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

n

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?