Diodes - Ous-research.no

The philosophie….Systematic errors in dose delivery for anindividual patient may arise due to theinfluence of– patient contours– patient mobility– inhomogeneities– internal organ motionMoreover, errors can be introduced by:– transferring treatment data from the treatmentplanning system or simulator– the treatment machine settings and calibration– positioning the patient and beam modifiers

The philosophie…..The ultimate check of the actual dose delivered toan individual patient can only be performed at thepatient level, by means of in vivo dosimetry.Several national and international organizations(AAPM, ICRU and NACP) recommend that invivo dose measurements should be made.The purpose of this is to detect errors inindividual treatment sessions due to equipmentmal-functioning and human mistakes.This approach serves to detect unexplained, nonstatisticalfluctuations in dose delivery

SYSTEM AND MEASUREMENTCONFIGURATION

System & measurementconfigurationRadiation detectors– Thermoluminescent dosimeters (TLD)– Semiconductors (diods)– …..Dose registration system– Electrometers for diods– TLD readers–….Measurement and action protocol‣ Action levels‣ How to correct

System & measurementconfigurationMeasurement of the entranceand exit dose, D ent. and D exit ,provide information aboutwhether:Calculated out-put dose iscorrect according to pts.anatomyTarget volume will receive theintended dose

System & measurementconfigurationBased on themeasurements ofthe entrance andexit dose, D ent. andD exit , actual midplane or target dosecan be found.

System & measurementconfigurationEntrance dosemeasurements, on axis,will verify correct outputand correct SSD.wedgeEntrance dosemeasurements, off axis,will verify appropriateuse of wedge accordingto anatomyheterogeneityPts. contour

System & measurementExit dosemeasurements, onaxis, will verifywhether calculatedmonitor units arecorrect according tothickness and tissuedensity.configurationwedgePts. contourheterogeneity

System & measurementCombined exit andentrancemeasurement canprovideinformation aboutmid plan doseconfigurationwedgePts. contourheterogeneity

System & measurementconfigurationIn vivo measurementsDose registration andautomatic comparisonwith expected valuesAction suggested andtaken according topredefined protocol

DETECTORS

DETECTORSSemiconductors (diods)Thermoluminescent dosimeters(TLD)Electronic portal device (EPID)Electron paramagnetic dosimetry

DiodesThe diodes in use for in vivo dosimetry are silicondetectors.The base material can be n- or p-type silicon.The dependence of the diode response on accumulateddose, dose rate, and temperature is related to severalcrystal characteristics, for example, the doping level.P-type diodes resist radiation much better than n-typedetectors; the decrease in sensitivity with irradiation ismuch smaller for p-type than for n-type diodes.

DiodesIllustration of a diode detectorcircuit. Radiation produces electronsand ’holes’These are attracted to the positiveand negative sideA current in the circuit is thusinduced

DiodesIn vivo dosimetry semiconductor detectors consist of a diodesurrounded by a build-up cap.hemispherical build-up cap is equivalent in attenuation andbuild-up to 2 cm water and can be used with full build-up fordose measurements in high energy x-ray beams up to 8 MV.surface dose will be increased to at least 90% of maximumdosedose at larger depths will be reduced by at least 5%cylindrical build-up cap, the beam attenuation on the centralbeam axis is up to 4%, 8%, and 13% for a 4 MV, 6 MV, and15 MV x-ray beam, respectively

DiodesMain advantages of diodes are:– a high sensitivity to radiation, small size, goodmechanical stability– The sensitivity per unit volume of a diode is about18,000 times higher than for an air-filled ionizationchamber.– absence of external voltage, and immediate availabilityof the measured dose.– On-line dose verification allow dose adaptation duringthe treatment session

Diodeshttp://www.scanditronix-wellhofer.com/Detectors

DiodesAExample of detector design;a) sphericalb) dropletB

DiodesDose response relationshipfor a given detector;a linear response ispreferable.otherwise correctionfactors have to beintroduced.

Variation in responseas function ofcollimator size fordifferent energiesand build up capDiodes

DiodesSensitivity of the diode decreaseswith accumulated dose:• Pre-irradiation reduces thechange in response• Regularly calibration isdemanded

DiodesResponse of the detector isdependent on thetemperatureDetector temperature aftertaping onto the patient.

A unique calibrationfactor for both exitand entrancemeasurements is unsufficient,as theresponse have beenshown to differ.Diodes

DiodesEntrance and exit dosecorrection factor, C field size,as a function of the side of asquare field, for an 8-MVand 18-MV photon beam.D diode=R diode·N D· iC iMeijer GJ, et.al. Int J RadiatOncol Biol Phys. 49:1409-18,2001.

DiodesEntrance and exit dosecorrection factor, C SSD, asa function of the SSD foran 8-MV and 18-MVphoton beam.D diode=R diode·N D· iC iMeijer GJ, et.al. Int J RadiatOncol Biol Phys. 49:1409-18,2001.

DiodesEntrance and exit dosecorrection factor, C thickness, as afunction of the phantomthickness for an 8-MV and 18-MV photon beam.D diode=R diode·N D· iC iMeijer GJ, et.al. Int J RadiatOncol Biol Phys. 49:1409-18,2001.

TLDTLDs have been in use for in vivo dosimetry for avery long time. They are based on the principlethat imperfect crystals can absorb and store theenergy of ionizing radiation.Free electrons and holes are formed. The electronsmay be trapped at defects in the crystalinestructure.When heated to a temperature which is typical forthe detector material, electrons return to theconduction band and then may recombine with ahole, while emitting energy in the form ofelectromagnetic radiation.

Free electrons andholes are formed andtrapped at defects inthe crystalinestructure.When heated theelectrons return to theconduction bandwhile emitting energyTLD

TLDThis radiation, mainly in the visible wavelengthregion, is detected by a photomultiplier andcorrelated to the absorbed dose received by thematerial.After annealing, the TLD can be used again.

Schematicillustrationof a TLDreaderTLD

TLDTL materials used for in vivo dosimetry are:lithium fluoride (LiF), lithium borate (Li 2B 4O 7),and calcium sulphate (CaSO 4).TL detectors can either be powders or soliddosimeters, in the form of rods, chips, or pellets.Their small dependence on dose rate, temperature,and energy in the therapeutic range, and their wideapplicable dose range make TLDs suitable for invivo dosimetry purposes.TLDs are small with a good spatial resolution.

TLDDose responserelationship:a linearresponse ispreferable.is used out sidethe linear region,correction factorsmust be applied.

The response isdependent on energyto a certain extent, butvaries betweendifferent TLmaterials.TLD

TLDTLDs have the main advantage over diodes thatthey do not have to be connected to anelectrometer with a cable, and that they are easy totransport.The dose information can be stored over a longperiod of time, TLDs are suitable detectors formailing, which implies they can be used for intercomparisonof dose values delivered in differentinstitutions, such as in a multi-centre trial.They can also be tissue or bone equivalent.Disadvantages are that they cannot be used for onlinein vivo dosimetry

TLD

EPID dosimetryM. Essers, et al. Int J RadiatOncol Biol Phys 34:931-41,1996.EPID can determine transmissiondose in the entire irradiationfield,and from these measurements,the exit or midline dose in a planemight be obtained.Using this information, the dosehomogeity in the e.g. target volumecan be assessed.

EPID dosimetryExit dose rate measurements under inhomogeneousphantomM. Essers, et al. Int J Radiat OncolBiol Phys 34:931-41, 1996

EPID dosimetryRelative dose distributions in the mid-plane of a lung cancer patient irradiated with ananterior-posterior field at the Netherlands Cancer Institute, Amsterdam, TheNetherlands. (a) Calculated with the 3D treatment planning system; and (b) measuredwith an EPID behind the patient and converted to patient dose values.Essers M, er al. Int J Radiat OncolBiol Phys. 43:245-59, 1999.

EPRElectron paramagneticresonance spectroscopeRadiaion induced radiocalproportional to absorbed doseThe amount of radicals can bedetermined from electronparamagnetic resonancespectroscpyThe intensity of the spectrumcan be converted to dose

EPRThe relative response EPR and TL dosimeters for 4, 6, 10and 15 MV X-rays, as evaluated at different dose levelsVestad,TA, Malinen, E and Olsen DR.Phys. Med.Biol. (in press)

Clinical examplePatient with an EDP-20 diode withextra build-up cap.The diode is slightly shifted withrespect to the central beam axis, toavoid shielding by the entrancediode, which is shifted in theopposite direction.Meijer GJ, et.al. Int J RadiatOncol Biol Phys. 49:1409-18,2001.The electronic portal imagingdevice (EPID) on the right is usedto verify the correct diode position

Clinical exampleIn vivo dosimetryresults of 225 prostatepatients. The opencircles correspond tothe IVD results aftercorrection.Meijer GJ, et.al. Int J RadiatOncol Biol Phys. 49:1409-18,2001.

Clinical exampleVoordeckers M, et al. RadiotherOncol. 1998 47:45-8, 1998.Entrance dose measurements:overall results.650 entrance dose measurements forlung, H&N, breast and pelvicirradiation.Scanditronix, EDE type, detectorscovered with a hemi-sphericalperspex build-up cap with a waterequivalentthickness of 5 mm.Dosimeters were connected to aScanditronix DPD 10-channelelectrometer

Voordeckers M, et al. RadiotherOncol. 1998 47:45-8, 1998.Clinical example