introduction to Intensity modulated radiation therapy

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  • Rahim GoharMedical PhysicistDr Ziauddin Hospital*

  • ObjectivesDefine IMRTExplain the terminology used in IMRTIntroduce new advances in IMRT*

  • Forward Planning3D Conformal Therapy (not IMRT) is forward based planning.

    Planner chooses number and position of beams, shape, weighting and wedging, calculates the resulting distribution, & adjusts the beam parameters as needed

    Dose to structures is NOT specified

    *

  • Why IMRT?*Radiation Field

  • *InputOutput

  • What is IMRT ?Intensity Modulated Radiation Therapy (IMRT) is a method of radiation delivery using beams of varying intensitiesPlanner chooses Number of beams Energy DVH

    *Sharp Dose Gradient

  • *Intensity-Modulated Radiation TherapyModulate: change, vary, alter radiation intensity

    The different fluence mapsMLC specially designed pattern during treatments

    Fluence or Intensity Map

  • Work flowCT SimulationTPS PlanningPlanning Approval Pre treatment quality AssurancePass or failPre treatment Image VerificationTreatment Delivery

    *

  • Inverse Planning

    IMRT uses a different method of planning.

    The final goal - in terms of Dose/Volume for each structure is defined at the outset.

    The DVHs- Dose volume Histograms are adjusted to achieve the desired plan (rather than the beams)*

  • 9 beamsDepends on target shape and locationBeam Placement

  • Ideal objectivesPTV Lower objective:100% volume = 100% prescription doseUpper objective: None of PTV volume receive more than 100% dose OARNone of the OAR volume receive any doseNon realistic: Never practically achievableOARPTVPlanning Objectives (Constraints)

  • Realistic objectivesPTV Lower objective100% volume = 95% prescription doseUpper objective None of PTV volume receive more then 107% of prescription dose OAR (serial organs)None of the OAR volume receive more then tolerance dose

    OARPTVPlanning Objectives (Constraints)

  • Dose Volume Histogram

    *BrainstemBrStem

  • Dose and Volume Constraints*

  • *InputOutput

  • Comparisons Forward planning 3D conformal1. Beam angles2. Field size, weight & energy3. Wedges, compensators4. Blocking

    Inverse planning IMRT1. Beam angles2. Energy3. Adjusting dose constraints & priorities

    *

  • Optimization

    Process where many different flounces are tried, in order to find the best (optimum) one

    *

  • *Fluence Map H&N

  • Fluence

    Levels of radiation intensity that the linac outputsOptimal FluenceThe pattern of radiation intensity that delivers the best plan - determined by the software during optimization

    *

  • Actual fluence What the treatment unit is able to deliver considering physical parameters of the mlc (max.leaf speed, leaf transmission etc.)

    *

  • Optimization constraints

    Define desired plan in terms of the Dose/Volume each structure can receive

    Assign a priority to each point

    *

  • Priorities

    The priority is specified for each dose constraint points

    It defines the importance of that point relative to all other points for all structures

    *

  • Dose and Volume Constraints*

  • Iterations

    The beam intensities are adjusted many times during optimization, and many plans calculated

    Each adjustment is 1 iteration*

  • Leaf Motion Calculator

    MLC movement used during treatment is calculated by the Leaf Motion Calculator the output from this is the actual fluence*

  • Dose Volume Histogram (post planning)*

  • Pre treatment verification*Patient specific QA Can be done usingPortal dosimetryExternal deviceFilm base dosimetry

  • Delivery2 main ways of delivering the treatment;Segmented Delivery Also described as Step and Shoot - the beam is OFF as the MLCs move to their next position*

  • DeliverySliding Window (Dynamic Delivery).

    The beam remains ON as MLCs move automatically to their next position*

  • SummaryForward planningIMRTDose volume histogramInverse planningOptimizationDose constraintsPrioritiesIterationsCost functions and penaltiesFluence actual and optimalLeaf motion calculatorSegmented deliveryStep and shoot

    *Take home message IMRT is all about Volumes

  • *Questions?Comments?

    **Today I am going to talk about defining IMRT, explaining the terminology used in IMRT and new advances in IMRT. However the focus of my of my presentation is going to the terminology that is used in IMRT planning.*Before we get into IMRT terminology I want to explain what we do when planning 3d conformal radiation therapy. In 3d conformal planning for eg a ct based 3 fld rectum plan the beam parameters are inputted & manipulated to get a desired dose distribution.it is by manipulating the beam parameters for example wedges ,we get a dose distribution.*Beautiful B.C*This slide demonstrates what happens when a 3 fld pelvis is planned. beam parameters such as the wedges angles, etc are inputted into the comp system and the resultant output that is dose distribution is obtained.* In IMRT the beam is modulated by using different MLC patterns that the planning system produces after going through hundreds of calculations . These modulations correlate directly to the dose that is permitted to the determined volume of each structure specified by the planner...therefore if the planner says cord 50% of the cord can get 2500cGy for eg the system will try its best to achieve that. Of course the huge advantage of IMRT is the sparing of critical structures and as there are many structures that need sparing ,the planner must decide how to best give that information to the system to get the optimal plan.

    *Radiation pattern for a beam ,showing the intensity pattern formed by the MLCs*Dose Volume Histogram or DVH is defined as graph where each structure in the plan, can be viewed for the volume and dose that it is receiving. In IMRT by changing the DVH of particular structure the beam intensities are manipulated and the desired plan is acheivedBeam angle optimizn not used as the more modulated beams used, the less imp is their direction of incidenceNo significant benefit, its a non-convex problem, trapped in local minima**The dose volume histogram is used in two steps in IMRT planning. 1 is when we interactively adjust the doses and priorities of structures during optimization as the graph shows, and the other is when we have the final plan where we are able check exactly the volume and dose a particular structure is receiving. Brainstem*The table on the left specifies the volume & dose to be received by each structure . On the right the software tries to match these criteria as set by the planer.*So just as when you want to go form vanc to Coquitlam you tell the comp that Coquitlam is the end product and it finds the most efficient route to get you there, in Inverse planning you tell the planning system what doses you will accept to all the different structures, and it tells you the best and most efficient way to get those.So just as when you want to go form vanc to Coquitlam you tell the comp that Coquitlam is the end product and it finds the most efficient route to get you there, in Inverse planning you tell the planning system what doses you will accept to all the different structures, and it tells you the best and most efficient way to get there..So just as when you want to go form vanc to Coquitlam you tell the comp that Coquitlam is the end product and it finds the most efficient route to get you there, in Inverse planning you tell the planning system what doses you will accept to all the different structures, and it tells you the best and most efficient way to get there..So just as when you want to go form vanc to Coquitlam you tell the comp that Coquitlam is the end product and it finds the most efficient route to get you there, in Inverse planning you tell the planning system what doses you will accept to all the different structures, and it tells you the best and most efficient way to get those..*So while with forward planning the beam is modulated by wedges etc , with IMRT it is modulated by adjusting the dose constraints and priorities.*So hundreds of different mlc patterns and beam weights are tried by the computer to get to the specified dose goals that are set to the different structures by the planner.*This slide shows a fluence map in IMRT* Fluence is radiation intensity pattern & is calculated as the number of photons per given area .Optimal Fluence is what the planning system would like to deliver in a perfect world. * These are set to tell the planning system how important it is to cover the PTVs and the spare the critical structures also known as Organs At Risk OARs.*The priority is used as a weighting factor in the optimization process.For example the sparing of the Optic Chiasm could be more important than the spinal cord and therefore the Optic chiasm would have a higher priority. Only ptvs have upper & lower priorities. Upper for max dose & lower for minimum dose-all critical structures have upper only to limit dose.The highest priority is most often set on the PTVs as it is essential that these are covered with the appropriate dose*The table on the left specifies the volume & dose to be received by each structure . On the right the software tries to match these criteria as set by the planer.*Plans are run with hundreds of iterations , so that lots of different options are tried.*This is what the Linac can actually deliver taking into account the MLC limitations ie leaf size, rounded edge, tongue & groove. etc.*DVH to show exactly

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