Download - Introduction to radiation therapy
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Introduction to Medical Technology: Radiotherapy
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Contents
• Why radiotherapy?• Some basic principles• Approaches to radiotherapy
– external beam: x-rays, electrons, p, n, heavy ions
– brachytherapy• Stages in the radiotherapy process
– QA, imaging, planning, simulation, treatment, verification, modelling outcome
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Why Radiotherapy?
• Radiotherapy used to treat cancers– In conjunction with surgery and
chemotherapy• Proven benefit• Cells display strong dose-effect relationship• Physics, engineering, imaging, technology
based
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Benefits of Radiotherapy
• Breast Cancer• Mastectomy• Compare surgery and
chemotherapy (CMF) with and without radiotherapy
• 10 year survival improvement at 10 years
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Contents
• Why radiotherapy?• Some basic principles• Approaches to radiotherapy
– external beam: x-rays, electrons, p, n, heavy ions
– brachytherapy• Stages in the radiotherapy process
– QA, imaging, planning, simulation, treatment, verification, modelling outcome
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Radiobiology:Linear-quadratic model
• Effects of radiation on cell survival described by:
linear-quadratic model• Linear
– single hit sufficient • Quadratic
– multiple events
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Radiobiology:Tumour and normal tissue
• Different responses to radiation / different
• Deliver one large dose– effects similar
• Deliver many smaller doses– effects very different
• Fractionation– e.g. 60 Gy in 2 Gy
fractions
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Radiobiology: Tumour and normal tissue
• Radiation effect vs. dose– sigmoid behaviour– stochastic process
• Tumour control lower dose than normal tissue damage– Makes radiotherapy
possible!• Radiotherapy goals and
research– separate two curves
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Some Nomenclature
• Dose response curve– plot of radiation effect vs. dose
• Radical treatment– treatment with curative intent (cf. palliation)
• RBE (relative biological effect)=quality factor• Tumour staging: Tumour, Nodal, Metastasis• Necrosis and hypoxia
– regions of tumour dead or dormant due to low oxygen level
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Contents
• Why radiotherapy?• Some basic principles• Approaches to radiotherapy
– external beam: x-rays, electrons, p, n, heavy ions
– brachytherapy• Stages in the radiotherapy process
– QA, imaging, planning, simulation, treatment, verification, modelling outcome
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Approaches to Radiotherapy
• External beam– Fire radiation beam into patient
• Usually several beams from various directions• Cross-fire effect
• Brachytherapy– Place set of sealed radioactive sources or
seeds into patient• Often contained in applicator tube
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External Beam Treatment
• Most commonly Cobalt 60 rays (1.25 MeV), x-rays, electrons– Co 60 radioactive source– x-rays and e-, electron linac
• Sometimes orthovoltage (up to 300 kV)– Superficial tumours
• More rarely hadrons (many MeV)– protons, neutrons, heavy ions
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Electron Linac for X-ray and Electron Radiotherapy
• X-rays (4-50 MV), electrons (4-50 MeV)• X-rays polychromatic
– Bremsstrahlung• Electrons scatter in air
– Need for collimation• Cross-fire effect
– Several beams aimed at tumour/target– High dose to tumour/small dose to
surrounding tissue
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Radiotherapy Linacs
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X-Ray Beam Characteristics
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Beam Shaping Devices
• Main Rectangular Collimators• Wedge• Multileaf Collimator
– Field edge shaping• Block• Compensator• Multileaf Collimator
– IMRT (intensity modulated radiotherapy)
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Lead Compensator
• Sheets of lead– 0.5 mm thick
• Used in conjunction with wedge
• Shapes to breast and shields lung
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Granulate Compensator
• Stainless steel granulate
• Used in conjunction with wedge
• Shapes to breast and shields lung
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Intensity Modulated Radiotherapy
• Most common delivery method MLC (multileaf collimator)– Dynamic
• Scan MLC leaves across field with radiation on
– Multiple segment• Delivery set of irradiations at same position with
different field shapes
• Compensators
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Patient Image
Cumulative Intensity
Field 1
Field 4Field 3
Field 2
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Electron Beam Characteristics
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Brachytherapy
• Sources placed in patient using metal tubes or applicators
• e.g. Ir192
• Afterloading• High dose rate (HDR) and low dose rate
(LDR), pulsed dose rate (PDR)• Fractionated• Prescription schemes for location of sources
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Hadron Radiotherapy
• Large accelerators– High energies needed– Large magnetic rigidity
• Protons most common– Over 20 in the world
• Neutrons• Heavy ions
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