiation detectors for diagnostic
radiology. Appl Radiat Isot 1999;50:125-36.
3. Burke K, Sutton D. Optimization and deconvolution of lithium fluoride TLD-100 in
diagnostic radiology. Br J Radiol 1997;70: 261-71.
4. Jursinic PA. Characterization of optically stimulated luminescent
dosimeters, OSLDs, for clinical dosimetric measurements. Med Phys 2007;34:45945. Yukihara EG, McKeever SW. Optically stimulated luminescence (OSL) dosimetry in
medicine. Phys Med Biol 2008;53:R351-79.
6. Frederiksen NL, Benson BW, Sokolowski TW. Effective dose and risk assessment
from computed tomography of the maxillofacial complex. DMFR 1995;24:55-8.
7. Hirsch E, Wolf U, Heinicke F, Silva MA. Dosimetry of the cone beam computed
tomography Veraviewepocs 3D compared with the 3D Accuitomo in different fields ofview. DMFR 2008;37:268-73
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