Rahim GoharMedical PhysicistDr Ziauddin Hospital

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ObjectivesDefine IMRTExplain the terminology used in IMRTIntroduce new advances in IMRT

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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

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Why IMRT?

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Radiation Field

Prostate

Rectum

Prostate

Rectum

Prostate

Rectum

Pre-IMRT

IMRT IGRT

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70%

80%

90%

40%

Input

Output

What is IMRT ?Intensity Modulated Radiation Therapy (IMRT) is a

method of radiation delivery using beams of varying intensities

Planner chooses Number of beams Energy DVH

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Sharp Dose Gradient

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Intensity-Modulated Radiation Therapy

Modulate: change, vary, alter radiation intensity

The different fluence mapsMLC specially designed pattern during treatments

Fluence or Intensity Map

Work flow

CT SimulationTPS PlanningPlanning Approval Pre treatment quality AssurancePass or failPre treatment Image VerificationTreatment Delivery

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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)

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9 beams

80

120

150180210

240

280

7 beams

•Depends on target shape and location

Beam Placement

Ideal objectives PTV

Lower objective:

100% volume = 100% prescription dose

Upper objective: None of PTV volume receive

more than 100% dose

OAR

None of the OAR volume receive any dose

Non realistic: Never practically achievable

OAR

PTV

Planning Objectives (Constraints)

Realistic objectivesPTV

Lower objective 100% volume = 95% prescription dose

Upper 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

OAR

PTV

Planning Objectives (Constraints)

Dose Volume Histogram

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BrainstemBrStem

Dose and Volume Constraints

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70%

80%

90%

40%

!

Input

Output

Comparisons Forward planning 3D conformal1. Beam angles2. Field size, weight &

energy3. Wedges,

compensators4. Blocking

Inverse planning IMRT1. Beam angles2. Energy3. Adjusting dose

constraints & priorities

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Optimization

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

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Fluence Map H&N

FluenceLevels of radiation intensity that the linac outputsOptimal FluenceThe pattern of radiation intensity that delivers the

best plan - determined by the software during optimization

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Actual fluence What the treatment unit is able to deliver –

considering physical parameters of the mlc (max.leaf speed, leaf transmission etc.)

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Optimization constraints

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

Assign a priority to each point

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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

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Dose and Volume Constraints

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Iterations

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

Each adjustment is 1 iteration

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Leaf Motion CalculatorMLC movement used during treatment is calculated by the Leaf

Motion Calculator – the output from this is the actual fluence

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Optimal Fluence

Leaf Motion Calculator

Actual Fluence

Optimization

Plan

Dose Volume Histogram (post planning)

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Pre treatment verification

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Patient specific QA Can be done usingPortal dosimetryExternal deviceFilm base dosimetry

Delivery

2 main ways of delivering the treatment;1. Segmented Delivery Also described as Step and Shoot - the beam is OFF as the MLCs move to their next position

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Delivery2. Sliding Window (Dynamic Delivery).

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

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Summary Forward planning IMRT Dose volume histogram Inverse planning Optimization Dose constraints Priorities Iterations Cost functions and penalties Fluence – actual and optimal Leaf motion calculator Segmented delivery Step and shoot

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Questions?Comments?