Radiation Protection in Radiotherapy

Radiation Protection inRadiotherapyPart 2Radiation PhysicsLecture 2: Dosimetry and EquipmentIAEA Training Material on Radiation Protection in Radiotherapy

Part 2, lecture 2: Dosimetry and equipment

Radiation Protection in Radiotherapy

RationaleRadiation dose delivered to the target and surrounding tissues is one of the major predictors of radiotherapy treatment outcome (compare part 3 of the course). It is generally assumed that the dose must be accurately delivered within +/-5% of the prescribed dose to ensure the treatment aims are met.



ObjectivesTo understand the relevance of radiation dose and dosimetry for radiotherapy To be able to explain the difference between absolute and relative dosimetryTo be able to discuss the features of the most common dosimeters in radiotherapy: ionization chambers, semiconductors, thermoluminescence dosimeters (TLD) and film



Contents of lecture 21. Absolute and relative dosimetry2. The dosimetric environment: phantoms3. Dosimetric techniquesphysical backgroundpractical realization



1. Absolute and relative dosimetryAbsolute dosimetry is a technique that yields information directly on absorbed dose in Gy. This absolute dosimetric measurement is also referred to as calibration. All further measurements are then compared to this known dose under reference conditions. This means relative dosimetry is performed. In general no conversion coefficients or correction factors are required in relative dosimetry since it is only the comparison of two dosimeter readings, one of them being in reference conditions.



Absolute dosimetryRequired for every radiation quality onceDetermination of absorbed dose (in Gy) at one reference point in a phantomWell defined geometry (example for a linear accelerator: measurements in water, at 100cm FSD, 10x10cm2 field size, depth 10cmFollows protocols (compare part 10)



Absolute dosimetryRequired for every radiation quality onceDetermination of absolute dose (in Gy) at one reference point in a phantomWell defined geometry: Eg. water phantom, 100cm FSD, 10x10cm2 field size, depth 10cmFollows protocols (compare part 10)Of tremendous importance:If the absolute dosimetryis incorrect EVERYTHINGwill be wrong


Quick QuestionA dose of 1Gy delivers a huge quantity of energy to the patient - is it true or false?



AnswerFALSE 1Gy = 1J/kg. Delivering this amount of energy would raise the temperature of tissue by less than 0.001oC. Even for a 100kg person it is much less than the energy consumed with a bowl of muesli please note the amount of energy in food is often listed on the package.



Relative dosimetryRelates dose under non-reference conditions to the dose under reference conditionsTypically at least two measurements are required:one in conditions where the dose shall be determinedone in conditions where the dose is known



Examples for relative dosimetryCharacterization of a radiation beampercentage depth dose, tissue maximum ratios or similarprofilesDetermination of factors affecting outputfield size factors, applicator factorsfilter factors, wedge factorspatient specific factors (e.g. electron cut-out)



Percentage depth dose measurementVariation of dose in a medium (typically water) with depthIncludes attenuation and inverse square law components



Percentage depth doseRelates doseat differentdepths in water(or the patient)to the dose at the depth of dose maximum - notethat the y axis isrelative!!!



TAR, TMR, TPRRelative dosimetry for isocentric treatment set-up (compare part 5)All can be converted into percentage depth doseTAR = ratio of dose in phantom with x cm overlaying tissue to dose at the same point in airTMR = ratio of dose with x cm overlaying tissue to dose at dose maximum (detector position fixed)TPR as TMR but as a ratio to dose at a reference point (e.g. 10cm overlaying tissue)



TMR, TPRMimics isocentric conditionsTMR is a special case of TPR where the reference phantom depth is depth of maximum dose



PDD and TMRPercentage depth dose (PDD) changes with distance of the patient to the source due to variations in the inverse square law (ISL), TAR, TMR and TPR do not.Strong ISLdependenceWeak ISLdependence



Output factorsCompare dose with dose under reference conditionsdifferent field sizeswedge factortray factorapplicator factorelectron cutout factor



Example: wedge factorDose under referenceconditionsCould also involve different field sizes and/ordifferent depths of the detector in the phantom


Quick QuestionIs a Half Value Layer measurement for the determination of X Ray quality absolute or relative dosimetry?



AnswerRelative dosimetry:we relate the dose with different aluminium or copper filters in the beam to the dose without the filters to determine which filter thickness attenuates the beam to half its original intensitythe result is independent of the actual dose given - we could measure for 10s or 20s or 60s each time, as long as we ensure the irradiation is identical for all measurements



2. The dosimetric environmentPhantomsA phantom represents the radiation properties of the patient and allows the introduction of a radiation detector into this environment, a task that would be difficult in a real patient. A very important example is the scanning water phantom. Alternatively, the phantom can be made of slabs of tissue mimicking material or even shaped as a human body (anthropomorphic).



Scanning water phantom



Slab phantoms



Tissue equivalent materialsMany specifically manufactured materials such as solid water (previous slide), white water, plastic water, Polystyrene (good for megavoltage beams, not ideal for low energy photons)Perspex (other names: PMMA, Plexiglas) - tissue equivalent composition, but with higher physical density - correction is necessary.



Anthropomorphic phantomWhole bodyphantom: ART



Allows placement of radiation detectors in the phantom (shown here are TLDs)Includesinhomogeneities



RANDO phantomtorsoCT slice through lungHead withTLD holes



Pediatric phantom



Some remarks on phantomsIt is essential that they are tested prior to use physical measurements - weight, dimensionsradiation measurements - CT scan, attenuation checksCheaper alternatives can also be usedwax for shaping of humanoid phantomscork as lung equivalentAs long as their properties and limitations are known - they are useful



3. Radiation effects and dosimetry


PRIVATE Radiation effect

Dosimetric method

Ionization in gases

Ionization chamber

Ionization in liquids

Liquid filled ionization chamber

Ionization in solids

Semiconductors

Diamond detectors

Luminescence

Thermoluminescence dosimetry

Fluorescence

Scintillators

Chemical transitions

Radiographic film

Chemical dosimetry

NMR dosimetry

Heat

Calorimetry

Biological effects

Erythema

Chromosome damage


Principles of radiation detectionIonization chamberGeiger Mueller CounterThermoluminescence dosimetryFilmSemiconductors



Detection of Ionization in AirAdaptedfrom Collins2001Ion chamber



Detection of Ionization in AirAdaptedfrom Metcalfe1998



Ionometric measurementsIonization Chamber200-400VMeasures exposure which can be converted to dosenot very sensitiveGeiger Counter>700VEvery ionization event is countedCounter of events not a dosimetervery sensitive



Ionization ChambersThimble chambers600cc chamber



Cross section through a Farmer type chamber (from Metcalfe 1996)



Ionization ChambersFarmer 0.6 cc chamber and electrometerMost important chamber in radiotherapy dosimetry



ElectrometerFrom the chamber



Ionization chambersRelatively large volume for small signal (1Gy produces approximately 36nC in 1cc of air)To improve spatial resolution at least in one dimension parallel plate type chambers are used.



Parallel plate chambersFrom Metcalfe et al 1996



Parallel Plate Ionization ChambersUsed for low energy X Rays (< 60 KV)Electrons of any energy but rated as the preferred method for energies < 10 MeV and essential for energies < 5 MeVMany types available in different materials and sizesOften sold in combination with a suitable slab phantom



Parallel Plate Ionization Chambers - examplesMarkus chambersmalldesigned for electronsHolt chamberrobustembedded in polystyrene slab



Well type ionization chamberFor calibration of brachytherapy sourcesBrachytherapysource



Ionization chamber type survey metersnot as sensitive as G-M devices but not affected bypulsed beams such as occur with acceleratorsbecause of the above,this is the preferred device around high energy radiotherapyaccelerators



Geiger-Mueller CounterNot a dosimeter - just a counter of radiation eventsVery sensitiveLight weight and convenient to use Suitable for miniaturization



Geiger-Mueller (G-M) DevicesUseful for area monitoringroom monitoringpersonnel monitoring

Care required in regions of high dose rate or pulsed beams as reading may be inaccurate



Thermoluminescence dosimetry (TLD)Small crystalsMany different materialsPassive dosimeter - no cables requiredWide dosimetric range (Gy to 100s of Gy)Many different applications



Various TLD types



Simplified scheme of the TLD process



TLD glow curves



Glow curvesAllow researchAre powerful QA tools - does the glow curve look OK?Can be used for further evaluationMay improve the accuracy through glow curve deconvolution



The role of different dopants



Importance of thermal treatmentDetermines the arrangement of impuritiessensitivityfadingresponse to different radiation qualitiesMaintain thermal treatment constant...



Dose response of LiF:Mg,Ti:

wide dosimetric range

watch supralinearity



Variation of TLD response with radiation quality



Materials: oh what a choice... LiF:Mg,Ti (the gold standard) CaF2 (all natural, or with Mn, Dy or Tm) CaSO4 BeO Al2O3 :C (record sensitivity 1uGy) LiF:Mg,Cu,P (the new star?)



TLD reader photomultiplier based planchet and hot N2 gas heating available



What can one expect...Reproducibility: single chip 2% (0.1Gy, 1SD)Accuracy (4 chips standard, 2 chips measurement) 3% (0.1Gy, 95% confidence)about 30 minutes per measurement...



Radiographic filmReduction of silver halide to silverRequires processing ---> problems with reproducibilityTwo dimensional dosimeterHigh spatial resolutionHigh atomic number ---> variations of response with radiation quality



Radiographic filmCross sectionOften prepacked for ease of use



Film: dose responseEvaluation of film via optical densityOD = log (I0 / I)Densitometers are commercially available



Radiographic film dosimetry in practiceDepends on excellent processor QACommonly used for demonstration of dose distributionsProblems with accuracy and variations in response with X Ray energy



Radiochromic filmNew developmentNo developingNot (very) light sensitiveBetter tissue equivalenceExpensive



Semiconductor DevicesDiodesMOSFET detectorsDiodes for water phantommeasurements



DiodesFrom Metcalfe et al. 1996Mostly used likea photocell generatinga voltage proportionalto the dose received.



Metal Oxide SemiconductorField Effect TransistorFrom Metcalfe et al. 1996MOSFETs = extremelysmall sensitive volume



1. irradiation2. Charge carriers trapped in Si substrate3. Currentbetween sourceand drain altered



Gate bias duringirradiation: determines sensitivityReadoutafter irradiation:gate bias requiredto maintainconstant current



Diodes and other Solid State DevicesAdvantagesdirect readingsensitivesmall sizewaterproofing possibleDisadvantagestemperature sensitivesensitivity may change --> re-calibration necessaryregular QA procedures need to be followed



Summary of lecture 2


Ion chambers

Semiconductors

TLDs

Film

Advantages

Well understood, accurate, variety of forms available

Small, robust

Small, no cables required

Two dimensional, ease of use

Disadvantages

Large, high voltage required

Temperature dependence

Delayed readout, complex handling

Not tissue equivalent, not very reproducible

Common use

Reference dosimetry, beam scanning

Beam scanning, in vivo dosimetry

Dose verification, in vivo dosimetry

QA, assessment of dose distributions

Comment

Most common and important dosimetric technique

New developments (MOSFETs) may increase utility

Also used for dosimetric intercomparisons (audits)

New developments (radiochromic film) may increase utility


General Summary: PhysicsIn radiotherapy, photons (X Rays and gamma rays) and electrons are the most important radiation typesAccuracy of dose delivery is essential for good practice in radiotherapyAbsolute dosimetry determines the absorbed dose in Gray at a well-defined reference point. Relative dosimetry relates then the dose in all other points or the dose under different irradiation conditions to this absolute measurement.There are many different techniques available for dosimetry - none is perfect and it requires training and experience to choose the most appropriate technique for a particular purpose and interpret the results



Where to Get More InformationMedical physicistsTextbooks:Khan F. The physics of radiation therapy. 1994.Metcalfe P.; Kron T.; Hoban P. The physics of radiotherapy X-rays from linear accelerators. 1997. Cember H. Introduction to health physics. 1983Williams J; Thwaites D. Radiotherapy Physics. 1993.


Any questions?


Question:Which radiation detectors could be useful for in vivo dosimetry and why?



In radiotherapy the dose delivered to the patient is typically too large for radiographic film which in addition to this is light sensitive. Ionisation chambers are often fragile and require high voltage, both not ideal when working with patients. Therefore, TLDs are often used as detectors for in vivo dosimetry. They are small, do not require cables for the measurement and there are materials which are virtually tissue equivalent. TLDs can be complemented by diodes if an immediate reading (= active dosimetry) is required. As TLDs, diodes are solid state dosimeters and therefore sensitive and small. Other detectors of interest in this group would be MOSFETs.A different class of in vivo dosimeters are exit dose detectors in the form of electronic portal imaging (compare part 5). They may prove very useful for on-line verification.


Part No 2, Lesson No 2Part 2: Radiation PhysicsLesson 2: Dosimetry and equipmentLearning objectives: Upon completion of this lesson, the students will be able to:To understand the relevance of radiation dose and dosimetry for radiotherapy To be able to explain the difference between absolute and relative dosimetryTo be able to discuss the features of the most common dosimeters in radiotherapy: ionization chambers, semiconductors, thermoluminescence dosimeters (TLD) and filmActivity: lectureDuration: 2 hoursReferences:Khan F. The physics of radiation therapy. 2nd ed. Williams and Wilkins, Baltimore 1994, ISBN 0683-04502-4Peter Metcalfe, Tomas Kron and Peter Hoban. The physics of radiotherapy X-rays from linear accelerators. Medical Physics Publishing, Wisconsin 1997, ISBN 0-944838-76-6.Greening JR 1981 Fundamentals of radiation dosimetry. HPA Medical Physics Handbook 6. Adam Hilger, BristolTomas Kron. Dosimetric Tools. In: Modern Technology of Radiation Oncology (Ed.: J Van Dyk) Chapter 19. Medical Physics Publishing, Wisconsin 1999, ISBN 0-944838-38-3, pp. 753-821.Williams, J.R. and Thwaites, D.I. (eds.) Radiotherapy physics. Oxford: Oxford University Press, 1993IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2This rational follows directly from the previous lecture in which the dose response curve was shown.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Please note the two statements are linked - the lecturer can point out that one is not possible without the other (eg to perform absolute dosimetry one needs to know the radiation quality which is determined using relative dosimetry (eg percentage depth dose measurements)IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2More information on calibration can also be found in part 10 of the courseIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2This question is relatively simple and it should take only a very short time for the participants to reply either yes or no. The lecturer can use a show of hands to avoid lengthy discussions.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2It is not essential that the participants are familiar with all the concepts behind these measurements - some of them are explained in the followingIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The relative axis is important to note - the lecturer should point out the process of normalisation where every curve is normalised to 100 at its maximum.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The lecturer can point out that the TPR for depth of maximum dose is the same as TMR.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The lecturer can point out that the TPR for depth of maximum dose is the same as TMR as discussed on the previous slide.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The lecturer can point out that this measurement is nevertheless required to perform absolute calibration of a kilovoltage X-ray beam.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Shown is a Wellhoeffer waterphantom (cost > $US 100,000.-)The waterphantom is an essential part of a radiotherapy physics set-up. It is used for beam scanning and required for commissioning of radiation equipment as well as treatment planning systems. Water is convenient to use - it is readily available, cheap and clear.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Shown is solid water, a water equivalent polyraisin material specifically developed for radiotherapy aplications.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The custom made materials are typically quite expensive - a set of slabs of different thickness in total 20cm costs around $US 10,000.-IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Again ther are many types of anthropomorphic phantoms available. Some older designs contain real human bone which is undesirable as they dry out.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The phantom shown is manufactured by CIRSThe lecturer can point out that the phantom can be set-up as a patient and diagnostic procedures can be performed as if it was a patient. Therefore, it is possible to verify the whole treatment chain in one experiment.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The important message here is:- there are many different radiation effects- virtually all have been used for quantification of radiation, ie. For dosimetry

The lecturer could also point out that biological dosimetry, while not featured prominently here, has not only historic importance. Even today, radiation schedules are adjusted to account for biological effects - eg very significant side effects form radiotherapy.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2This is a schematic drawing onlyIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The lecturer can use this graph to slowly introduce the relevant physics:no voltage - no signala bit of voltage - sufficient to attract some ions to electrodes, however many still recombine with opposite charge carriersat a certain voltage all ions are collected - here one can measure a signal which is proportional to dose, the ionisation chamber regionmore voltage will allow charged particles to gather so much energy that they can produce new charges in secondary collisions - useful range for amplification of initial chargesat even higher voltage one creates an avalanche of charges, basically a discharge, the height of which is NOT related anymore to the initial number of charges created and therefore not related to dose anymore. One can count all interactions in the chamber: Geiger Mueller rangeabove this voltage there is damage to the chamber due to continuous discharge.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Shown are Farmer type thimble chambers. All chambers manufactured by NEIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Shown is a NE 2570 electrometer with a Farmer type chamberIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Shown is a Keithley 614 electrometerIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Well type chambers are ideal for small sources - they increase the signal due to excellent geometry: nearly the whole 4pi space is covered.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Relatively large volume - may use gas under pressure to increase sensitivityIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Each interaction in the chamber leads to a large single signal, a count. After a count the GM tube is not active for a certain period of time, the dead time. This can leads to problems in high dose rate radiation or pulsed beams.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Photograph of some commercially available thermoluminescence dosimeters shown is also an aluminum storage and annealingtray with 50 recesses for TLDs). IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Thermoluminescence dosimeters (TLDs) are crystals that can store some of the energy deposited by ionising radiation in a retrievable form. The figure illustrates the principle of Thermo , (one applies heat) Luminescence (and the crystal emits light) and Dosimetry (of which the intensity is related to the dose of ionizing radiation absorbed by the crystal prior to heating",). While the emitted light is proportional to the absorbed radiation the proportionality constant varies with radiation energy, total dose, TLD material and - most difficult to account for - thermal history of the crystals. As such, TLD is mostly used as a relative dosimetric technique in which the dose to be determined is compared to a similar known dose given to the same or a similar TL detector. TLDs have the advantages of small physical size and that no cables are required during irradiation. As such they are particularly well suited for measurements within solid phantoms and in vivo dosimetry. The chief disadvantages are the delay between irradiation and the readout process and the complexity of the whole TLD set-up.

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2This and the following 6 slides introduce the participants further to TLD theory - the lecturer may choose to omit these slides.

Shown here is the fact that different trap depths lead to different temperatures required for freeing the electrons from the traps, leading to differnet glow peaks.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Different impurities facilitate different processes. Shown here are:Traps (yellow) andRecombination centres which allow recombination under the emission of light.Both are important and their details depend on the structure of the impurities within the crystal lattice.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2This is a double log plotIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The overresponse at kilovoltage X-rays is due to the slightly higher atomic number of LiF (8.3) compared to water (7.5).IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2One of the major advantages of TLD is the wide choice of materials - the slide illustrates that different applications may require different materials.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Shown is a ToLeDo readerIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2This is the important take-home message from the TLD slides.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Radiographic film typically consists of a clear polyester base that is coated on one or both sides with a radiosensitive emulsion. The sensitive layer consists normally of silver halide (most commonly bromide) crystals (diameter 0.2 - 2m m) embedded in gelatin. The absorption of ionising radiation causes the following reaction (simplified):

photon + Br- Br + e- e- + Ag+ Ag.

The elemental silver is black and produces the latent image. During the development of the film other silver ions are reduced in the presence of silver atoms. Therefore, if one (in practice few) silver ion is reduced in a silver bromide crystal, all silver in this crystal (or grain) will be reduced during development. The rest of the silver halide (in undeveloped grains) is then washed away from the film during fixation and only the areas of the film which were hit by radiation appear black.Advantages of film are its good spatial resolution and the fact that a whole two dimensional dose distribution is acquired in one measurement. The disadvantage is its poor reproducibility (which depends on radiation energy as well as the processing) and the non-linearity of its dose response.

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2This slide is more of a teaser - to go into details with radiochromic film dosimetry is beyond the scope of the course. The lecturer can add as additional information:Expensive ($100 for 10x10cm2)Not fully tissue equivalent - underresponse at low X-ray energies (fat equivalent) - still much better than radiographic filmrequires relatively high doses (typically >2Gy)accuracy limited by low dose response and determination of optical density - typically quoted as +/- 3% (1SD)development is ongoingIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2There are two main semiconductor devices used for dosimetry in clinical radiotherapyIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Most semiconductor radiation detectors are silicon based. Silicon itself is a very poor conductor. However the silicon matrix can be doped with impurities of electron donors (eg. P, As, Se) to produce n-type semiconductors or with electron acceptor atoms (eg. Ga, In) to produce a p-type semiconductor. The radiation sensitive diode is formed by a thin layer of one type of semiconductor on top of a substrate from the other type.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The details of radiation dosimetry with MOSFETs are beyond the scope of this course - the next slide illustrates the basic workings.Take home message: they are small and may be important in the futureIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2Wollongong MOSFET systemIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2A good slide to make a separate handout from...IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2The lecturer should point out that the present course is not designed to deliver all the knowledge required to be able to perform radiation dosimetry in radiotherapy - it is generally accepted that this requires at least a physics degree and 3 to 5 years specialist training and experience.

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 2IAEA Training Material: Radiation Protection in Radiotherapy

Radiation Protection in Radiotherapy

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