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3DCRT and IMRT
- 1. RADIATION PHYSICS –3DCRT AND IMRT DR KIRON G
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- 3. CONFORMAL THERAPY createshigh-dose volumes that are shaped to closely “conform” to the desired target volumes and prescription doses minimizing the dose to critical normal tissues maximizing tumor control probability (TCP) accuracy in localization of CTV is critical minimizing normal tissue complication probability (NTCP).
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- 5. 3-DCRT based on3D Anatomic information treatment planning system that is capable of calculating 3D dose distribution and dose volume statistics for contoured structures
- 6. TREATMENT PLANNING PROCESS Patient positioningand Immobilisation CT Simulation Image Segmentation Forward planning Plan evaluation Treatment Delivery
- 7. CT SIMULATION CTsimulator is a CT scanner equipped with laser localizers to setup the treatment isocenter, a flat couch Relationship between pixel value (CT number) and tissue density is established and this allows pixel by pixel correction for tissue inhomogeneities in computing dose distribution CT is related to electron density and atomic number CT provides the best geometric accuracy. MRI shows proton density distribution
- 8. IMAGE REGISTRATION Processof correlating different image data sets to identify corresponding structures or regions Eg: MRI and CT fusion
- 9. IMAGE SEGMENTATION Sliceby slice delineation of anatomic regions of interest
- 10. BEAM APERTURE DESIGN After image segmentation has been completed, next step is to select beam direction and designing beam apertures Multiplicity of Fields remove the need of using ultrahigh energy beams (>10MV) Multiple fields requires beam shaping blocks or MLC
- 11. PLAN OPTIMIZATION ANDEVALUATION Isodose curves and surfaces Dose Volume Histograms
- 12. PLAN OPTIMIZATION ANDEVALUATION – ISODOSE CURVES AND SURFACES Dose distribution of plans are evaluated by viewing isodose curves in individual slices or 3D ISO dose surfaces It shows regions of uniform dose, high dose or low dose and their anatomic location and extent
- 13. PLAN OPTIMIZATION ANDEVALUATION - DOSE VOLUME HISTOGRAMS Provides quantitative information about how much dose is absorbed in how much volume Also summarizes the entire dose distribution into a single curve for each anatomic structure of interest Can be represented in two forms Cumulative integral DVH Differential DVH
- 14. CUMULATIVE DVH plotof the volume of a given structure receiving a certain dose or higher as a function of dose found to be more useful Any point on the cumulative DVH Curve shows the volume of a given structure that receives the indicated dose or higher cumulativ e
- 15. DIFFERENTIAL DVH Plotof volume receiving a dose within a specified dose interval as a function of dose shows extent of dose variation within a given structure
- 16. DOSE COMPUTATION ALGORITHMS Correction Based Model based Convolution-Superposition Method Direct Monte Carlo
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- 18. DEFINED AS nonuniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution Optimal fluence profiles for a given set of beam directions are determined through inverse planning
- 19. IMRT To produceintensity modulated fluence profiles, accelerator must be equipped with a system that can change the given beam profile to a profile of arbitrary shape Computer controlled MLC is the most practical system for delivering intensity modulated beams
- 20. IMRT DELIVERY Fixedgantry angle Rotating fan beams (tomo therapy) Rotating cone beams IMRT Fixed gantry angle Segmental MLC delivery Dynamic MLC delivery Rotating fan beams Tomotherapy Rotating cone beams Intensity Modulated Arc therapy Volumetric Modulated Arc therapy
- 21. IMRT WITH FIXEDGANTRY ANGLES Segmental MLC delivery Dynamic MLC delivery
- 22. SEGMENTAL MLC DELIVERY/ STEP AND SHOOT / STOP AND SHOOT treated by multiple fields Each field is subdivided into a set of subfields irradiated with uniform beam intensity levels Subfields are created by MLC and delivered in a stack arrangement one at a time in sequence Accelerator is turned off while the leaves move to create the next subfield A nine field plan could be delivered in less than 20 min
- 23. PROS AND CONS Advantage ease of implementation Disadvantage instability of some accelerators when beam is switched off and on within a fraction of a second
- 24. DYNAMIC MLC DELIVERY/ SLIDING WINDOW accelerator beam is on while the leaves are moving period that the aperture between leaves remains open : dwell time Dwell time allows the delivery of variable intensity to different points in the field
- 25. IMRT WITH ROTATINGFAN BEAMS : TOMOTHERAPY treated slice by slice by Intensity modulated Beams similar to CT imaging special collimator designed to generate the beams as the gantry rotates around the longitudinal axis of the patient 2 devices In one device, couch moves one or two slices at a time In second device, couch moves continuously as in helical CT
- 26. IMRT WITH ROTATINGCONE BEAMS Intensity Modulated Arc therapy Volumetric Modulated Arc therapy
- 27. INTENSITY MODULATED ARCTHERAPY(IMAT) uses the MLC dynamically to shape the fields & rotate the gantry Similar to step & shoot in that each field is positioned along the arc and subdivided into subfields of uniform intensity But the MLC moves dynamically to shape each subfield while the gantry is rotating And Beam is on all the time
- 28. INTENSITY MODULATED ARCTHERAPY(IMAT) Multiple overlapping arcs are delivered with the leaves moving to new position at regular angular interval say 5 degree One dimensional intensity profile generated by stacking of fields defined by one leaf pair, Two dimensional profiles created by repeating the whole process for all the leaf pairs of the MLC
- 29. DISADVANTAGES OF IMAT inefficient due to necessity of treating several arcs to deliver a single IMAT treatment field only a little improvement in isodose distribution from other forms of IMRT
- 30. VMAT Delivery ofa rotational cone beam with variable shape and intensity gantry moves continuously while the MLC leaves and dose rate varying throughout the arc
- 31. CLINICAL APPLICATION –IMRT extra degree of freedom that is intensity modulation in achieving dose conformity not limited by target size or its location Superficial disease sites (e.g., parotid, neck nodes, chest wall), often treated with electrons, can also be treated with IMRT as effectively dose conformity is a “double-edged sword,” with more normal tissue sparing on the one hand and greater possibility of target miss on the other Large integral dose
- 32. CAVEATS - CONFORMALTHERAPY Highly Susceptible to motion and setup related errors Extensive physics manpower Planning time is increased Increased cost
- 33. COMPARISON 3DCRT Less conformal Forward planning Less strict QA Less expensive Less Time consuming Less reduction of normal tissue dose No dose intensity modulation Uniform dose IMRT More conformal Inverse planning More accurate QA More expensive More time consuming More reduction of normal tissue dose Dose intensity can be modulated within the target Gradient dose
- 34. INVERSE PLANNING OROPTIMIZATION Introduced by Brahme process by which the intensity distribution of each beam employed in a plan is determined such that the resultant dose distribution can best meet the criteria specified by the planner Criteria are typically specified in terms of dose and dose-volume requirements, or biological indices such as tumor control probability (TCP) and normal tissue complication probability (NTCP).
- 35. INVERSE PLANNING OROPTIMIZATION the desired dose distribution was first defined, and then an integral equation was solved to find an appropriate beam intensity to provide it
- 36. COMPARISON Conventional forward planning depends on geometric relationship between the tumor and nearby sensitive structures. Inverse planning is less dependent on the geometric parameters more on specification of volumes of tumor targets & sensitive structures, as well as their dose constraints
- 37. COMPARISON Forward planning -Sequence Decide dose to PTV Beam arrangement Field shaping Beam modification Dose calculation Inverse planning – sequence Dose constraints Beam arrangement Plan generation Technique selected
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