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Three dimensional conformal radiation therapy
Three Dimensional Conformal Radiation Therapy involves the following key steps: 1. Image acquisition using CT or MRI to obtain 3D anatomical information of the patient. 2. Target and critical structure delineation through image segmentation to define volumes of interest. 3. Treatment planning using a 3D planning system to design optimized beam arrangements and apertures that conform the dose distribution to the target volume while minimizing dose to surrounding tissues. 4. Plan evaluation using tools like isodose distributions, dose-volume histograms and color washes to evaluate the dose coverage of targets and sparing of critical structures before finalizing the plan.
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Overview of the presentation topics related to the 3D CRT and its treatment planning process.
Highlights the differences in treatment planning for 3D CRT compared to conventional therapy, emphasizing 3D anatomy and optimization.
Describes image acquisition techniques for 3D planning, focusing on CT simulations and their importance in identifying treatment positions.
Defines image registration and its importance, detailing various techniques for correlating different imaging modalities.
Details the process of image segmentation in treatment planning, its critical nature, and volume definitions.
Discusses beam aperture design, BEV and DRR usage, and considerations for multi-field setups in radiation therapy.
Explains 3D TPS capabilities, forward planning, and strategy for optimizing treatment plans based on multiple fields.
Describes various methods for evaluating treatment plans, including isodose curves and dose-volume histograms.
Details the steps required for plan implementation, including patient positioning and verification corrections.
Outlines the different dose calculation algorithms used in treatment planning, highlighting the accuracy of Monte Carlo methods.
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Three dimensional conformal radiation therapy
- 1. Three Dimensional Conformal RadiationTherapy -The treatment planning process Dr Deepika Malik JR III, Dept of Radiotherapy
- 2. • 3 DCRT is based on 3-D anatomic information and use dose distributions that conform as closely as possible to the target volume in terms of adequate dose to the tumor and minimum possible dose to adjacent normal tissue
- 4. Overwiew • Main distinctionbetween treatment planning of 3-D CRT and conventional radiation therapy – it requires : 3-D anatomic information : treatment-planning system that allows optimization of dose distribution which meets the clinical objectives.
- 5. 3 D treatmentplanning process
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- 8. • Preplanning process •proposed treatment position of the patient is determined. • immobilization device is fabricated. • It is an Important step--Errors may occur if patients are inadequately immobilized, with resultant treatment fields inaccurately aligned from treatment to treatment .
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- 11. • Image acquisitionis the foundation of 3 D panning. • The anatomic information is usually obtained in the form of closely spaced transverse images, which can be processed to reconstruct anatomy in any plane, or in three dimensions
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- 13. CT Image- mostcommonly used • CT image -reconstructed from a matrix of relative linear attenuation coefficients measured by the CT scanner.
- 14. CT SIMULATOR • Imagesare acquired on a dedicated CT machine called CT simulator with following features – A large bore (75-85cm) to accommodate various treatment positions along with treatment accessories. – A flat couch insert to simulate treatment machine couch. – A laser system consisting of • Inner laser • External moving laser to position patients for imaging & for marking - A graphic work station
- 15. • CT isdone with patient in the treatment position with immobilization device • Radio opaque fiducial are placed . Intention is to place these initial marks as close to final isocentre as possible • These fiducial assist in any coordinate transformation needed as a result of 3D planning and eventual plan implementation. • The planning CT data set is transferred to a 3D-TPS or workstation via a computer network.
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- 19. • The termimage registration -- a process of correlating different image data sets to identify corresponding structures or regions. • Allows full voxel to voxel intensity match • Image Fusion automatically correlates thousands of points from two image sets, providing true volumetric fusion of anatomical data sets.
- 20. • For example--,mapping of structures seen in MRI onto the CT images. • Various registration techniques include – Point-to-point fitting, – Line or curve matching – Surface or topography matching – Volume matching
- 21. MRI IMAGE CT IMAGE CONTOURINGON BLENDED IMAGE POINT TO POINT MATCHING IMAGE FUSION
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- 23. Segmentation • slice-by-slice delineationof anatomic regions of interest-- external contours, targets, critical normal structures, anatomic landmarks, etc. • The radiation oncologist draws the target volumes in each slice with appropriate margins to include visible tumor, the suspected tumor spread, and patient motion uncertainties. • The segmented regions can be rendered in different colors
- 24. • one ofthe most laborious but important processes in treatment planning. • It requires clinical judgment, which cannot be automated or completely image based. • It should not be delegated to personnel other than the physician in charge of the case, the radiation oncologist.
- 25. Manual segmentation Autosegmentation Contours drawn by auto-deformation from another contour
- 26. Volume definition • Prerequisitefor 3-D treatment planning. • ICRU reports50 & 62 define & describe target & critical structure volumes.
- 27. • volumes definedprior to treatment planning – Gross tumor volume (GTV). – Clinical target volume (CTV). • Defined during the treatment planning process – Planning target volume (PTV). – Organs at risk. • As a result of treatment planning, volumes described. – Treated volume (TV). – Irradiated volume (IRV).
- 43. Beam aperture design aidedby • the BEV ( Beam’s eye view)capability of the 3- D treatment-planning system • DRR
- 44. Digitally Reconstructed Radiograph-DRR •A synthetic radiograph produced by tracing ray-lines from a virtual source position through the CT data to a virtual film plane . • It is analogous to conventional simulation radiographs.
- 45. • DRR isused – for treatment portal design – for verification of treatment portal by comparison with port films or electronic portal images
- 46. Beam Eye View-BEV •In BEV observer’s viewing point is at the source of radiation looking out along axis of radiation beam. • Targets and critical normal structures visible in different colors through segmentation can be viewed from different directions in planes perpendicular to the beam's central axis. – Demonstrates geometric coverage of target volume by the beam – Shielding & MLCs are designed on BEV – Useful in identifying best gantry, collimator, and couch angles to irradiate target & avoid adjacent normal structures
- 48. Beam apertures canbe designed • Automatically - the user sets a uniform margin around the PTV. • Manual- when its needed to draw a nonuniform margin • Generally, a 2-cm margin between the PTV and the field edge ensures better than 95% isodose coverage of the PTV
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- 50. • For planning,the 3D TPS must have the capability to simulate each of the treatment machine motion functions, including – Gantry angle, – Collimator length, width & angle, – MLC leaf settings, – Couch latitude, longitude, height & angle
- 51. FORWARD PLANNING • For3D CRT forward planning is used. • Beam arrangement is selected. • Using BEV, beam aperture is designed • Dose is prescribed. • 3D dose distribution is calculated. • Then plan is evaluated. • Plan is modified based on dose distribution evaluation, using various combinations of – Beam , collimator & couch angle, – Beam weights & – Beam modifying devices (wedges, compensators) to get desired dose distribution.
- 52. • Three-dimensional treatmentplanning encourages the use of multiple fields because targets and critical structures can be viewed in the BEV configuration individually for each field. • Multiple fields removes the need for using ultra- high-energy beams (>10 MV), which are required when treating thoracic or pelvic tumors with only two parallel opposed fields
- 53. • Using alarge number of fields (greater than four) creates the problem of - designing an excessive number of beam- shaping blocks - requiring longer setup times -Carrying so many heavy blocks creates a nuisance for therapists who have to guard against dropping a block accidentally or using a wrong block.
- 54. • A goodalternative to multiple field blocking is the use of a multileaf collimator (MLC) • A field drawn on a BEV printout can be digitized to set the MLC setting. • BEV field outlines can also be transmitted electronically to the accelerator to program the MLC.
- 55. Plan optimization andevaluation
- 56. Plan Optimisation • Optimisationrefers to the technique of finding the best physical and technically possible treatment plan to fulfill the specified physical and clinical criteria • An optimal plan should deliver tumoricidal dose to the entire tumor and spare all the normal tissues.
- 57. PLAN EVALUATION • Toolsused in the evaluation of the planned dose distribution: • Isodose lines • Color wash • DVHs (Dose volume histograms ) – Dose distribution statistics
- 58. Isodose curves • Dosedistributions of competing plans are evaluated by viewing isodose curves in individual slices, orthogonal planes (e.g., transverse, sagittal, and coronal), or 3-D isodose surfaces.
- 59. Colour wash • Spectrumof colors superimposed on the anatomic information represented by modulation of intensity – Gives quick over view of dose distribution – Easy to assess overdosage in normal tissue that are not contoured. – To assess dose heterogeneity inside PTV • Slice by slice evaluation of dose distribution can be done
- 60. Dose volume histograms •DVHs summarize the information contained in the 3-D dose distribution & quantitatively evaluates treatment plans. • DVHs are usually displayed in the form of ‘per cent volume of total volume’ against dose. • The DVH may be represented in two forms: – Cumulative integral DVH – Differential DVH.
- 61. CUMULATIVE DVH-more useful •It is plot of volume of a given structure receiving a certain dose. • Any point on the cumulative DVH curve shows the volume of a given structure that receives the indicated dose or higher. • It start at 100% of the volume for zero dose, since all of the volume receives at least more than zero Gy.
- 62. DIFFERENTIAL DVH • Thedirect or differential DVH is a plot of volume receiving a dose within a specified dose interval (or dose bin) as a function of dose. • It shows extent of dose variation within a given structure. • The ideal DVH for a target volume would be a single column indicating that 100% of volume receives prescribed dose. • For a critical structure, the DVH may contain several peaks indicating that different parts of the organ receive different doses. DVH - target vol. DVH - OAR
- 63. 3-D DOSE CLOUD •Map isodoses in three dimensions and overlay the resulting isosurface on a 3-D display with surface renderings of target & other contoured organs.
- 64. Dose statistics • Itprovide quantitative information on the volume of the target or critical structure and on the dose received by that volume. • These include: – The minimum dose to the volume – The maximum dose to the volume – The mean dose to the volume • Useful in dose reporting.
- 68. PLAN IMPLEMENTATION • Oncethe treatment plan has been evaluated & approved, documentation for plan implementation must be generated. • It includes – beam parameter settings transferred to the treatment machine’s record and verify system, – MLC parameters communicated to computer system that controls MLC system of the treatment machine, – DRR generation & printing or transfer to an image database.
- 69. Plan implementation • AfterPhysician contuors target volumes and determination of treatment isocentre is done • Couch shifts ( distances in three directions ) between reference marks drawn on CT Scanner and treatment centre are then calculated • On first day of treatment , patient is first positioned to initial refernce marks and then shifted to treatment isocentre using the calculated shifts . • The treatment isocentre is then marked on the patient
- 70. Position verification • Patientposition is verified and thus corrected using EPID ( electronic portal imaging device) • Both the field and the bony anatomy are matched sequentially to give an estimate of error.
- 71. Online and offlinecorrections • refer to whether the patient is on the treatment couch while the verification is being done and whether the correction would be applied to the same or subsequent sessions
- 72. Offline corrections • imagesacquired before treatment and matched to the reference image at a later time point. • Aims to determine the individual systematic setup error and thus reduce it. • When combined with setup data of other patients treated under the same protocol, it helps define the population standard error for that treatment in that institution. • PTV margins in an institution depend on these determinations of individual and population systematic errors
- 73. Online corrections • Acquisitionof images and their verification and correction prior to the day’s treatment. • Aims to reduce both random and systematic errors. • The treatment site and the expected magnitude of error may determine the frequency of online imaging. • Sites where large daily shifts are anticipated (abdomen, pelvis, and thorax) or where even slight shifts will alter the dose distribution within adjacent critical structures (paraspinal tumors, intracranial tumors in close proximity to optic structures) are best managed with daily imaging
- 75. Dose calculation • Dosecalculation algorithms ….three broad categories: (a) correction based, (b) model based, and (c) direct Monte Carlo. • Direct Monte Carlo - most accurate method for treatment planning.
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