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? Least Patient Death Severe Complication Bad administration Major Treatment Deviation Minor Treatment Deviation Litigation Lost Revenue

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 Patient Positioning Organ Motion Delivery errors TPS commissioning TPS algorithm weaknesses Organ Motion Patient Positioning

Mechanical accuracy of LINAC • Gantry Collimator isocenter

Explanations for Failures Minimum # of occurrences incorrect output factors in TPS 1 incorrect PDD in TPS Software error inadequacies in beam modeling at leaf ends (Cadman, et al; PMB 2002) 14 not adjusting MU to account for dose differences measured with ion chamber 3 errors in couch indexing with Peacock system 2 mm tolerence on MLC leaf position setup errors 7 target malfunction

No compromise with accuracy What is the Optimal Tool? Reliable and Cost-effective QA No compromise with accuracy Save time for setup, measuring, and analysis Versatility to use Less need for human resource Excellent spatial resolution provide 3-D data

QA for IMRT: 4 Levels Pre-Clinical verification of IMRT treatment (patient related) Verification of fluence maps, individual IMRT fields on water phantom IMRT delivery specific QA Basic QA (LINAC, MLC) 4 3 2 1

(IMRT) – QA Plan

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? 14 14

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) 286,7 6) 355,2 15) 85,0 18) 10,0 7) 335,1 5) 392,0 4) 423,9 3) 459,3 2) 515,3 13) 124,5 16) 47,6 11) 196,9 9) 257,0 12) 147,4 10) 216,1 14) 79,3 17) 7,7 Planned dose value pattern (18 steps, dose values in cGy)

+ = Multiple Beam ‘Segments’ Resultant IMRT Each with a Different MLC Shape Resultant IMRT Beam Intensity Map + =

Measured Calculated

MLC QA - check the influence of gravity

MLC Delivery Error at Gantry 90 deg Individual segments Gantry 90 deg After error analysis & correction leaf positioning failure!

Error in jaw position: Plan measured difference Profiles __ Y1 jaw displaced by 1.8 mm

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 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

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.

All Leaves Closed Completely Radiation Leaks through between Leaves and Across Ends Interleaf Transmission Leaf End Collimator Covers Field Up to Outermost Leaf Leaks between Sides Reduced with Backup Collimator Treatment Field Collimator Jaw

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° X - Rotating Gantry

Comparison of predicted and measured MLC-Shapes Inverse Back-Projection Leaf Sequencer Delivered 3D-Dose- Distribution RTPS:Desired 3D-Dose- Distribution RTPS:Desired Fluence-Map Leaf- & Gantry sequence Deliveredfluence MC-2 MC-SW ArcCHECK

Patient-Specific validation of treatment plans Inverse Back-Projection Delivery System Treatment Planning MLC Segmentation 2D-Array /3D-Array Delivered Fluence

Predicted: --------Measured: Measured fluence map comparison Predicted: --------Measured: Predicted fluence map

DICOM_RT DOSE plan: Beam1: G=210 C=180 segments=20

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 7: Gantry 150 degree Beam 5: Gantry 50 degree Beam 6: Gantry 100 degree

Measured (composite) Beam 1 … Beam 7:

IMRT-Composite field verification (MC-SW): Pass-rate: 97.5 % Measured Calculated IMRT-Composite field verification (MC-SW): Pass-rate: 97.5 %

Discussions – Information Weight 0º: beam is normal to the array and all information is weighted equally 90º: 2D reduced to 1D, most information is lost Other angles: variable weights Dosimetric information is over or under-weighted based on beam angle and field size 0º 90º

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 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 patient with RTPS 2. Planning of same 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 exposed film 6. Comparison of planned versus measured dose

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. Calculate 3D-Dose Distribution Put Films in the Phantom. Process, Scan, Calibrate Films. Compose 3D-Dose Distribution 2D-Dose/Fluence Calculate Fluence Pattern or 2-D Dose Distribution Film, 2D-Array, 3D-Array Leaf Positions MLC QA Leaf Positions from TPS Film, 2D-Array, 3D-Array, MU/Dose Check Dose in a reference Point Ion-Chamber/Electrometer Penumbra measurement Needed for TPS Set-up Small Ion Chamber or Diode (SFD) in 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?