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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Photon percentage depth dose comparisonPHOTONSELECTRONSLinac
beams
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Different designsHand controlCouch withcontrolsTouch guardWall
panel tohide stand
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
The problem: require >4MeV electronsNot possible to achieve
this conventionally using a potential difference Electrons are
accelerate using microwaves
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Schematic drawing of a linac
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron Accelerators6 MV short waveguideNo bendingmagnet
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron Accelerators18 MV long waveguide
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron AcceleratorsWaveguides for acceleration of electrons
using microwavesShort standingwave guideBuncher for initial
acceleration of electrons
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron AcceleratorsBending the electron beamAchromatic
magnet:All energies are focused onto the targetSlits for
selectionof electron energy
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Treatment head
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Monitor ion chamber designTwo independent chambers - redundant
check of dose deliveredEach chamber segmented - allows feedback for
flatness and symmetry
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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!!!
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron AcceleratorsHead complexity to handle multiple energies
and multiple modalitiesdifferent flattening filters and scattering
foils on a carouselmonitor chamberscollimators
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Important AccessoriesWedgesDynamic wedgesBlocksMultileaf
Collimator (MLC)Electronic Portal Imaging (EPID)
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron AcceleratorsWedges3 or more fixed
wedgesauto-wedgedynamic wedgeModify dose distributionangle
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron AcceleratorsDynamic wedge
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Wedgesstandard treatment accessoryrequired for example in breast
and head and neck treatmentdynamic wedge most popular because:no
weightany wedge angle possiblebut difficult to commission
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron AcceleratorsAsymmetric collimator
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron AcceleratorsAsymmetric collimatorsRead-outs on the
linac gantryLeft collimatorread-outRight
collimatorread-outCollimatorrotationread-outLower
collimatorread-outGantryread-out
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Linac mounted MLC
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Multileaf Collimator (MLC)
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in 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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Two approaches to IMRTMultiple individual fields, each of them
intensity modulated in two dimensionsLinac based IMRT
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Two approaches to IMRTContinuous rotation of a one dimensional
fan beam which consists of many beamlets which can be turned on or
offTomotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Components ofHelical Tomotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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...
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electronic portal imaging devices in practiceSiemensVarian
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electronic portal imageComparison of simulator and portal image
(right)
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron applicator on a modern Varian linear accelerator
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Model of an electron applicator for Monte Carlo
CalculationspatientElectronapplicator
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Electron AcceleratorsComplex control systemReliance on
computers
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
Varian Clinac operation screens Clinical modeService mode
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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%
Part 5, lecture 2 (cont.): Equipment - linacs
Radiation Protection in Radiotherapy
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 5, lecture 2 (cont.): Equipment - linacs
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