radiation protection in radiotherapy

Radiation Protection inRadiotherapy

Part 10

Good Practice in EBT

Lecture 1 (cont.): Equipment design

IAEA Training Material on Radiation Protection in Radiotherapy

Part 10, lecture 1 (cont.): Equipment design 2Radiation Protection in Radiotherapy

2. Features of safe design in practice

A General considerations

B Kilovoltage radiation units

C Telecurie units

D Megavoltage units

E Other irradiation units


A. General Safety Requirements

• Radiation Protection Measures include• Protection of the patient during treatment

• Equipment shielding

• Collimation system

• Patient comfort and control

• Protection of others• Room shielding (this was covered in part 7)


Equipment shielding

• Part of dose reduction strategy for patients

• Dose to patient other than target due to scatter and leakage


Equipment shielding

• X Ray equipment - only needed when machine is on• protects the patient during treatment

• Telecobalt units - shielding needed all the time• protects patient and staff during set-up

General design limit - leakage should be lessthan 0.1% of the primary radiation


Testing of shielding integrity of a linac head using film

About 2t of lead


Collimation

• Creates outlines of the radiation field which should conform to the target

• Can be done by a variety of different measures depending on the treatment unit type

• Always includes some leakage through the collimation - typically <2% of the primary beam


Collimation

• Aim to limit field to the target only

Customized blocksor prefabricated blocksin geometric shapes


Collimation

• Applicators • electron beams

• superficial beams

• Movable jaws

• Lead blocks• fixed shapes

• customized

• Multileaf collimator


Custom shielding may reduce the dose to critical organs

• e.g. scrotal shields to reduce dose to scrotum due to scattered radiation


Patient comfort and control

• The best collimation does not help if the patient is not stable• need good immobilization devices

• need to put patient in a reasonably comfortable position (this is often difficult with very sick patients)

• need to make them feel comfortable


Immobilization/set-up devices

• There are innumerable systems - many of them home built and designed

• A good mould room is essential - they are responsible for both,• immobilization and

• block making



• Head rests


Head and Neck Immobilization

All MedTec

Head rests to fit

Prone head rest


Lateral Head position



• The more accuracy is required, the more effort one must make e.g.:

• Stereotactic head frame with repositioning accuracy better than 2mm



• Immobilization shells for head

• Vacuum bag for body immobilization


Various body immobilisation devices

All MedTec

Body fix with external markers for set-up


Belly board for prone position

• Allows ‘belly’ to move into space

• Some of the bowel can be moved out of the field


Vacuum bags

All MedTec

Customized for every patient



• Board for set-up of breast patients

Arm rest to get arm out of the treatment field

Slope to straighten sternum in order to minimize lung dose

Leg rest

Head rest


… sometimes movement is difficult to control...

• e.g. rectal and bladder filling in prostate treatment • determine location of the prostate prior to each

treatment fraction using ultrasound


… sometimes movement is difficult to control...

• e.g. lung motion due to breathing• determine motion and

gate radiation beam

External markers on the patient which can be

tracked by a video system


Low cost solutions

• Ask patients to• hold still

• have reproducible bladder filling (e.g. always full or always empty)

• provide dietary advise

• breath shallow

• Make patients feel comfortable and secure


A note on intercom systems

• Need to be able to see the patient - is he/she comfortable? Is she/he moving?

• Need to be able to talk to the patient

• Need to be able to hear if the patient is in distress


B. Kilovoltage Equipment (10 - 150 kV)

• Dose rate is approximately proportional to the nth power of the accelerating potential as kVn where 2 < n < 3

• Dose rate is approximately proportional to current (mA)

• Therefore important that kV and mA are stable.

• It is obviously important that the timer is accurate and stable


Kilovoltage Equipment (10 – 150 kV)

• Dose control is achieved by a dual timer system as it is usually not practical to use a transmission ionization chamber

• Interlocks should be present to prevent incorrect combinations of kV, mA, and filtration

Quick Question

What are the fluctuations of the mains voltage in your hospital? What would be the consequence in dose if these would not be filtered out before generating the

high voltage for the X Ray tube?


Answer

• A +/- 10% voltage variation is not uncommon due to loading of the net at different times of the day or heavy occasional uses on the same mains (e.g. a lift)

• This translates into 40% dose variation which is unacceptable

• Mains stabilization is a MUST


Kilovoltage Equipment (10 - 150 kV)

• Leakage from the tube housing, the Air Kerma Rate (AKR) shall not exceed• 10 mGy h-1 at 1 metre from focus• 300 mGy h-1 at 5 cm from housing or accessory

equipment• if the tube is designed to operate in the range

10 - 50 kV then a special housing is required with a maximum leakage of 1 mGy h-1

• Testing for hot spots should be carried out using film-wrap techniques


Patient shielding

• May be done on the skin using lead sheets cut into customized shapes

• Special shields may be used - e.g. eye shields


Kilovoltage Equipment (150 - 400 kV)Orthovoltage irradiation units

• It is practical to use a transmission ionization chamber with this equipment and the primary dose control system should be an integrating dosemeter.

• The backup (secondary) dose control system can be either an independent integrating dosemeter or a timer


Kilovoltage Equipment (150 - 400 kV)

• Leakage from the tube housing, the Air Kerma Rate (AKR) shall not exceed• 10 mGy h-1 at 1 metre from focus

• 300 mGy h-1 at 5 cm from housing or accessory equipment (including the beam collimation system such as cones)

• Testing for hot spots should be carried out using film-wrap techniques


C. Telecurie units

• 137-Cs or more importantly 60-Co

• High activity in treatment head

• Termination of exposure is usually by dual independent timers


Timers

• Need two completely independent timers

• One should count time up, one down


Gamma-ray equipment

• The source should be sealed such that the container can withstand temperatures likely to be obtained in building fires.

• Wipe tests should be carried out initially at installation and at regular intervals to check for surface contamination. This test need not be carried out directly on the source surface and can be carried out on a surface which comes into contact with the source during normal operation of the equipment.


Cobalt unit designs


Gamma-ray equipment

• At commissioning, cross-sectional drawings of the head should be examined to identify possible locations where radiation leakage could be a problem.

• Film wrap techniques can be used to identify positions of ‘hot’ spots.

• Accurate integrated ionization chamber readings should be made at the location of any hot spots and also in a regular pattern around the head.


Gamma-ray equipment

• Leakage from the head with the source in the Off position: the Air Kerma Rate (AKR) shall not exceed• 10 Gy h-1 at 1 metre

from source

• 200 Gy h-1 at 5 cm from housing or accessory equipment


Gamma-ray equipment

• Leakage from the head with the source in the On position: the Air Kerma Rate (AKR) shall not exceed• 10 mGy h-1 at 1 metre from source or

• 0.1% of the useful beam AKR

• whichever is the greater


Gamma-ray equipment

• The beam control mechanism shall be of the ‘fail to safety’ type and will return to the Off position in the event of:• end of normal exposure• any breakdown situation• interruption of the force holding the beam

control mechanism in the On position, for example failure of electrical power or compressed air supply


Gamma-ray equipment

• In case of failure of the automatic source return section of the beam control mechanism, it shall be possible to interrupt the exposure by other means, for example, a manual return system

• It shall be possible to unload or repair the treatment head without exceeding the dose limit for occupational exposure recommended by regulation

Mechanical source position indicator


Gamma-ray equipment

• Collimation, patient immobilisation and blocking as described in first section of part 10 and the case of linacs.

• Two particularities• No commercial MLC available (but several

home built systems)

• Due to large source size and wide penumbra: penumbra trimmers (collimation close to the patient can be employed)


Specific design for Co units

• Penumbra trimmers - collimation close to patient reduces penumbra width


Beam stopper

• Metal disk at the exit side:• reduces primary beam shielding requirements• may make set-up of patients more cumbersome


D Megavoltage units

• Electron linear accelerators - linacs

• Capable of X Ray (4 to 25MV) and electron (4 to 25MeV) irradiation


Linacs

• Radiation exposure is usually 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

• The other system is termed the secondary system and is usually set to terminate the exposure after an additional dose, typically set around 0.25 Gy

• Most modern accelerators also have a timer which will terminate the exposure if both ionization chamber systems fail


Linacs

• Modern accelerators have a lot of treatment options as discussed in part 6, for example• X Rays or electrons (dual mode)• multiple energies

• 2 X Ray energies• 5 or more electron energies

• wedges• 3 or more fixed wedges• auto-wedge• dynamic wedge


Linacs

• 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 provide a clear indication of what functions have been set


A linac control example

Varian

Active selection

Parameter display


Linacs

• 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


Linacs - a note on MLCs

• X Ray Collimators may be• rectangular (conventional)

• 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 - this can be done using radiographic film


Linacs - electrons

• 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

• should be checked for leakage • adjacent to the open beam

• on the sides of the applicators

Cut-out at theend of the applicator


Electron collimation

• Done at the end of the applicator using customized cut-outs

Cut-out foam where field should be

PourLMAaroundit


IEC 601.2

• Limit values at different locations around the useful field


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 metres centred 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 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 metre from the path of the electrons between their origin and the target or electron window shall not exceed 0.5%


IEC standard 601.2

• Leakage in from linac head particularly of concern if the radiation can reach the patient


Guidance on leakage levelsin different parts of the field


Also consider

• Treatment in different patient positions – e.g. sitting or standing next to the linac for treatment of a hand


Linacs - a note on neutrons

• Neutrons will only be a problem if the X Ray energy is greater than 10 MV - in practice consideration MUST be given to neutrons if the energy is greater than or equal to 15MV

• The rate of equivalent dose of the neutrons should not exceed 1% of the dose-equivalent rate of the X Rays - measured in sievert

• The radiation weighting factor for the neutrons should be taken as 20. The above limit means that the neutron absorbed dose rate is always less than the X Ray absorbed dose rate


Accidents due to equipment design...

• An operator of an accelerator quickly selected X Ray mode and quickly changed to electron mode before the machine was able to complete the first request (to operate in X Ray mode) and it operated with hybrid instructions. The same accident occurred in six different hospitals and two patients died due to doses as high as about 160-180 Gy


This should not have happened...

• Contributing factors:• The computer controlled accelerators were not

tested for the extreme conditions that occurred in practice at the hospitals.

• It took too long for the manufacturer to identify the problem and disseminate the information and by then six hospitals had experienced the same failure and two patients had died from radiation


E Other irradiation units

• Diagnostic units in radiotherapy• CT scanner

• Simulator

• Other therapy irradiation units• heavy particles


Diagnostic units in radiotherapy

• Essential and often integral part of a modern radiotherapy department

• Essential for adequate target definition - therefore important also for optimization of medical exposure from a radiation protection point of view

• Includes not only X Ray equipment but may be MRI, ultrasound and nuclear medicine

• Beyond the scope of this course - however, covered in separate courses on diagnostic radiology and nuclear medicine


A note on simulators

• The simulator should be capable of reproducing all motions and X Ray exposure types (not radiation energy and dose though) of the treatment units


Simulator control

Patient clearly visible throughlarge lead glass window

Varian Medical Systems

Control screen similar to linac

Fluoroscopy screen


Simulators and other diagnostic equipment

• Often the most important aspect of design is to ensure that the simulator patient set-up can be transferred without any modifications to the treatment unit. This includes ‘imperfections’ of the systems such as couch sag under patient’s weight.


Heavy Particles

• These could include • Neutrons

• Protons

• Helium nuclei (alpha particles)

• Other heavy nuclei (Carbon nuclei)

• Negative pi mesons

• Protons are most common and increasing in their use


Heavy Particles treatment facilities

• These are very specialized installations

• shielding with high neutron fluxes can be extensive and complex

• neutrons require hydrogen rich materials for good energy absorption for example wood and or plastics

• many neutron interactions produce high energy gamma rays requiring large thicknesses of concrete , or steel to absorb them


Additional note on heavy particles

• Many of the points covered for electron accelerators are also applicable for these installations

• Specialized systems for positioning patients may be required

• The charged particle accelerators are often multipurpose facilities which also serve research objectives (e.g. material research). These applications may require entirely different beam parameters (e.g. high particle flux) than medical treatment. More care has to be taken to ensure that only the correct beam can reach the patient.

• There may be several treatment rooms for one accelerator

radiation protection in radiotherapy

Documents

tube housing

dose variation

radiation field

mains voltage

voltage variation

exceed10 mgy h

focus300 mgy h

maximum leakage