jc on oslds

7/29/2019 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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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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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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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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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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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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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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