QA for the Radiotherapy Salih Arican, M.Sc.
Mar 28, 2015
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?
•Patient Death•Severe Complication•Bad administration•Major Treatment Deviation•Minor Treatment Deviation•Litigation•Lost Revenue
Worst
Least
FDA Adverse Event Report(06/16/2004):
Patient Overdosed by 13.8%
Patient subsequently died as aresult 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 FailuresExplanation 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 chamber
3
errors in couch indexing with Peacock system 3
2 mm tolerence on MLC leaf position 1
setup errors 7
target malfunction 1
No compromise
with accuracy
Less need for
human resource
Save time for setup, measuring, and analysis
Versatility to use
Reliable and
Cost-effective QA
What is the Optimal Tool?
Excel
lent
spa
tial
reso
lutio
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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
IMRT-QA Plan
Comissioning and testing of the treatment planning and delivery system
Routine QA of the delivery system
Patient-Specific validationof treatment plans
Dose-per-MU constancy
Transmission characteristics (leakage) of the leafs
Accuracy of relative MLC leaf position
Speed of each leaf
The flatness and symmetry of the beam Penumbra of the leaf ends
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 thetreatment 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’Each with a Different MLC Shape
Resultant IMRTBeam Intensity Map
+ + =
Measured Calculated
MLC QA - check the influence of gravity
MLC Delivery Error at Gantry 90 deg
Individual segments
Gantry 0 deg
leaf positioning failure!After error analysis & correction
Gantry 90 deg
Error in jaw position:
Plan
measured difference
Profiles __ plan __ measuredY1 jaw displaced by 1.8 mm
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
MLC pairs form a narrow slot moving across the field, stopping and reaccelerating at predefined positions (garden fence technique)
Leaf positioning accuracy:
Regular Pattern(golden standard)
Regular Pattern Measured Pattern
1.0 mm
0.9 mm
0.8 mm
0.7 mm
0.6 mm
0.5 mm
0.4 mm
0.3 mm
0.2 mm
0.1 mm
1.0 mm
0.5 mm
Leaf speed accuracy
The accuracy of dynamic MLC delivery depends on the accuracy with which thespeed 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
InterleafTransmission
Leaf EndTransmission
Collimator Covers Field Up to Outermost Leaf
Leaks between Sides Reduced with Backup Collimator
InterleafTransmission
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
X-
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
TreatmentPlanning
MLCSegmentation
DeliverySystem
2D-Array/3D-Array
InverseBack-
Projection
Delivered Fluence
Measured fluence map
Predicted fluence map
comparison
Predicted: --------Measured:
Beam1: G=210 C=180 segments=20Beam2: G=260 C=180 segments=12Beam3: G=310 C=180 segments=18Beam4: G=0 C=180 segments=18Beam5: G=50 C=180 segments=22Beam6: G=100 C=180 segments=10Beam7: G=150 C=180 segments=16
DICOM_RT DOSE plan:
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%
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!
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
Film dosimetry: Plan oriented workflow
6. Comparison of planned versus measured dose
3. Exposure of film in Body Phantom to IMRT cycle
4. Development and digitization of exposed film
5. Import of planned and measured data in analysis SW
1. Planning of IMRT cycle for patient with RTPS
2. Planning of same IMRT cycle but nowwith Body Phantom
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 PositionsMLC 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 TPSSet-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 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.
Conclusions
Quality assurance allows a reliable comparison of results amongdifferent radiotherapy centers, ensuring a more uniform and accurate dosimetry and treatment delivery.
Conclusions
Improved technology and more complex treatments in modern radiotherapycan only be fully exploited if a high level of accuracy andconsistency is achieved.
Thank you,Questions?