MOSFET dosimetry in radiotherapy · PDF fileMOSFET dosimetry in radiotherapy Joanna E.Cygler1...

MOSFET MOSFET dosimetry in dosimetry in MOSFET MOSFET dosimetry in dosimetry in radiotherapyradiotherapy

Joanna E.Cygler1 and Paolo Scalchi2Joanna E.Cygler and Paolo Scalchi

1Th Ott H it l R i l C C t Ott C d1The Ottawa Hospital Regional Cancer Centre, Ottawa, Canada

2Department of Medical Physics, San Bortolo Hospital, Vicenza, Italy

The Ottawa L’HopitalThe Ottawa L HopitalHospital d’OttawaRegional Cancer Centre

DisclosureDisclosureDisclosureDisclosure

The authors have received research support

from Thomson-Nielsen, Best Medical Canada

d Si l T h l i Iand Sicel Technologies, Inc.

Cygler, MOSFET dosimetry, AAPM Summer School 2009

OutlineOutline• Principles of MOSFET dosimetry• Brief description of commercially available MOSFET

systemssystems• Dosimetric characteristics of MOSFET detectors

– Temperature dependencep p– Energy dependence– Dose and dose rate dependence– Time dependence (other than the dose-rate dependence)Time dependence (other than the dose-rate dependence)– Angular dependence

• Advantages and disadvantages• Clinical dosimetry applications• Summary


MOSFET structureMOSFET structureMOSFET structureMOSFET structure• Metal Oxide Semiconductor

Field Effect Transistor

• Capable of dose measurements

immediately after irradiation or

can be sampled in predefined

time intervals (on-line

dosimetry)

• Can operate in active (negative

bias on gate during radiation) or


passive modeSoubra, Cygler, Mackay, Med. Phys. Soubra, Cygler, Mackay, Med. Phys. 21(4)21(4), 567, 567--572, 1994572, 1994

Threshold voltage shift / Threshold voltage shift / ΔΔVT VT Threshold voltage shift / Threshold voltage shift / ΔΔVT VT

AfterBefore exposure

After exposure • VT is a function of

absorbed dose • That function is linear That function is linear

when the MOSFET operates in the biased m d d in th mode during the irradiation

• Absorbed dose linearity yregion increases with the increase of the bias voltage


bias voltageSoubra, Cygler, Mackay, Med. Phys. Soubra, Cygler, Mackay, Med. Phys. 21(4)21(4), 567, 567--572, 1994572, 1994

Types of MOSFETs availableTypes of MOSFETs availableSingle bias, single MOSFET• Temperature dependenceTemperature dependence• Instability of responseDual-bias, dual-MOSFET

P d b S b C l t l i M d Ph (1994)• Proposed by Soubra, Cygler et. al. in Med. Phys. (1994)• Two MOSFETs on same silicon chip operating at two different

gate biases• Better sensitivity, reproducibility, and stability than single

MOSFET• Minimal temperature effectsUnbiased single MOSFET• Temperature dependence• Instability of response frequently used as disposable detectors


Instability of response, frequently used as disposable detectors• Shorter linearity range than biased MOSFETs

DualDual--MOSFETMOSFET--dualdual--bias detectorbias detectorDualDual MOSFETMOSFET dualdual bias detectorbias detector


Courtesy of Ian Thomson

Commercial MOSFET systems Commercial MOSFET systems il bl th k til bl th k tavailable on the marketavailable on the market

• BEST Medical (Thomson-Nielsen)– Mobile MOSFET systemMobile MOSFET system

• Sicel Technologies Inc.S cel echnolog es Inc.– OneDose system

– DVS (Dose Verification System)– DVS (Dose Verification System)


Mobile MOSFET systemMobile MOSFET systemMobile MOSFET systemMobile MOSFET system

MOSFETReader / bias box

MOSFET array

TN detectors come in two physical sizes:


TN detectors come in two physical sizes: standard and microMOSFETs, see Table 29-I

Wireless setupWireless setupWireless setupWireless setup


Nominal sensitivities in highNominal sensitivities in high--energy photon beams of various energy photon beams of various TN detector/bias combinations TN detector/bias combinations TN detector/bias combinations. TN detector/bias combinations.

Nominal sensitivityTN MOSFET type Bias Nominal sensitivity (mV/cGy)

Standard sensitivity Standard 1

Standard sensitivity High 3Sta da d se s t v ty g 3

High sensitivity Standard 3


High sensitivity High 9

OneDose MOSFET systemOneDose MOSFET systemOneDose MOSFET systemOneDose MOSFET system

OneDose MOSFET

buildup cap

OneDosePlus MOSFET


OneDose reader OneDose MOSFET detectors

Courtesy of Sicel Technologies

DDoseose VVerificationerification SSystemystem ComponentsComponents

11-gauge needle

l bl DImplantable Dosimeter

ReaderDVS Reader

Plan and Review Software

DVS Database

Cygler, MOSFET dosimetry, AAPM Summer School 2009 Courtesy of Sicel Technologies

Implantable MOSFET detector (DVS)Implantable MOSFET detector (DVS)p ( )p ( )

• Electronics assembly Electronics assembly contains 2 MOSFETs and support circuitry

• Bi-directional antenna coil provides dosimeter power and communications channeland communications channel

• Hermetically sealed in bio-compatible glass capsule

MOSFETS

compat ble glass capsule

• Filled with medical grade epoxy


p y

CMRP MOSFET Dosimetry Systemy y

MOSkin detectors, thickness 0.07 mm, th ckness 0.07 mm, see Table 29-III

MOSFET Clinical Dosimetry System:


designed and distributed by CMRPCourtesy of Anatoly Rosenfeld

MOSFET calibrationMOSFET calibrationMOSFET calibrationMOSFET calibration• Purpose – to establish calibration coefficient of the p

detector

1),(

)()(0det

0

QDMQDQCF

)(1)(, QCF

QS wAD

Units: mV/cGyUnits: cGy/mV

kVQCFQD )()(

),(),( 00det QDVQDM


iith kVQCFQD )()(

Calibration processCalibration process

Calibration process dosimetric characterization of d lib i ffi i detectors calibration coefficient

• In principle, the users are responsible for the calibration of new detectors

• Some companies, e.g. Sicel sell pre-calibrated detectors.

– the user is still responsible for checking the calibration coefficients, so errors are avoided in patient dosimetry


Dosimetric characterization of Dosimetric characterization of MOSFET d t tMOSFET d t tMOSFET detectorsMOSFET detectors

• MOSFET detectors should be fully characterized before use

• How it is done, depends on the intended use of the detectorC lib ti i f ll b ild diti s• Calibration in full buildup conditions– in phantom, at a standard depth, e.g. dmax, 5 cm,

etcetc.– in a linac beam, simultaneous measurement of the

detector and ionization chamber signals


g

SSD calibration setSSD calibration set--upup

SSD1 = 80 cm (Co-60)

SSD2 = 100 cm (linac)Field Size = 10x10 cm2

SSD2 = 100 cm (linac)

MOSFET depth = 5.0 cm

Ion chamber Ion chamber depth = 11.3 cm Backscatter = 12.3 cm


*Diagrams not to scale

Clinical calibration processClinical calibration process• 50-100 MU delivered several times, threshold

voltage recorded before and after each trialg

• Simultaneous ion chamber measurement used to d t min th d s f h t i l)(QD

TG-51 PDD curves

determine the dose for each trial)(0 QD

Mraw Dose @ depth = 11.3 cm Dose to water at MOSFET location

TG 51 PDD curves

),()(

0

0

QDVQDCF

th

mVcGy


Calibration Process for DVSCalibration Process for DVS

• Calibration performed by the fmanufacturer

•The response (or radiation sensitivity) of each dosimeter is first determined using a

Sicel 60Co Irradiator

60Co source

• Calibration is performed in a Calibration is performed in a phantom (“in vitro” ) at body temperature (37°C) In Vitro Water Tank Testing


t mp ratur ( 7 ) In-Vitro Water Tank Testing

Courtesy of G. Beyer

Calibration Process for DVS Calibration Process for DVS (cont.)(cont.)Calibration Process for DVS Calibration Process for DVS (cont.)(cont.)• A cumulative dose response calibration curve is obtained for a

Lot Dose Response - RADFET Radiation Sensitivity

0.5

vity

specific lot by irradiating a statistically significant representative sample from the lot up to 80Gy(0 2

0.25

0.3

0.35

0.4

0.45

Rad

iatio

n Se

nsiti

v(m

V/cG

y)

(maximum dose range of the dosimeter). Dose response curve

• Verified by UW ADCL (sample lot sent for testing)

0.2201 1004 2008 3008 4009 5011 6018 7026 8036

C umul a t i v e Dose ( c Gy )

• Verified by UW-ADCL (sample lot sent for testing)

• Calibration is valid for use with daily doses of 150-250 cGy

• Reported accuracy for each lot has a calibration certificate with values Reported accuracy for each lot has a calibration certificate with values within:

•<5.5% (2σ) up to 20 Gy


•<6.5% (2σ) up to 74 Gy (accuracy decreases for doses > 74 Gy).


Correction factors for MOSFETsCorrection factors for MOSFETs

• Environmental – temperature (no pressure c cti n)correction)

• Energy dependence– beam energym gy– modality (photons, electrons, particles)

• Accumulated doseD • Dose rate

• Field size• SSDSSD• Directional dependence


Temperature dependenceTemperature dependenceTemperature dependenceTemperature dependence

• TN dual-MOSFET-dual-bias detector –temperature independentp p

• Other currently available MOSFET detectors Other currently available MOSFET detectors need corrections to be applied when used at a temperature different from the one at calibrationtemperature different from the one at calibration


Temperature dependenceTemperature dependence

The DVS is calibrated at 37oC 0.46

0.4723CThe DVS is calibrated at 37oC

for use at body temperature.

DVS dosimeter is 0.42

0.43

0.44

0.45

0.46

nsiti

vity

(mV

/cG

y) 37CLinear (23C)

DVS dosimeter is approximately 3.3% more sensitive (higher dosereading for same applied dose)

y = -1.0054E-02x + 4.5635E-010.38

0.39

0.40

0.41

Aver

age

sen

g pp )when irradiated at 37oC vs. 23oC

0.370 1 2 3 4 5 6 7 8

Irradiation session #

A multiplicative correction factor of 1.033 can be used for room temperature phantom measurements



MOSFET energy dependenceMOSFET energy dependenceMOSFET energy dependenceMOSFET energy dependence

Edwards et al 1997

Wang et al MC

Wang et al, MC, 2005

Air-kerma sensitivity

Wang et al MC 2005Kron et al 1998

Kron et al, 1998

Edwards et al, 1975

Air-kerma sensitivity

Kron et al, 1998

Absorbed dose ensitivity

photon energy / keV

Cygler, MOSFET dosimetry, AAPM Summer School 2009Wang et al Radiat. Prot. Dos. 2005

p gy

MOSFET energy dependenceMOSFET energy dependenceTNTN 502 RDM502 RDMTNTN--502 RDM502 RDM

1.09

)

1.05

1.07

cGy/

mV)

1 01

1.03

Fact

or (c

0.99

1.01

brat

ion

0.95

0.97

Cal

i


C0-60 4 MV 6 MV

Beam Energy Courtesy of T. Woods

Energy dependence Energy dependence -- TNTN--502502--RDRD

* experiment* experiment MC high energy X-rays

● MC mono-energetic beams


Panettieri et al Phys Med Biol. 52(1):303-16.2007

Absorbed dose linearity Absorbed dose linearity –– dual dual MOSFETMOSFET d ld l bibiMOSFETMOSFET--dualdual--biasbias

6 MV

Consorti et al, Int. J. rad. Onc. Biol. Phys.63, 952-60, 2005

Ramaseshan et al, Phys. Med. Biol. 49 , 4031–4048, 2004


Effect of accumulated doseEffect of accumulated doseEffect of accumulated doseEffect of accumulated dose

Ramani et al Int. J. Rad. Onc. Biol. Ramani et al, Int. J. Rad. Onc. Biol. Phys., 37, 959-64, 1997


Effect of accumulated dose Effect of accumulated dose ––Eff f mEff f munbiased MOSFET unbiased MOSFET -- DVSDVS

Lot Dose Response - RADFET Radiation Sensitivity

0.4

0.45

0.5

nsiti

vity

y)

0.3

0.35

0.4

diat

ion

Sen

(mV/

cGy

0.2

0.25

201 1004 2008 3008 4009 5011 6018 7026 8036

Cumul a t i v e Dose ( c Gy )

Rad


( y )

Directional dependenceDirectional dependenceDirectional dependenceDirectional dependence

• Ideally it should be isotropic

• In practice, angular response of p , g pthe detector depends on

- its design

• It can be different for different energies

• Should be evaluated in-phantom to derive correction factors


Directional dependenceDirectional dependenceof microMOSFETof microMOSFET


Rowbottom & Jaffray. Med. Phys. 31, 609-15, 2004

Directional dependence phantomDirectional dependence phantomDirectional dependence phantomDirectional dependence phantom

Example of a spherical phantom that can be used to pmeasure angular dependence of MOSFET response


Directional dependence

Energy: 100 kVp , FS:10x10 cm2 ,FSD:50 cm, Depth:1.5 cm

1.05

1.10

onseXray Tube

0.90

0.95

1.00

ativ

e re

spo

Rotating insert

0.80

0.85

0 30 60 90 120 150 180 210 240 270 300 330R

ela

3 cm

Polystyrene phantom Angle / deg(25 cm x 25 cm x 3 cm )

Isotropic within 2.5% (1SD) Cygler et al, Radiother. Onc. 80, 296–301, 2006


Time dependence Time dependence -- CyberknifeCyberknifeTime dependence Time dependence CyberknifeCyberknife•• Cyberknife: long treatment timesCyberknife: long treatment times

•• HighHigh--dose DVS dosimeter initial design dose DVS dosimeter initial design --significant dependence of response on irradiation significant dependence of response on irradiation g p pg p ptime: 1time: 1--h and 2.5h and 2.5--h multiple irradiations caused h multiple irradiations caused mean overmean over--responses of 4% and 8%, respectively, responses of 4% and 8%, respectively, when compared to a single irradiation of when compared to a single irradiation of 1.5 min.1.5 min.

•• Improved DVS design Improved DVS design -- no time dependence no time dependence p gp g pp(response within 2 %)(response within 2 %)


MOSFET detectorsMOSFET detectorsAdvantages Advantages vs vs disadvantagesdisadvantagesAdvantages Advantages vs. vs. disadvantagesdisadvantages

Advantages Disadvantagesdvantages• Very small active volume • Dual-MOSFET-dual bias

g• Finite lifetime(~100 Gy)• Energy dependence

system eliminates most correction factors

• Instantaneous readouts

• Temperature dependence for single-MOSFET-detector• Instantaneous readouts

(on-line dosimetry)• Permanent dose storage

• Sensitivity change with accumulated dose for unbiased MOSFETsg

(Can be read multiple times)

• Waterproof

unbiased MOSFETs


• Waterproof• Efficient in use

Clinical applicationsClinical applications• In-phantom measurements

– Build-up curves for high energy photon beams– Interface dosimetry– Small field output factors (radiosurgery)

• In-vivo dosimetry– External beam: entrance and exit doses, skin dose,

peripheral dose, tumor dose– TBI

IMRT– IMRT– IGRT

IORT


– IORT– brachytherapy

BuildBuild--up depth dose curves 6 MVup depth dose curves 6 MVFS 10 10 FS 10 10 2 2 SSD 100 SSD 100 FS=10x10 cmFS=10x10 cm2, 2, SSD=100 cmSSD=100 cm

MOSkin chip developed at CMRP allows measurements allows measurements of the surface dose at a depth of 0.07mm

Cygler, MOSFET dosimetry, AAPM Summer School 2009Courtesy of Anatoly Rosenfeld

Entrance and exit dosimetryEntrance and exit dosimetryEntrance and exit dosimetryEntrance and exit dosimetry

• Use of build-up caps is recommendedp p

• OneDose Plus detectors – come equipped with caps

• TN MOSFETs – one can fit inside special buildup

capscaps


Radiosurgery dosimetryRadiosurgery dosimetry

1

Radiosurgery dosimetryRadiosurgery dosimetry

0.9

1ct

or

0.8

tput

fac

li i l0.7ou

t clinical

MOSFET

0.60 20 40 60 80 100

i /


cone size / mm

Courtesy of J. Wojcicka

IMRT in vivo dosimetryIMRT in vivo dosimetryyy

Marcie et al, Int. J. Rad. Onc. Biol. Phys., 61, 1603–6, 2005 • In some cases BB’s placed on top


In some cases, BB s placed on top of patient’s mask for original CT

Results from Results from in vivoin vivo measurementsmeasurementshhTomotherapyTomotherapy

Example: Patient E


Cherpak et al Radiother. Oncol. 86, 242-250, 2008

IORTIORTIORTIORT

Pancreas treatment with Novac7Pancreas treatment with Novac7


Consorti et al, Int. J. rad. Onc. Biol. Phys.63, 952-60, 2005

Detector calibration for Detector calibration for brachytherapy brachytherapy TGTG 43 formalism43 formalismbrachytherapy brachytherapy --TGTG--43 formalism43 formalism

)()()(/)()( FGGSD

•Solid water phantom

),()(),(/),(),( 00 rFrgrGrGSrD K

5.5 cm

1.0cm

MOSFET

Solid water phantom Reference distance 1 cm

SrD )( 6 cm

125I (6702) Source KSrD ),(

)(cGyDosefSolid Water )(mVV

fth

cal

Energy Dependence:Sensitivity (25I/ 60Co) ~ 3


Sensitivity (25I/ 60Co) ~ 3

Cygler et al, Radiother. Onc. 80, 296–301, 2006

In VivoIn Vivo Measurements Measurements –– prostate prostate 7

implantsimplants

SetscrewSetscrew

seed

1

US probe

12

34

US probe

MOSFETMOSFETMOSFET positionMOSFET position


Fluoroscopy image of the prostate after implant

••MOSFET reading taken every 1cm MOSFET reading taken every 1cm

Use of MOSFET detectors during Use of MOSFET detectors during i l d i l dprostate implant procedureprostate implant procedure

l l t d i iti l l () d t i l t ( )

10

12

/h)

150% mPD

calculated initial pre-plan () measured post implant ()

6

8

Rat

e (c

Gy/

90% mPD

mPD

2

4

Initi

al D

ose

Prostate length = 50 mm

Prostate Base

Prostate Apex

90% mPD

0

2

0 10 20 30 40 50 60 70 80 90 100 110Distance (mm)

g


Cygler et al Radiotherapy and Oncology 80: 296-301; 2006

Distance (mm)

BNCT at BNL medical research BNCT at BNL medical research ttreactorreactor

0.80

1.00

dose

0.20

0.40

0.60

elat

ive

boro

n

0.00

0.20

0 2 4 6 8 10 12Depth in phantom (cm)

Re

Thermal neutron depth dose distribution in a perspex phantom in a BNCT epithermal neutron beam facility at BNL obtained with paired MOSFET detectors with 10B converter Subtracting on line


paired MOSFET detectors with 10B converter. Subtracting on-line the response of paired MOSFET eliminate effect of gamma dose.

Courtesy of Anatoly Rosenfeld

ConclusionsConclusionsConclusionsConclusions

• MOSFET detectors are very useful for dosimetry • MOSFET detectors are very useful for dosimetry,

especially

– High dose gradient fields

• Accurate if properly characterized and usedAccurate if properly characterized and used

• Phantom (in vitro) dosimetry

• In vivo dosimetry in external beam and brachytherapy

treatments


m

AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements

• Anatoly Rosenfeld• Anatoly Rosenfeld• Ian Thomson• Nuria Jornet• G. Beyer, Sicel Technologies Inc.y , g• A. Hallil, Best Medical Canada• Amanda Cherpak• Amanda Cherpak• Tara Woods


ThankThank youyouMerci


Date post:	30-Jan-2018
Category:	Documents
Upload:	phamnhi
View:	233 times
Download:	1 times

MOSFET dosimetry in radiotherapy · PDF fileMOSFET dosimetry in radiotherapy Joanna E.Cygler1...

Documents