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    radiotherapy Superficial physics equipment radiotherapy Superficial physics equipment Presentation Transcript

    • Superficial &Orthovoltagetreatment unitsRadiotherapy PhysicsDesign and operationByYasmeen Malik
    • Presentation includes1. Structure and function of the component parts of asuperficial/orthovoltage unit2. Heat dissipation3. Inherent and added filtration4. Beam definition5. Beam characteristics of a superficial/orthovoltage beam6. Basic principles of clinical practiceBy Y Malik
    • External Beam RT MV Dual energy Photons and electron therapy Linear accelerator kV x-rays Superficial treatment SXR/DXR/orthovoltage/Gulmay/skinsunit/pantak/therapexBy Y Malik
    • GantrySupportLocking systemTreatmentCouchHeat exchangerX-ray tube housingApplicatorSecondary collimatorBy Y Malik
    •  CeilingmountedgantryBy Y Malik
    • Energies Diagnostic X-rays 50 -125 kV Superficial X-ray 50 - 150 kV Orthovoltage X-ray 150-300 kVBy Y Malik
    • Gulmay therapy unit 40-300kVApplicatorPortAnode endCathode endFilterBy Y Malik
    • X-ray tube Glass envelope Cathode AnodeBy Y Malik
    • Cathode assembly Negative Filament Tungsten coil 1-2 mm wide High melting point (3370 C) Good thermionic emitter at 2000 C Focusing cup Shallow depression Nickel Focus electrons towards targetBy Y Malik
    • Anode assembly Positive charge Target Stationary/rotating Tungsten High melting point 3370 C High Z = 74 Thick copper block Good conductor Cavities filled with oil Focal areaBy Y Malik
    • Efficiency in X-ray Production kV 1% X-rays 99% heat MV Efficiency x-ray production increases with energy 4 MV = 40% X-rays Incident electron has higher KE which is given to theorbital electronBy Y Malik
    •  Diagnostic kV X-rays Large KE deposited in very short exposure time Rotating target RT kV X-rays Large KE deposited in longer exposure time Stationary target Conventional simulator Fluoroscopy/images = rotating typeBy Y Malik
    • Rotating Anode TargetBy Y Malik
    • Anode assembly... Radiation shield protects glass envelope Beryllium exit window Primary collimationBy Y Malik
    • Tube housing X-ray tube contained in X-ray head (outer tube housing) Lead lined casing – radiation protection Insulating Oil Sensitive devices Temperature control Shock proof Electrical interlock systemBy Y Malik
    • Heat Dissipation Tungsten target Anode copper block Cavities filled with oil Heat exchanger Tube housing Bellows absorb heat and expand Air convection Fan assisted Overheating interlock systemBy Y Malik
    • Heat Dissipation... RT Total energy per exposure is large but spread over alonger period Longer exposure time Stationary anode Diagnostic Total energy per exposure in very short time Very short exposure time Rotating anode Increase surface areaBy Y Malik
    • X-ray production1. Cathode - Filament heated by current2. Thermionic emission3. Electrons attracted to positive anode4. Strike the tungsten target5. X-ray and heat production6. Heat dissipation7. X-rays exit via window of the tube8. Surrounding insulating oil9. Exit via Port (window) of the metal casing10. Filter11. ApplicatorBy Y Malik
    • Spatial distribution of x-rays atTargetBy Y Malik
    • Filtration modifies the beam Heterogeneousbeam 1% X-rays 80-90% Brems. 10-20% Ch.rad Filtration Improve quality Scatter radiation Patient doseTypical Photon Energy Spectrum from aMachine Operating at KV = 80
    • Characteristic radiation oftungsten (k-shell lines)By Y MalikOrbital e- gains KEExceeds BE of K-shell 69.5keVTherefore ejectedLeaving behind avacancyIncident e-strikes innershell e-e.g. Machineoperating at80kVIncident e-KE – BE(80-69.5=10.5keV)K-shell vacancy filled byL-shell e-Energy difference emittedas ch.rad.69.5keV (K-shell) -10.2keV(L-shell) =59.3keV69.5keV10.2keV3keV
    • Bremsstrahlung radiation –continuous spectrumBy Y MalikIncident e-e.g. Machineoperating at80kVe.g. Initial KE of 80keV – lost KE of 60keV asBremsstrahlung photon =20keVBremsstrahlung photon. Inthis example it has KEof 60keV (can have arange of energies) hencethe continuous spectrum
    • Filtration Attenuation Inherent Added Harden the beam Improve quality of beam Simple and Compound filtersBy Y Malik
    • Half Value Layer (HVL) Beam quality expressed as the HVL HVL is that amount of absorbing material that will reducethe intensity of the primary beam to one-half its originalvalue HVLs are usually expressed in terms of aluminiumfiltration equivalency for example, HVL = 2.0 mm ALBy Y Malik
    • Beam Quality at kV Energies Superficial Aluminium Orthovoltage Tin Copper AluminiumBy Y Malik
    • Inherent Filtration Pathway of the beam Anode copper block Tube housing Beryllium window exit Insulating oil Glass window of tube housing(exit port)By Y Malik
    • Compound filter – Thoreus filterBy Y MalikPrimary BeamFiltrationimproves beamquality.Low energyphotons areattenuated.Scatter/Patientdose29.3keV9keVCharacteristic photons:1.5keV
    • Beam definition Primary collimation Radiation shielding Window of x-ray tube Secondary collimation Applicators Collimate beam Define Field Size FSD Lead cutout Field shape Conformal techniqueBy Y Malik
    • Applicators - SXR Lead base plate Steel/lead sides Open Perspex end FSD 10-30 cm Field size Field shape Lead cutoutBy Y Malik
    • Applicators - DXR Metal base plate Lead collimator Steel sides lead lined Close ended 5 cm perspex FSD 50 cm Heavy/bulkyBy Y Malik
    • Lead cut out Additional beam shaping Skin surface Immobilisation Thickness Shielding Lead face mask Back scatterBy Y Malik
    • Control panel Operation Tube voltage kV Tube current mA Exposure timer /monitor units dual counting Beam modification - filter selection Two operatorsBy Y Malik
    • Dosimetry300 kV 6 MVNot toScale• Lateral side scatter• Round isodose lines• 100% = 0.0 cm• 80% = 3.2 cm• 50% = 7.3 cm• Less scatter• Flat isodose lines• 100% = 1.5 cm• 80% = 7 cm• 50% = 16 cmBy Y Malik
    • Dosimetry...300 kV 6 MeVNotToScale• Lateral side scatter• Round isodose lines• 100% = 0.0 cm• 80% = 3.2 cm• 50% = 7.3 cm• Lateral scatter at depth• Rapid dose fall off• 100% = 0.0 cm• 80% = 2.1 cm• 50% = 3.0 cmBy Y Malik
    • Principles of Clinical PracticekV therapy 50–300 kV Applicators Aluminium filter (up to 150 kV) Compound filter (150-300 kV) 100% skin surface 90% at 0.5cm depth (sxr) 90% at 2.0cm depth (dxr)Electron therapy 4 – 18 MeV Applicators No filters 100% dependent on MeV 90% at E/2.3cm Eg. 6 MeV = 2.6cmBy Y Malik
    • kV X-ray Therapy Unit Low cost compared with MV unit Simple in design and operation Simple collimation and beam shaping Housing of the equipment OptimisationBy Y Malik
    • Planning Stage Diagnosis Gross Tumour Volume Planning Target Volume Beam shaping Shielding Internal/external Fractionation regime 40 Gy in 10# daily Single 1GyBy Y Malik
    • Treatment Patient positioning Patient care Side effects Erythema Desquamation Crusting Skin pigmentation Follow upBy Y Malik
    • Exit beam blocking Consider underlying structures Back scatter dose Deep tissues can be spared by blocking the exitbeam Exit beam can give additional dose to underlyingstructures Lead sheet of few mm encased in wax or dentalacrylic used Absorb back scatterBy Y Malik
    • BCC PinnaExit beam shield1mm thickCoated with waxLead cut outDefine field shape(field size defined by applicator)By Y Malik
    •  BCC left ala nasi Superficial therapy unit Exit beam Oral shielding Nasal shielding with wax External eye shield 2mmthick Depending on close proximityto lesion Lead face maskBy Y Malik
    • Skin apposition Applicator Parallel to skin surface Equal distance & pressure Stand off Stand in Dosimetry calculations Target volumeapplicatorBy Y Malik
    • Clinical Application Squamous Cell Carcinoma Basal Cell Ca Melanoma Kaposi Sarcoma Mycosis FungoidesBy Y Malik