Radiation Protection in Radiotherapy Part 5 External Beam Radiotherapy Lecture 2 (cont.): Equipment...

Radiation Protection inRadiotherapyPart 5External Beam RadiotherapyLecture 2 (cont.): Equipment and safe designIAEA Training Material on Radiation Protection in Radiotherapy

Part 5, lecture 2 (cont.): Equipment - linacs

Radiation Protection in Radiotherapy

3. Medical Linear AcceleratorsShort: linacMost radiotherapy patients are treated using linacsSeveral manufacturersCourtesy Siemens



Photon percentage depth dose comparisonPHOTONSELECTRONSLinac beams



Different designsHand controlCouch withcontrolsTouch guardWall panel tohide stand



The problem: require >4MeV electronsNot possible to achieve this conventionally using a potential difference Electrons are accelerate using microwaves



Schematic drawing of a linac



Electron Accelerators6 MV short waveguideNo bendingmagnet



Electron Accelerators18 MV long waveguide



Electron AcceleratorsWaveguides for acceleration of electrons using microwavesShort standingwave guideBuncher for initial acceleration of electrons



Electron AcceleratorsBending the electron beamAchromatic magnet:All energies are focused onto the targetSlits for selectionof electron energy



Treatment head



Electron AcceleratorsRadiation exposure is controlled by two independent integrating transmission ionization chamber systems.One of these is designated as the primary system and should terminate the exposure at the correct number of monitor unitsThese also steer the beam via a feedback loop



Monitor ion chamber designTwo independent chambers - redundant check of dose deliveredEach chamber segmented - allows feedback for flatness and symmetry



Electron AcceleratorsThe other system is termed the secondary system and is usually set to terminate the exposure after an additional 0.4 GyMost modern accelerators also have a timer which will terminate the exposure if both ionization chamber systems failDefense in depth at work!!!



Electron AcceleratorsModern accelerators have a lot of treatment options, for exampleX Rays or electrons (dual mode)multiple energies2 X Ray energies5 or more electron energies



Electron AcceleratorsHead complexity to handle multiple energies and multiple modalitiesdifferent flattening filters and scattering foils on a carouselmonitor chamberscollimators



Electron Accelerators X Ray Collimators may be (1)rectangular (conventional) the transmission through the collimators should be less than 2% of the primary (open) beam



Electron Accelerators X Ray Collimators may be (2)Multi-Leaf collimators (MLC)the transmission through the collimators should be less than 2% of the primary (open) beamThe transmission between the leaves should be checked to ensure that it is less than the manufacturers specificationSiemens MLC



Electron Accelerators Electron applicators, these may beopen sided for modern accelerators using double scattering foils or scanned beamsenclosed for older accelerators using single scattering foilsboth types should be checked forleakage adjacent to the open beamon the sides of the applicatorsVarian open sidedelectron cone



Important AccessoriesWedgesDynamic wedgesBlocksMultileaf Collimator (MLC)Electronic Portal Imaging (EPID)



Electron AcceleratorsWedges3 or more fixed wedgesauto-wedgedynamic wedgeModify dose distributionangle



Electron AcceleratorsDynamic wedge



Wedgesstandard treatment accessoryrequired for example in breast and head and neck treatmentdynamic wedge most popular because:no weightany wedge angle possiblebut difficult to commission



Electron AcceleratorsAsymmetric collimator



Electron AcceleratorsAsymmetric collimatorsRead-outs on the linac gantryLeft collimatorread-outRight collimatorread-outCollimatorrotationread-outLower collimatorread-outGantryread-out



Multileaf Collimator (MLC)Used to define any field shape for radiation beamsSeveral variations to the theme:different leaf widths (1cm to 0.4cm)replaces collimators or additional to normal collimators



Linac mounted MLC



MLCThe quality of the field definition depends on the width of the leafsThere is always some interleaf leakage Typically the transmission through the MLC is larger than through a standard collimator



Multileaf Collimator (MLC)



Dynamic MLCConcept similar to dynamic wedgeWhen MLC moves during treatment different parts of the field are shielded resulting in different overall radiation levels delivered in different parts of the beam: Intensity modulated radiotherapy



Intensity ModulationAchieved using a Multi Leaf Collimator (MLC)The field shape is altered step-by-step or dynamically while dose is deliveredMLC pattern 1MLC pattern 3MLC pattern 2Intensitymap



Two approaches to IMRTMultiple individual fields, each of them intensity modulated in two dimensionsLinac based IMRT



Two approaches to IMRTContinuous rotation of a one dimensional fan beam which consists of many beamlets which can be turned on or offTomotherapy



Components ofHelical Tomotherapy



Comments on IMRTBest possible dose distribution with photonsNo intuitive link between MLC settings, monitor units and the delivered dose distributionImpossible without computers in diagnostics, planning and deliveryChallenges for QA



Electronic Portal ImagingImaging device at the beam exit side of the patient to record the treatment fieldAllows to verify that the field was delivered to the correct location in the patientMany different systems available...



Electronic portal imaging devices in practiceSiemensVarian



Electronic portal imageComparison of simulator and portal image (right)



Electron beamsNo target requiredScattering foil used to produce larger beam - alternative would be to scan the pencil beam using electromagnetic fieldsApplicator required to produce good field delineation on the patientElectron BeamScattering FoilIon ChamberSecondary CollimatorElectron applicatorPatientPrimary Collimator



Electron applicator on a modern Varian linear accelerator



Electron applicatorsmay beopen sided for modern accelerators using double scattering foils or scanned beamsenclosed for older accelerators using single scattering foilsmust be checked for leakage adjacent to the open beamon the sides of the applicators



Model of an electron applicator for Monte Carlo CalculationspatientElectronapplicator



Electron AcceleratorsWith such a large number of possible settings it is essential that interlocks be provided to prevent inappropriate combinations from being selectedIt is also essential that the control console provides a clear indication of what functions have been set



Electron AcceleratorsComplex control systemReliance on computers



Varian Clinac operation screens Clinical modeService mode



Electron Accelerators Verification systemsAll accelerator manufacturers now produce computer controlled verification systems which provide an additional check that the settings on the accelerator console are correct forproper accelerator function andcorrespond exactly with the parameters determined for the individual patient during the treatment planning process



Electron Accelerators Head leakagethe Air Kerma Rate (AKR) due to leakage radiation at any point outside the maximum useful beam, but inside a plane circular area with a radius of 2 meters centered around, and perpendicular to, the central axis of the beam at the normal distance of treatment shall not exceed 0.2% of the AKR at the central axis of the open beam. The measurement shall be done with a thick shielding block covering the open beam



Electron AcceleratorsHead leakage Except in the area defined in the previous slide the Air Kerma Rate (AKR) due to leakage radiation (excluding neutrons) at any point 1 meter from the path of the electrons between their origin and the target or electron window shall not exceed 0.5%



Electron AcceleratorsNeutronsthese will only be a problem if the X Ray energy is equal to or greater than 15 MVissues which need to be considered when neutrons are presents include:neutron activationshielding problems


Part No...., Module No....Lesson NoPart 5: External Beam RadiotherapyLesson 2: EquipmentLearning objectives: Upon completion of this lesson, the students will be able to:To review physics and technology of external beam radiotherapy equipmentTo understand the design and functionality of the equipmentTo appreciate the role of international standards such as IEC 601-2-1 for equipment designActivity: Lecture(s)Duration: 3 hours

IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoSeveral manufacturers alerts to the fact that there are only few in the world. All of them are highly developed products. Shown here is a Siemens linac.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoA reminder of beam characteristicsIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoA Philips/Elekta linac- the touch guard is an important safety interlock, once it touches anything the motion of the machine is interrupted. This avoids collisions.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThe picture of the lightning illustrates what would happen if there was a potential difference of 4MV between two points.Microwave technology developed for radar was instrumental for creation of the first linacs some 50 years ago.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoA drawing for which the lecturer should take some time. It is assumed that the lecturer is familiar with the basic layout of a linac - he/she will then be able to take the participants through the components identified in the drawing.The components of the beam delivery section are discussed in a few slides.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThe waveguide shown in red is the accelerating structure within the linac - here the electrons are accelerated using microwaves.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoTwo pictures of accelerating waveguides - shown is a short guide for standing wave acceleration with cavities placed aside and a travelling wave guide (lower left). It is not necessary that the participants will understand the difference between the two concepts - depending on the group of participants and the background of the lecturer, he/she may want to omit the slide.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoA 270deg bending magnet is typically better than a 90deg one.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThe lecturer should take time to take the participants through this slide. It is recommended to follow the beam...IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis system is not identical for all manufacturers - important is that a variety of feedback loops allow to steer the beam and turn it off if certain parameters are not met.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoExample shown is a Varian linac with two photon and 5 electron energies.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis picture from VARIAN illustrates the head design and is somewhat a summary. The next issue shown on the next slides is the collimation systemIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThe illustration shows two orthogonal sets of jaws. In most cases they are made from tungsten and approximately 10cm thickIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoAs in orthovoltage applicators the open side makes it easier to set-up the patient and also reduces the weight.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThese will be discussed in more detail in the followingIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoTypically linacs are equipped with 4 wedges: 15, 30, 45 and 60deg.The wedge angle can be defined in multiple ways - it is typically the angle of the 80% isodose line.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis has been discussed in the previous lecture.In the context of the present lecture, the lecturer can point out that this is a software option available now on most linacs.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThe asymmetric collimator is a very useful feature on linacs which now also becomes available on some Cobalt units. The idea is that the two jaws in a collimator set can move independently. The lecturer can point out that this is obviously also a prerequisite for the dynamic wedge.Asymmetric collimation is most commonly used for half beam blocking where a beam is blocked at central axis with the resulting field edge having no divergence. This is very useful for beam matching and junctioning.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThe tongue and groove design reduces interleaf leakage.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoA picture from a Varian linac comparison of 80 leaf MLC (1cm projected width at isocentre) and 120 leaf MLC (0.5cm projected width).IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis picture illustrates the variations in field size possible with MLCs - combined all these segments allow intensity modulation.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoSimilar slide to previous lecture - The lecturer can take participants through the right part of the slide step by step, adding up the fluence distribution in all three fields shown. In practice, there will be often more than 50 subfields or segments.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis is the most common approach to IMRT - the insert shows a dose distribution which can be achieved this way. The dose distribution is recorded on a film an illustrates both a half moon shaped high dose region and excellent sparing in the dark centre area. One could think of a paraspinal tumor.

IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis slide illustrates a tomotherapy approach which was first introduced by the NOMOS Peacock corporation. The treatment is slice by slice similarly to a CT scanner. While the treatment fan beam rotates around the patient the leafs open and shut as required. The MLC is often termed a binary MLC as it can only be open or shut.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis is a complicated slide which is currently hidden.It would go beyond the scope of the present course to give a detailed description of tomotherapy. However, the lecturer may want to show it to the participants ifa) he/she is familiar with the conceptb) the participants will benefit from the informationIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis slide leads to the next device, EPID, which is primarily a verification and QA tool.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIllustrated in the next slideIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoWhile it is not required for the participants to understand the operation of these devices, the lecturer can point out that the Siemens system is based on a fluorescent screen with the resulting light bend around towards the gantry suing a mirror. In the gantry a camera is detecting the light.The Varian system is based on an array of radiation detectors, fluid filled ionization chambers and m,ore recently amorphous silicon semiconductor detectors.The difference in approach can be seen from the design of the two systems.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThe result of the EPID is an immediate image similar to a port film.More information on its use is provided in part X, lecture 3 of the course.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis is a fancy slide which shows the whole linear accelerator beam delivery structure. Monte Carlo Calcualtions can be used to track particles such as electrons through these structures and display the beam as shown.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoShown is the control area of a Varian Clinac 1800IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis is an important bit of information - record and verify systems should be part of every radiotherapy department. IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoIAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation SourcesPart No...., Module No....Lesson NoThis will be discussed in more detail in part VII of the course.IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Radiation Protection in Radiotherapy Part 5 External Beam Radiotherapy Lecture 2 (cont.): Equipment...

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