2Quality Assurance Why do we need (IMRT) QA? Do I really need to do QA for each IMRT patient?If I use an independent Monitor Unit calculation program do I still need QA for each Patient?Will I still need do IMRT QA after we’ve treated 500 patients?If I expand my monthly machine QA can I eliminate IMRT QA for each patient?
3What’s the Worst that Could Happen? LeastPatient DeathSevere ComplicationBad administrationMajor Treatment DeviationMinor Treatment DeviationLitigationLost Revenue
4FDA Adverse Event Report (06/16/2004): Patient Overdosed by 13.8%Patient subsequently died as aresult of complications relatedto the mistreatment
5FDA Adverse Event Report (04/07/2005) : Medical center reported that between 2004 and pts received radiation approx 52% in excess of their prescribed doseThe excess radiation was a result of a calculation error by the medical center physicist during calibrationThis incident has been recognized/identified as "human error"
6FDA Adverse Event Report (04/22/2005) Prostate IMRT patient treated to a higher dose than prescribedReported as Medical Physics user error
7The overall accuracyof (IMRT) treatmentdepends on …
8Reasons for errors Patient Positioning Organ Motion Delivery errorsTPS commissioningTPS algorithm weaknessesOrgan MotionPatient Positioning
9Mechanical accuracy of LINAC • GantryCollimatorisocenter
10Explanations for Failures Minimum # of occurrencesincorrect output factors in TPS1incorrect PDD in TPSSoftware errorinadequacies in beam modeling at leaf ends (Cadman, et al; PMB 2002)14not adjusting MU to account for dose differences measured with ion chamber3errors in couch indexing with Peacock system2 mm tolerence on MLC leaf positionsetup errors7target malfunction
11No compromise with accuracy What is the Optimal Tool?Reliable andCost-effective QANo compromise with accuracySave time for setup, measuring, and analysisVersatility to useLess need for human resourceExcellent spatialresolutionprovide3-D data
12QA for IMRT: 4 LevelsPre-Clinical verification of IMRT treatment (patient related)Verification of fluence maps, individual IMRT fields on water phantomIMRT delivery specific QABasic QA (LINAC, MLC)4321
26Error in jaw position: Plan measured difference Profiles __ Y1 jaw displaced by1.8 mm
27Leaf position uncertainties Beam widths of 1 cm, uncertainties of a few tenths of a millimeter in leaf position can cause dose uncertainties of several percent.e.g. 0.5mm >5%
28MLC QA: Accuracy of relative MLC leaf position Leaf positioning accuracy:MLC pairs form a narrow slot moving across the field, stopping and reaccelerating at predefined positions (garden fence technique)
311.0 mm 0.5 mm 1.0 mm 0.9 mm 0.8 mm 0.7 mm 0.6 mm 0.5 mm 0.4 mm 0.3 mm
32Leaf speed accuracyThe accuracy of dynamic MLC delivery depends on the accuracy with which thespeed of each leaf is controlled.
33MLC QA – Leaf Speed TestLeaf pairs form gaps moving with different speedDelivery with beam interrupts
34Leaf transmission characteristic The transmission characteristics (leakage) of the MLC are important for IMRT because the leaves shadow the treatment area for a large fraction of the delivered MU.
35All Leaves Closed Completely Radiation Leaks through between Leaves and Across EndsInterleafTransmissionLeaf EndCollimator Covers Field Up to Outermost LeafLeaks between Sides Reduced with Backup CollimatorTreatment FieldCollimator Jaw
38Patient-specific Verification ? What is missing :Does the plan give correct dose distribution ?Does it fulfill the therapeutic requirements ?What is the influence of inter-fraction variation ?In case of 2D verificationWhat is the influence of revealed discrepancies on the dose distribution?
39Pre-Treatment Verification Field oriented Plan orientedGantry =0°X-Rotating Gantry
40Comparison of predicted and measured MLC-Shapes Inverse Back-ProjectionLeaf SequencerDelivered 3D-Dose- DistributionRTPS:Desired D-Dose- DistributionRTPS:Desired Fluence-MapLeaf- & Gantry sequenceDeliveredfluenceMC-2MC-SWArcCHECK
41Patient-Specific validation of treatment plans InverseBack-ProjectionDeliverySystemTreatmentPlanningMLCSegmentation2D-Array/3D-ArrayDelivered Fluence
48IMRT-Composite field verification (MC-SW): Pass-rate: 97.5 % Measured CalculatedIMRT-Composite field verification (MC-SW): Pass-rate: 97.5 %
49Discussions – Information Weight 0º: beam is normal to the array and all information is weighted equally90º: 2D reduced to 1D, most information is lostOther angles: variable weightsDosimetric information is over or under-weighted based on beam angle and field size0º90º
53RD-Oxford Cancer Center H&N. dcm converted in AC_PLAN RD-Oxford Cancer Center H&N.dcm converted in AC_PLAN.txt RD-Oxford Cancer Center H&N.dcm converted in AC_PLAN.txt importedas 2D composite planThe difference is clear: Cold-spot value at the gantry angle x1 degree might be balanced with hot-spot value at the gantry angle degree x2. That effect can't be seen in composite analysis result but with ArcCHECK measured and unrolled fields!
54Film dosimetry: Plan oriented workflow 1. Planning of IMRT cycle for patient with RTPS2. Planning of same IMRT cycle but nowwith Body Phantom3. Exposure of film in Body Phantom to IMRT cycle5. Import of planned and measured datain analysis SW4. Development and digitization of exposed film6. Comparison of planned versus measured dose
55FilmThe choise of film is very important. But even more important is the calibration of the film and the stability of the film processing environment and chemistry
56QuantityCalculationMeasurement3D-Dose DistributionApply Plan to Phantom. Calculate 3D-Dose DistributionPut Films in the Phantom. Process, Scan, Calibrate Films. Compose 3D-Dose Distribution2D-Dose/FluenceCalculate Fluence Pattern or 2-D Dose DistributionFilm, 2D-Array, 3D-ArrayLeaf PositionsMLC QALeaf Positions from TPSFilm, 2D-Array, 3D-Array,MU/Dose CheckDose in a reference PointIon-Chamber/ElectrometerPenumbra measurementNeeded for TPSSet-upSmall Ion Chamber or Diode (SFD) in 3D-Phantom
57ConclusionsQuality assurance reduces uncertainties and errors in dosimetry, treatment planning, equipment performance, treatment delivery, etc., thereby improving dosimetric and geometric accuracy and the precision of dose delivery.
58ConclusionsQuality assurance not only reduces the likelihood of accidents and errorsoccurring, it also increases the probability that they will be recognized and rectified sooner if they do occur, thereby reducing their consequences for patient treatment.
59ConclusionsQuality assurance allows a reliable comparison of results amongdifferent radiotherapy centers, ensuring a more uniform and accurate dosimetry and treatment delivery.
60ConclusionsImproved technology and more complex treatments in modern radiotherapycan only be fully exploited if a high level of accuracy andconsistency is achieved.