Radiation Protection in Radiotherapy

Radiation Protection in Radiotherapy
Part No...., Module No....Lesson No Module title IAEA Training Material on Radiation Protection in Radiotherapy Radiation Protection in Radiotherapy Part 5: External Beam Radiotherapy Lesson 2: Equipment Learning objectives: Upon completion of this lesson, the students will be able to: To review physics and technology of external beam radiotherapy equipment To understand the design and functionality of the equipment To appreciate the role of international standards such as IEC for equipment design Activity: Lecture(s) Duration: 3 hours Part 5 External Beam Radiotherapy Lecture 2 (cont.): Equipment and safe design IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

3. Medical Linear Accelerators
Part No...., Module No....Lesson No Module title 3. Medical Linear Accelerators Short: “linac” Most radiotherapy patients are treated using linacs Several manufacturers Several 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. Courtesy Siemens Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Photon percentage depth dose comparison
Part No...., Module No....Lesson No Module title Photon percentage depth dose comparison PHOTONS ELECTRONS A reminder of beam characteristics Linac beams Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No
Module title Different designs Touch guard Wall panel to hide stand A 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. Couch with controls Hand control Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

The problem: require >4MeV electrons
Part No...., Module No....Lesson No Module title The problem: require >4MeV electrons Not possible to achieve this conventionally using a potential difference Electrons are accelerate using microwaves The 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. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Schematic drawing of a linac
Part No...., Module No....Lesson No Module title Schematic drawing of a linac A 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. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Electron Accelerators
Part No...., Module No....Lesson No Module title Electron Accelerators No bending magnet 6 MV short waveguide The waveguide shown in red is the accelerating structure within the linac - here the electrons are accelerated using microwaves. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators 18 MV long waveguide Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Waveguides for acceleration of electrons using microwaves Short standing wave guide Two 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. Buncher for initial acceleration of electrons Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Bending the electron beam Achromatic magnet: All energies are focused onto the target A 270deg bending magnet is typically better than a 90deg one. Slits for selection of electron energy Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Treatment head The lecturer should take time to take the participants through this slide. It is recommended to follow the beam... Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Radiation 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 units These also steer the beam via a feedback loop Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Monitor ion chamber design
Part No...., Module No....Lesson No Module title Monitor ion chamber design This 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. Two independent chambers - redundant check of dose delivered Each chamber segmented - allows feedback for flatness and symmetry Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators The other system is termed the secondary system and is usually set to terminate the exposure after an additional 0.4 Gy Most modern accelerators also have a timer which will terminate the exposure if both ionization chamber systems fail Defense in depth at work!!! Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Modern accelerators have a lot of treatment options, for example X Rays or electrons (dual mode) multiple energies 2 X Ray energies 5 or more electron energies Example shown is a Varian linac with two photon and 5 electron energies. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Head complexity to handle multiple energies and multiple modalities different flattening filters and scattering foils on a ‘carousel’ monitor chambers collimators This picture from VARIAN illustrates the head design and is somewhat a summary. The next issue shown on the next slides is the collimation system Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title 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 The illustration shows two orthogonal sets of jaws. In most cases they are made from tungsten and approximately 10cm thick Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title 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) beam The transmission between the leaves should be checked to ensure that it is less than the manufacturer’s specification Siemens MLC Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Electron applicators, these may be open sided for modern accelerators using double scattering foils or scanned beams enclosed for older accelerators using single scattering foils both types should be checked for leakage adjacent to the open beam on the sides of the applicators As in orthovoltage applicators the open side makes it easier to set-up the patient and also reduces the weight. Varian open sided electron cone Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Important Accessories
Part No...., Module No....Lesson No Module title Important Accessories Wedges Dynamic wedges Blocks Multileaf Collimator (MLC) Electronic Portal Imaging (EPID) These will be discussed in more detail in the following Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators angle Wedges 3 or more fixed wedges auto-wedge dynamic wedge Modify dose distribution Typically 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. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Dynamic wedge This 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. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Wedges ‘standard’ treatment accessory required for example in breast and head and neck treatment dynamic wedge most popular because: no weight any wedge angle possible but difficult to commission Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Asymmetric collimator The 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. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Asymmetric collimators Read-outs on the linac gantry Collimator rotation read-out Right collimator read-out Lower collimator read-out Left collimator read-out Gantry read-out Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Multileaf Collimator (MLC)
Part No...., Module No....Lesson No Module title Multileaf Collimator (MLC) Used to define any field shape for radiation beams Several 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 IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Linac mounted MLC The tongue and groove design reduces interleaf leakage. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title MLC The quality of the field definition depends on the width of the leafs There is always some interleaf leakage Typically the transmission through the MLC is larger than through a standard collimator A picture from a Varian linac comparison of 80 leaf MLC (1cm projected width at isocentre) and 120 leaf MLC (0.5cm projected width). Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Multileaf Collimator (MLC)
Part No...., Module No....Lesson No Module title Multileaf Collimator (MLC) This picture illustrates the variations in field size possible with MLCs - combined all these segments allow intensity modulation. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Dynamic MLC Concept similar to dynamic wedge When 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 IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Intensity Modulation MLC pattern 1 MLC pattern 2 Achieved using a Multi Leaf Collimator (MLC) The field shape is altered step-by-step or dynamically while dose is delivered MLC pattern 3 Similar 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. Intensity map Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Two approaches to IMRT Linac based IMRT Multiple individual fields, each of them intensity modulated in two dimensions This 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. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Two approaches to IMRT Continuous rotation of a one dimensional fan beam which consists of many beamlets which can be turned on or off This 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. Tomotherapy Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Binary MLC Components of Helical Tomotherapy Helical Scanning This 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 if a) he/she is familiar with the concept b) the participants will benefit from the information Ring detector at exit side Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Comments on IMRT Best possible dose distribution with photons No intuitive link between MLC settings, monitor units and the delivered dose distribution Impossible without computers in diagnostics, planning and delivery Challenges for QA This slide leads to the next device, EPID, which is primarily a verification and QA tool. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Electronic Portal Imaging
Part No...., Module No....Lesson No Module title Electronic Portal Imaging Imaging device at the beam exit side of the patient to record the treatment field Allows to verify that the field was delivered to the correct location in the patient Many different systems available... Illustrated in the next slide Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Electronic portal imaging devices in practice
Part No...., Module No....Lesson No Module title Electronic portal imaging devices in practice While 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. Siemens Varian Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Electronic portal image
Part No...., Module No....Lesson No Module title Electronic portal image The 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. Comparison of simulator and portal image (right) Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Electron beams Electron Beam Primary Collimator No target required Scattering foil used to produce larger beam - alternative would be to scan the pencil beam using electromagnetic fields Applicator required to produce good field delineation on the patient Scattering Foil Ion Chamber Secondary Collimator Electron applicator Patient Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Electron applicator on a modern Varian linear accelerator
Part No...., Module No....Lesson No Module title Electron applicator on a modern Varian linear accelerator Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Module title Electron applicators may be open sided for modern accelerators using double scattering foils or scanned beams enclosed for older accelerators using single scattering foils must be checked for leakage adjacent to the open beam on the sides of the applicators Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Model of an electron applicator for Monte Carlo Calculations
Part No...., Module No....Lesson No Module title Model of an electron applicator for Monte Carlo Calculations This 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. Electron applicator patient Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators With such a large number of possible settings it is essential that interlocks be provided to prevent inappropriate combinations from being selected It is also essential that the control console provides a clear indication of what functions have been set Shown is the control area of a Varian Clinac 1800 Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Complex control system Reliance on computers Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Varian Clinac operation screens
Clinical mode Service mode Part 5, lecture 2 (cont.): Equipment - linacs

Part No...., Module No....Lesson No Module title Electron Accelerators Verification systems All accelerator manufacturers now produce computer controlled verification systems which provide an additional check that the settings on the accelerator console are correct for proper accelerator function and correspond exactly with the parameters determined for the individual patient during the treatment planning process This is an important bit of information - record and verify systems should be part of every radiotherapy department. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Head leakage the 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 IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Head 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 IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Part No...., Module No....Lesson No Module title Electron Accelerators Neutrons these will only be a problem if the X Ray energy is equal to or greater than 15 MV issues which need to be considered when neutrons are presents include: neutron activation shielding problems This will be discussed in more detail in part VII of the course. Part 5, lecture 2 (cont.): Equipment - linacs IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

Radiation Protection in Radiotherapy

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