11/20/200611ENGG 167 MEDICAL IMAGINGLecture 21: Friday, Nov. 17Radiation Therapy IReferences: The Modern Technology of Radiation Oncology, Ed. J. van Dyk, Med. Phys. Publ., 1999. (Chapter 11)Principles and Practice of Radiation Therapy, Vol 1 Introduction to Radiation Therapy, C. M. Washington, D. T. Leaver, Mosby Year-Book Inc, 1996. 2Radiation Therapy – Part I1) Dose response2) Cobalt Therapy3) Linear Accelerators4) Depth Dose Curves5) Treatment Planning6) Model-based treatment planning11/20/200623Radiation TherapyRef: Van DykeAdditionalPhysicist functions:System specificationTreatment room planningConstruction supervisionSystem purchasingSystem installationSystem Quality Audit4Radiation dose-response curvesRef: Van Dyk11/20/200635Radiation treatment planningRef: Van Dyk6Orthovoltage X-ray therapy device – Pantak 300 (300 kVp)Ref: Van Dyk11/20/200647Cobalt-60 therapy device - older therapy systemRef: Van Dyk & Washington8Cobalt-60 therapy device - older therapy systemRef: Van Dyk & Washington11/20/200659Linear Accelerator (LINAC)Ref: Van Dyk10Linear Accelerator (LINAC) schematic Ref: Van Dyk11/20/2006611Different LINAC designsRef: Van Dyk12Electron gun pulses electrons into waveguideRef: Van DykThermionic emission of electrons. In static emission, typically near –20kV used for acceleration of beam into waveguide.In pulsed emission, pulses of –150V and +180V oscillate with respect to the cathode.11/20/2006713Waveguide acceleratorRef: Van Dyk14Waveguide acceleratorRef: Van DykEvacuated or dielectric filled cylindrical cavities, tuned for a specific frequency. (2.856 GHz)Must be modified such that phase velocity, νph< c, since νph= νpart, particle velocity.This is accomplished with cylindrical disks placed at quarter wavelength spacings11/20/2006815Waveguide acceleratorRef: http://www.howstuffworks.comThe spacing of the cavities is matched to the wavelength of the microwaves. The spacing allows the electric and magnetic fields to repeat their pattern every three cavities. Electrons or positrons in the beam come through the cavities in small bunches. The arrival of each bunch is timed so that it gets a “push” from the electric field across the cavities. 16RF generation for electron pulse acceleration –magnetrons and klystronsRef: Van DykMagnetronResonant cavities deliver energy to electrons as they spiral around a magnetic field. Coupling of electrons to aerial delivers RF power. Typically used for < 6MeV powers.KlystronResonant cavities deliver energy to electrons as they travel from cathode to beam dump. Design of buncher and catcher cavities provides maximal coupling of RF power to and from electron beam. Typically used for > 6 MeV powers.11/20/2006917Microwave power transfer to acceratorRef: Van DykTypical frequency is 2.856 GHz for medical linacs.TE01mode waveguides used to transfer RF powerPressurized with dielectric gas (freon or SF6) to 2 atm. pressureCirculator also required, which is a component allowing one way transfer of the RF (composed of a ferrite such as ceramic mixed with iron), making back reflections into klystron impossible.18Electron beam steering systemsRef: Van Dyk11/20/20061019LINAC head components – photon and electron modesRef: Van Dyk20LINAC beam parameters which need to be measuredRef: Van DykPercentage depth-dose – measured with small ionization chamber translated vertically down along the central line. Max value called dmax. Beam profiles – horizontal measure of dose falloff in penumbral regions between 20% and 80% maximum dose.Flatness - F = 100 x (Dmax-Dmin)/(Dmax+Dmin) across the region >80%Dose field factors – correction factor for doses generated from different field sizes. Most machines set for 1 MU = 1 cGy delivered at dmaxin water on the central axis for 10x10 cm2field11/20/20061121Depth-dose curves for different photon & electron energiesRef: Van Dykphotons electrons22Dose versus field sizeRef: Van Dyk11/20/20061223Iso-dose lines in water and inhomogeneous phantomsRef: Van Dyk24Cobalt-60 beam profile vs. 6 MeV beamRef: Van Dyk11/20/20061325Cobalt-60 beam profile vs 6MeV beamRef: Van Dyk26Treatment Planning Ref: Van Dyk• Prediction of dose delivery in heterogeneous patient tissues.• Each patient requires a separate dose plan.• Prediction of tissues to hit and tissues to spare• Model-based simulation based upon CT scan• Optimization11/20/20061427Model-based prediction of doseRef: Van Dyk28Treatment planning with CT scans - IRef: Van Dyk11/20/20061529Treatment planning with CT scans - IIRef: Van Dyk30Patient immobilization proceduresRef: Van Dyk11/20/20061631Accuracy limits in positioningRef: Van Dyk32Journal Club IV– Industrial/Product Review•Look through journals handed out , as well as web links on classwebsite. •Choose a product/company to present.•Give me the name of the product/company before Wednesday. Things you need to present:•Overview of product (pictures)• technical specifications•How is it used ? (main medical imaging procedures)•Who has used it ? (search360 or medline ?)•What competition is there for this product? (other companies