QA for the Radiotherapy Salih Arican, M.Sc. Quality 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? What’s the Worst that Could Happen? Worst •Patient Death •Severe Complication •Bad administration •Major Treatment Deviation •Minor Treatment Deviation •Litigation •Lost Revenue Least FDA Adverse Event Report (06/16/2004): Patient Overdosed by 13.8% Patient subsequently died as a result of complications related to the mistreatment FDA Adverse Event Report (04/07/2005) : •Medical center reported that between 2004 and 2005 77 pts received radiation approx 52% in excess of their prescribed dose •The excess radiation was a result of a calculation error by the medical center physicist during calibration •This incident has been recognized/identified as "human error" FDA Adverse Event Report (04/22/2005) •Prostate IMRT patient treated to a higher dose than prescribed •Reported as Medical Physics user error The overall accuracy of (IMRT) treatment depends on … Reasons for errors Delivery errors TPS commissioning TPS algorithm weaknesses Organ Motion Patient Positioning Mechanical accuracy of LINAC • Gantry • Collimator • isocenter Explanations for Failures Explanation Minimum # of occurrences incorrect output factors in TPS 1 incorrect PDD in TPS 1 Software error 1 inadequacies in beam modeling at leaf ends (Cadman, et al; PMB 2002) 14 not adjusting MU to account for dose differences measured with ion 3 chamber errors in couch indexing with Peacock system 3 2 mm tolerence on MLC leaf position 1 setup errors 7 target malfunction 1 What is the Optimal Tool? Reliable and Cost-effective QA provide 3-D data QA for IMRT: 4 Levels • Pre-Clinical verification of IMRT treatment (patient related) 4 • Verification of fluence maps, individual IMRT fields on water phantom 3 • IMRT delivery specific QA 2 • Basic QA (LINAC, MLC) 1 (IMRT) – QA Plan IMRT-QA Plan Comissioning and testing of the Routine QA of Patient-Specific validation treatment planning the delivery system of treatment plans and delivery system Accuracy of relative Transmission characteristics MLC leaf position (leakage) of the leafs The flatness and symmetry Penumbra of the leaf ends of the beam Dose-per-MU constancy Speed of each leaf Routine QA of the delivery system • Does the radiation delivered have: The correct energy? The correct place? The correct dose? The correct intensity? The correct time? Beam Stability: Flatness,Symmetry Stability of flatness and symmetry affects dose rate for small fields directed off the central axis. Beam Stability: Dose Rate With IMRT delivery, there is the potential for short irradiation times (MUs). Dose rate stability influences the treatment precision. Linac-QA: Dose Rate Linac-QA: Dose / Pulse Linac-QA: Beam Start-Up Linac-QA: Beam Position Stabilization Time LINAC-QA: Dose delivery 8) 6) 15) 18) 286,7 355,2 85,0 10,0 7) 5) 335,1 392,0 4) 3) 2) 13) 16) 423,9 459,3 515,3 124,5 47,6 11) 9) 196,9 257,0 12) 10) 14) 17) 147,4 216,1 79,3 7,7 Planned dose value pattern (18 steps, dose values in cGy) + + = Multiple Beam ‘Segments’ Resultant IMRT Each with a Different MLC Shape Beam Intensity Map Measured Calculated MLC QA - check the influence of gravity MLC Delivery Error at Gantry 90 deg Gantry 0 deg Individual segments Gantry 90 deg After error analysis & correction leaf positioning failure! Error in jaw position: Plan measured difference 1.8 mm Y1 jaw displaced by 1.8 mm Profiles __ plan __ measured Leaf 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% MLC 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) Regular Pattern (golden standard) Regular Pattern Measured Pattern 1.0 mm 1.0 mm 0.9 mm 0.8 mm 0.7 mm 0.6 mm 0.5 mm 0.4 mm 0.5 mm 0.3 mm 0.2 mm 0.1 mm Leaf speed accuracy The accuracy of dynamic MLC delivery depends on the accuracy with which the speed of each leaf is controlled. MLC QA – Leaf Speed Test Leaf pairs form gaps moving with different speed Delivery with beam interrupts Leaf 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. Interleaf Transmission Leaf End Transmission All Leaves Closed Radiation Leaks through between Completely Leaves and Across Ends Interleaf Transmission Treatment Field Collimator Jaw Collimator Covers Field Leaks between Sides Reduced with Up to Outermost Leaf Backup Collimator Transmission (Leakage) Check Patient-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 verification – What is the influence of revealed discrepancies on the dose distribution? Pre-Treatment Verification Field oriented Plan oriented Gantry =0° Rotating Gantry X- X- Comparison of predicted and measured MLC-Shapes MC-2 MC-SW Deliveredfluence RTPS:Desired Leaf- & Gantry 3D-Dose- Distribution RTPS:Desired Fluence- Leaf Sequencer sequence Map Inverse Back-Projection Delivered 3D-Dose- Distribution ArcCHECK Patient-Specific validation of treatment plans Inverse Back-Projection Treatment MLC Delivery 2D-Array Planning Segmentation System /3D-Array Delivered Fluence Measured fluence map comparison Predicted: -------- Measured: Predicted fluence map DICOM_RT DOSE plan: Beam1: G=210 C=180 segments=20 Beam2: G=260 C=180 segments=12 Beam3: G=310 C=180 segments=18 Beam4: G=0 C=180 segments=18 Beam5: G=50 C=180 segments=22 Beam6: G=100 C=180 segments=10 Beam7: G=150 C=180 segments=16 Plan oriented verification with 2D-Array Beam 1: Gantry 210 degree Beam 2: Gantry 260 degree Beam 3: Gantry 310 degree Beam 4: Gantry 0 degree Beam 5: Gantry 50 degree Beam 6: Gantry 100 degree Beam 7: Gantry 150 degree Measured (composite) Beam 1 … Beam 7: Measured Calculated IMRT-Composite field verification (MC-SW): Pass-rate: 97.5 % Plan oriented verification with ArcCHECK ArcCHECK in action VARIAN RapidArc Inselspital Bern-Switzerland Arc-1: Pass-Rate: 98%; Gamma: 3mm/3% RD-Oxford Cancer Center H&N.dcm converted in AC_PLAN.txt RD-Oxford Cancer Center H&N.dcm converted in AC_PLAN.txt imported as 2D composite plan The 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! Film dosimetry: Plan oriented workflow 1. Planning of IMRT cycle for 2. Planning of same patient with RTPS IMRT cycle but now with Body Phantom 3. Exposure of film in Body Phantom to IMRT cycle 5. Import of planned and measured data in analysis SW 4. Development and digitization of 6. Comparison of planned versus measured dose exposed film Film The 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 Quantity Calculation Measurement 3D-Dose Distribution Apply Plan to Phantom. Put Films in the Phantom. Process, Calculate 3D-Dose Scan, Calibrate Films. Compose 3D- Distribution Dose Distribution 2D-Dose/Fluence Calculate Fluence Pattern or Film, 2D-Array, 3D-Array 2-D Dose Distribution Leaf Positions Leaf Positions from TPS Film, 2D-Array, 3D-Array, MLC QA MU/Dose Check Dose in a reference Point Ion-Chamber/Electrometer Penumbra Needed for TPS Small Ion Chamber or Diode (SFD) in measurement Set-up 3D-Phantom Conclusions Quality 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. Conclusions Quality assurance not only reduces the likelihood of accidents and errors occurring, it also increases the probability that they will be recognized and rectified sooner if they do occur, thereby reducing their consequences for patient treatment. Conclusions Quality assurance allows a reliable comparison of results among different radiotherapy centers, ensuring a more uniform and accurate dosimetry and treatment delivery. Conclusions Improved technology and more complex treatments in modern radiotherapy can only be fully exploited if a high level of accuracy and consistency is achieved. Thank you, Questions?
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