Part VIII:Medical Exposures in Radiotherapy Design consideration of equipment Lecture 4 : Design consideration of equipment IAEA Post Graduate Educational

Part VIII:Medical Exposures in Part VIII:Medical Exposures in RadiotherapyRadiotherapy

Lecture 4 : Design consideration of equipment Design consideration of equipment

IAEA Post Graduate Educational Course on Radiation Protection and Safe Use of Radiation Sources

Module 3 Optimization of Protection for Medical Exposure

Part VIII.3.4 : Design consideration of equipment Design consideration of equipment 2

ObjectivesObjectives

• To understand the design considerations for Radiotherapy To understand the design considerations for Radiotherapy equipmentequipment

• To understand the radiation Safety requirements for To understand the radiation Safety requirements for Linear acceleratorLinear accelerator

• To understand the concept of Primary & secondary To understand the concept of Primary & secondary barriersbarriers

• To learn calculation of barrier thickness required for To learn calculation of barrier thickness required for Teletherapy room design.Teletherapy room design.

• Learn the regulations for radioactive source transferLearn the regulations for radioactive source transfer


Radiation safety issues with linear Radiation safety issues with linear acceleratoraccelerator

• Higher energy beams (6 to 21 MV) requires Higher energy beams (6 to 21 MV) requires larger shielding thicknesslarger shielding thickness

• Beam should be monitored as it could change Beam should be monitored as it could change due to electrical variationsdue to electrical variations

• Positional variation in target, flattening filter, Positional variation in target, flattening filter, scattering foil could result in undesired beam scattering foil could result in undesired beam output.output.

• Neutron production for high energy beamsNeutron production for high energy beams


Design considerationsDesign considerations

• Equipment used in medical exposure shall be so Equipment used in medical exposure shall be so designed thatdesigned that– Failure of a single component of the system be Failure of a single component of the system be

promptly detectable so that any unplanned promptly detectable so that any unplanned medical exposure of patients in minimized; andmedical exposure of patients in minimized; and

– The incidence of human error in the delivery of The incidence of human error in the delivery of unplanned medical exposure be minimisedunplanned medical exposure be minimised

– BSS Appendix IIBSS Appendix II


Prevention of Failures and human Prevention of Failures and human errorserrors

• Taking into account information provided by suppliers, Taking into account information provided by suppliers, identify possible equipment failures and human errors identify possible equipment failures and human errors that could result in unplanned medical exposures;that could result in unplanned medical exposures;

• Take all reasonable measures to prevent failures and Take all reasonable measures to prevent failures and errors, including the selection of suitably qualified errors, including the selection of suitably qualified personnel, the establishment of adequate procedures personnel, the establishment of adequate procedures for the calibration, quality assurance and operation of for the calibration, quality assurance and operation of therapeutic equipment, and provision to personal for therapeutic equipment, and provision to personal for appropriate training and periodic retraining procedures, appropriate training and periodic retraining procedures, including protection and safety aspectsincluding protection and safety aspects


Emergency response plansEmergency response plans

• Take all reasonable measures to minimize Take all reasonable measures to minimize the consequences of failures and errors that the consequences of failures and errors that may occur; andmay occur; and

• Develop appropriate contingency plans for Develop appropriate contingency plans for responding to events that may occurresponding to events that may occur

• display plans prominently display plans prominently • and periodically conduct practice drillsand periodically conduct practice drills


Source ON / OFF safetySource ON / OFF safety

• The beam control mechanism shall be a ‘fail The beam control mechanism shall be a ‘fail to safety’ type.This means the source will to safety’ type.This means the source will return to the return to the OffOff position in the event of: position in the event of:– end of normal exposureend of normal exposure– any breakdown situationany breakdown situation– interruption of the force holding the beam interruption of the force holding the beam

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


Emergency – Source retraction Emergency – Source retraction failurefailure

• Mechanical Rod connected to front of source Mechanical Rod connected to front of source drawer is visible when beam-ONdrawer is visible when beam-ON

• Failure indicated by red “beam-ON” light Failure indicated by red “beam-ON” light stays lit and audible alarm on entry.stays lit and audible alarm on entry.

• Policy posted in console areaPolicy posted in console area

Do Do NOTNOT turn unit off or press Emergency button turn unit off or press Emergency button


Cobalt Stuck SourceCobalt Stuck Source

• Emergency Tee bar to be kept at maze Emergency Tee bar to be kept at maze entrance or control panelentrance or control panel


Manual retraction of source to OFF Manual retraction of source to OFF positionposition

Tee - rods used to push the source to OFF position in Theratron units


Maintaining standards (IEC & ISO)Maintaining standards (IEC & ISO)

• Ensure that the radiation generator and sealed Ensure that the radiation generator and sealed sources sources – The equipment should conform to applicable The equipment should conform to applicable

standards of the International Electro technical standards of the International Electro technical Commission (IEC) and the ISO or to Commission (IEC) and the ISO or to equivalent national standardsequivalent national standards

– Performance specification and operating and Performance specification and operating and mainenance instruction including protection mainenance instruction including protection and safety instructions, be provided in a major and safety instructions, be provided in a major world language understandable to the users world language understandable to the users and in compliance with the relevant IEC or and in compliance with the relevant IEC or ISO standardsISO standards

– Translate to local language where appropriateTranslate to local language where appropriate


Design requirements for radiation Design requirements for radiation generators in Radiotherapygenerators in Radiotherapy

• Ensure thatEnsure that– Radiation generators and irradiation installations Radiation generators and irradiation installations

include provisions for selection, reliable indication include provisions for selection, reliable indication and confirmation of operation parameters such as and confirmation of operation parameters such as • Type of radiationType of radiation• Energy, Energy, • treatment distance (SSD), treatment distance (SSD), • field size, beam orientation and either treatment field size, beam orientation and either treatment

time or present dosetime or present dose


Design requirements for radiation Design requirements for radiation generators in Radiotherapygenerators in Radiotherapy

• Ensure thatEnsure that– Irradiation installation using radioactive Irradiation installation using radioactive

source be fail-safe in the sense that the source be fail-safe in the sense that the source will be automatically shielded in the source will be automatically shielded in the event of an interruption of power and will event of an interruption of power and will remain shielded until the beam control remain shielded until the beam control mechanism is reactivated from the control mechanism is reactivated from the control panel;panel;


Design requirements for high energy Design requirements for high energy Radiotherapy equipmentRadiotherapy equipment

• Have at least tow independent ‘fail to safety’ Have at least tow independent ‘fail to safety’ systems for terminating the irradiation; andsystems for terminating the irradiation; and

• Be provided with safety interlocks or other Be provided with safety interlocks or other means designed to prevent the clinical use of means designed to prevent the clinical use of the machine in condition other than those the machine in condition other than those selected at the control panel;selected at the control panel;


Safety FeaturesSafety Features

• Controlled Access AreasControlled Access Areas

– Control RoomControl Room– Product Handling AreaProduct Handling Area

• Authorized (Restricted) Access AreasAuthorized (Restricted) Access Areas

– Machine RoomMachine Room


Design consideration – Safety Design consideration – Safety interlocksinterlocks

• Safety interlocks be such that operation of the Safety interlocks be such that operation of the installation during maintenance procedures, if installation during maintenance procedures, if interlocks are bypassed, could be performed interlocks are bypassed, could be performed only under direct control of the maintenance only under direct control of the maintenance personnel using appropriate devices, codes personnel using appropriate devices, codes or keysor keys


Design consideration – Radiation Design consideration – Radiation sourcesource

• Radioactive sources for either teletherapy or Radioactive sources for either teletherapy or brachytherapy bed so constructed that they brachytherapy bed so constructed that they conform to the definition of a sealed source;conform to the definition of a sealed source;

• When appropriate, monitoring equipment be When appropriate, monitoring equipment be installed or be available to give warning of an installed or be available to give warning of an unusual situation in the use of radiation unusual situation in the use of radiation generators and radio nuclide therapy generators and radio nuclide therapy equipmentequipment



• Lighted Entrance SignsLighted Entrance Signs– machine room machine room

entranceentrance– processing maze area processing maze area

entranceentrance• Timed Entry and Start-up Timed Entry and Start-up

ProceduresProcedures



• Unauthorized Door or Gate OpeningsUnauthorized Door or Gate Openings– Pressure Mat DetectionPressure Mat Detection– Photo Cell DetectionPhoto Cell Detection

• Pull CordsPull Cords• Emergency Shut-down ButtonsEmergency Shut-down Buttons


Points to NotePoints to Note

• When the linear accelerator is off, there is When the linear accelerator is off, there is NONO radioactive hazard.radioactive hazard.

• NONO radioactive materials are used in the radioactive materials are used in the production of the electron beam.production of the electron beam.

• The product that has been irradiated is The product that has been irradiated is notnot radioactive.radioactive.


Teletherapy Room designTeletherapy Room design

• Teletherapy room design Teletherapy room design – Telecobalt unitsTelecobalt units– Linear acceleratorsLinear accelerators


Advantage of large treatment roomAdvantage of large treatment room

• Distance is effective shieldingDistance is effective shielding• Consider special treatments such as TBIConsider special treatments such as TBI• Need storage space for accessories and Need storage space for accessories and

patient immobilizationpatient immobilization• Allows for future upgrades of equipment (FAD Allows for future upgrades of equipment (FAD

80 ---> 100cm) and increases in shielding80 ---> 100cm) and increases in shielding


Design considerations: External Beam Design considerations: External Beam RadiotherapyRadiotherapy

• Placement of the treatment unitPlacement of the treatment unit• Primary beam directionPrimary beam direction• Operator locationOperator location• Surrounding areas - should have low occupancySurrounding areas - should have low occupancy

• Costs Costs • can be reduced if the design is goodcan be reduced if the design is good• for extension is usually much larger than for for extension is usually much larger than for

allowing for expansion already during the allowing for expansion already during the building phasebuilding phase

Design criteria: External Beam Design criteria: External Beam treatment areatreatment area

• Clear signs are required in areas leading to Clear signs are required in areas leading to treatment unitstreatment units

• Patient and visitor waiting areas should be Patient and visitor waiting areas should be positioned so that patients are unlikely to positioned so that patients are unlikely to enter treatment areas accidentallyenter treatment areas accidentally

• Patient change areas should be located so Patient change areas should be located so that the patient is unlikely to enter a treatment that the patient is unlikely to enter a treatment area accidentallyarea accidentally


Megavoltage RoomMegavoltage Room

• Is a door needed?Is a door needed?• Door interlocksDoor interlocks

– Protocol for closing the door and activating Protocol for closing the door and activating radiationradiation

– Door interlocksDoor interlocks• Radiation warning signs are neededRadiation warning signs are needed


The concrete walls of the new therapy vaults up to 2 m thick.The ceilings are 2.7 m thick.

The accelerator treatment room doors contain 4 inches of lead shielding for x-rays and 6 inches of polyethylene neutron absorber.

Design of high energy linear Design of high energy linear accelerator roomaccelerator room

From Cancer Care Manitoba web site


Shielding ParametersShielding Parameters

• Three sources of radiation to be considered Three sources of radiation to be considered for shieldingfor shielding

• These are :These are :– primary radiation (the x-ray beam)primary radiation (the x-ray beam)– scattered radiation (from the patient)scattered radiation (from the patient)– leakage radiation (from the head)leakage radiation (from the head)


Primary barrier thickness CalculationPrimary barrier thickness Calculation

• Effective dose/yr = WUT /dEffective dose/yr = WUT /d2 2 Sv/yrSv/yr

T = occupancyT = occupancy

W= workloadW= workload

U = use factor (1/4 for primary beams)U = use factor (1/4 for primary beams)

d = distance from source (outside wall)=4md = distance from source (outside wall)=4m



T - Occupancy T - Occupancy • T = fraction of time a particular place is T = fraction of time a particular place is

occupied by staff, patients or publicoccupied by staff, patients or public• Ranges from 1 for all work areas to 0.06 for Ranges from 1 for all work areas to 0.06 for

toilets and car parkstoilets and car parks• Occupancy factorsOccupancy factors

– Work areas (offices, staff roomsWork areas (offices, staff rooms ) – 1 ) – 1– Corridors Corridors = ¼ (0.25)= ¼ (0.25)



W - WorkloadW - Workload• A measure of the radiation outputA measure of the radiation output• Measured inMeasured in

– mA-minutes for x-ray unitsmA-minutes for x-ray units– Gy for cobalt 60 units, linear accelerators Gy for cobalt 60 units, linear accelerators

and brachytherapyand brachytherapy• Usually a gross overestimationUsually a gross overestimation


Shielding Parameters (cont.)Shielding Parameters (cont.)

W - WorkloadW - Workload• Work load usually increased by 20% for Work load usually increased by 20% for

physics QA and measurementsphysics QA and measurements• Other uses must also be accounted for and Other uses must also be accounted for and

included if significant egincluded if significant eg– researchresearch– blood irradiationblood irradiation


WorkloadWorkload

• For example, take a cobalt 60 or linear For example, take a cobalt 60 or linear acceleratoraccelerator

• Assume a dose of 2 Gy per patient per day at Assume a dose of 2 Gy per patient per day at the isocentrethe isocentre

• Assume 30 patients per day for 5 days a Assume 30 patients per day for 5 days a weekweek

• So how do we estimate ‘W’ the work load?So how do we estimate ‘W’ the work load?


Calculation of WorkloadCalculation of Workload

• Assume average dose at isocentre is 2.5Gy / Assume average dose at isocentre is 2.5Gy / patientpatient

• Thus W = 2.5 x 30 patients x 5 days Thus W = 2.5 x 30 patients x 5 days x 52 weeks = 19500 Gy per year x 52 weeks = 19500 Gy per year

W at isocentre = 1.95 x 10W at isocentre = 1.95 x 1044 Sv/year Sv/year

• In actual case increase by 20% for QA and In actual case increase by 20% for QA and other irradiationsother irradiations


Primary barrier calculationPrimary barrier calculation

• Effective dose /yr outside wall Effective dose /yr outside wall • Assume d= 3metersAssume d= 3meters• Effective Dose = WUT/dEffective Dose = WUT/d22

=1.950x 10=1.950x 104 4 (Sv/yr) x 1/4 x 1 / 3(Sv/yr) x 1/4 x 1 / 322

= 542 Sv/yr= 542 Sv/yr


Primary Calculation (cont.)Primary Calculation (cont.)

• Permissible dose for radiation worker is Permissible dose for radiation worker is 50mSv/yr taking 1/10 of it for ALARA50mSv/yr taking 1/10 of it for ALARA

• HHlimit limit = 5mSv / yr= 5mSv / yr

and Hand Hlimitlimit= WUT/d= WUT/d22 x B (B is transmission) x B (B is transmission)

B= HB= Hlimit limit / (WUT/d/ (WUT/d22) = 5 x ) = 5 x 10-310-3/ (542(Sv/yr))/ (542(Sv/yr))

Tenth Value Thickness(TVT)= log(1/B)Tenth Value Thickness(TVT)= log(1/B)

TVT TVT 5.02 5.02 • Look up talble for TVT for the energy for Look up talble for TVT for the energy for

various mateirals various mateirals


During Equipment InstallationDuring Equipment Installation

• Test that primary beam is contained within Test that primary beam is contained within the primary barrierthe primary barrier


Secondary BarrierSecondary Barrier

• Secondary barriers are calculated in a similar Secondary barriers are calculated in a similar fashion except fashion except – that the leakage rate from the head at 1 that the leakage rate from the head at 1

meter is used meter is used • Leakage rate is typically 0.1 to 0.2% of Leakage rate is typically 0.1 to 0.2% of

primary work loadprimary work load• Use factor is always 1Use factor is always 1

Secondary barriers


SummarySummary

• Design considerations should be followed for Design considerations should be followed for radiation generating equipmentradiation generating equipment

• Emergency procedures should be planned and Emergency procedures should be planned and displayed prominently displayed prominently

• No radiation safety issue in linac when the unit is off No radiation safety issue in linac when the unit is off • Target, Flattening filter and scattering foil positions Target, Flattening filter and scattering foil positions

are criticalare critical• High energy protons produce neutronsHigh energy protons produce neutrons• The walls facing the beam are primary barriers rest The walls facing the beam are primary barriers rest

are secondary barriers in a teletherapy roomare secondary barriers in a teletherapy room


QuestionsQuestions

• Why linac should be surveyed for neutronsWhy linac should be surveyed for neutrons• What is Work loadWhat is Work load• What are Occupancy factor & use factorWhat are Occupancy factor & use factor• What is a type A package?What is a type A package?


ReferencesReferences

• Basic Safety Standards Basic Safety Standards – Safety series No 115 IAEA publicationSafety series No 115 IAEA publication

• Lecture Material of Lee Collins prepared for IAEALecture Material of Lee Collins prepared for IAEA• Lecture Material of Tomas Kron, Prepared for IAEA Lecture Material of Tomas Kron, Prepared for IAEA

safety coursesafety course• Design and implementation of a radiotherapy

programme: clinical, medical physics, radiation protection and safety aspects - IAEA Tec Doc 1040

• ‘‘The Modern Technology of Radiation Oncology’ The Modern Technology of Radiation Oncology’ – Editor Jacob Van DykEditor Jacob Van Dyk

Documents

Part VIII:Medical Exposures in Radiotherapy Design consideration of equipment Lecture 4 : Design consideration of equipment IAEA Post Graduate Educational