1

Tolerance Limits and Action Levels for Planning and

Delivery of IMRT QA

Jatinder R. Palta PhDDepartment of Radiation Oncology

University of FloridaGainesville, Florida

Introduction

Many sources of error and uncertainty factor into the Many sources of error and uncertainty factor into the final dose distribution actually realized for a patientfinal dose distribution actually realized for a patient

Patient treatment planning (target delineation and dose Patient treatment planning (target delineation and dose calculation)calculation)setup uncertaintysetup uncertaintyinterinter--fraction or intrafraction or intra--fraction organ motionfraction organ motiontreatment deliverytreatment delivery

The sharp dose gradients in IMRT amplify the The sharp dose gradients in IMRT amplify the importance of these uncertainties, as even small importance of these uncertainties, as even small deviations from a planned treatment can compromise deviations from a planned treatment can compromise outcomeoutcome

ObjectivesProcess of IMRT planning and deliveryPotential sources of error in IMRT planning and deliveryQA strategy for:

IMRT planningIMRT deliveryPatient specific QA

Tolerance limits and action levels for planning and delivery of IMRTResource requirements for IMRT QA

The Overall Process of IMRT Planning and Delivery

IMRT Treatment Planning

Image Acquisition(Sim,CT,MR, …)

Structure Segmentation

Positioning and Immobilization

File Transfer and Management

IMRT Treatment Delivery Plan Validation

Position Verification

1 2 3 4

5 6 7 8

Positioning and Immobilization

Image Acquisition

Structure Segmentation

IMRT Treatment Planning and Evaluation

File Transfer and and Management

Plan ValidationIMRT Treatment Delivery and Verification

‘Chain’ of IMRT Process

Position Verification

Potential Sources of Error in IMRT

CT/MR/PET roomLasersSkin markersImagesRadio. anatomyTumorDelineationMarginPlanned beams

Treatment roomLasersSkin markersRadio. anatomyTumor

BeamsAcceleratorTreatment room

17 steps with a lot of room for errors!

Patient setup parameters

beam delivery data

patient

2

Types of Errors

17 small errors add up. Sometimes large errors occur: when most of these 17 errors are in the same direction

Errors that are made once per patient:– Treatment preparation (Planning) errors– Also called systematic errors - but stochastic in nature

Errors that are made for each treatment fraction:– Treatment execution errors– Also called random errors

What is the dose distribution received by the CTV ?

Treatment execution errors blur the cumulative dose distribution

Planning errors shift the cumulative dose distribution

Dose

CTV

General Elements of IMRT Quality Assurance

Clinical QA of: Consistent target volume and organs-at-risk delineationQuantitative assessment of organ motion during imaging and treatmentQuantitative assessment of setup variation during imaging and treatment

General Elements of IMRT Quality Assurance

Technical QA of:IMRT planning systemTreatment delivery equipmentPatient specific QA

IMRT Planning System Issues

Dosimetric issues:Beam modeling (focal and extra-focal radiation)Small field outputTransmission through the leavesInterleaf and leaf-end leakage

Computational issues:Dose calculation matrix resolution

Beam Modeling(Cross beam profile with inappropriate modeling of extra-focal radiation)

Measured vs. Calculated

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-15 -10 -5 0 5 10 15

Off-Axis (cm)

Relat

ive P

rofile

ADAC(4mm)wobkmlctrDiff

3

Beam Modeling(Cross beam profile with appropriate modeling of extra focal radiation)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-15 -10 -5 0 5 10 15

Off-Axis (cm)

Rel

ativ

e D

iffer

ence

diff(w/bk)diff(wobk)wbkmlctrwobkmlctr

Beam Modeling(Cross beam profile with inappropriate modeling of extra-focal radiation)

Measured vs. Calculated

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-15 -10 -5 0 5 10 15

Off-Axis (cm)

Relat

ive P

rofile

ADAC(4mm)wobkmlctrDiff

Beam Modeling and IM Field Output

Segment 1 Segment 2

Segment 3 Segment 8

Segment 4-7, 9-13

13 segment IM Field

Consequences of Inadequate Beam Modeling

5.056.7%69.974.95Total0.390.5%29.9%1.31.694-7,9-13

3.915.2%13.7%28.532.413,8

0.650.9%1.6%40.240.851-2

Diff(abs)cGyDiff(abs)%Diff(%)CalculatedMeasuredSegments

Leaf Transmission

0

0.2

0.4

0.6

0.8

1

-0.5 0 0.5 1

AnalyticMonte Carlo

Tran

smis

sion

Position relative to leaf tip (cm)

Window width

LeadingLeaf

FollowingLeaf

Leaf motion

Discrepancies in dose delivered increases for small window width and large number of MU. This is because of approximate nature of corrections for leaf transmission, rounded leaf tips, and leaf scatter

Tongue and Groove EffectMLC Elekta Varian

FWHM 3.3 mm 2.0 mmAvg. Min 0.76 mm 0.83 mm

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

-15 -10 -5 0 5 10 15

Off-Axis Distance (cm)

Rel

ativ

e Pr

ofile

MLC upper jawMLC Tertiary Collimator

4

Tongue and Groove EffectWithout T&G Correction With T&G Correction

Calculation Grid Size effect6MV Photon step size effect for 20x20 fields

both data set use trilinear interpolation

-20

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Relative OFF-Axis Distance (cm)

Rel

ativ

e O

utpu

t

0.2 cm0.4 cmdiff(%)

IMRT Planning System Validation

Center 100% Meas. 183.2 Plan 180.0 Diff. 1.75%

Center 20% Meas. 42.6 Plan 43.0 Diff. .23%

Center 0% Meas. 11.6 Plan 8.1 Diff. 1.9%

Strip intensity pattern with varying intensities

IMRT Planning System Validation

IMRT Guidance Document; Ezzell et.al.

A strip pattern A pyramid pattern A well pattern

A Pyramid Pattern with Equal MU A Pyramid Pattern with Equal MU(The difference plot)

5

MLC positional and leaf speed accuracy MLC positional and leaf speed accuracy (MLC positional and (MLC positional and leaf speed inaccuracies can cause significant dose delivery unceleaf speed inaccuracies can cause significant dose delivery uncertainties)rtainties)

Linac Linac performance for small MU delivery performance for small MU delivery (low MU per (low MU per segment and high dose rate can be an issue in IMRT dose deliverysegment and high dose rate can be an issue in IMRT dose deliveryaccuracy)accuracy)

MLC control issues MLC control issues (Large number of segments per field can be an (Large number of segments per field can be an issue for some delivery systems)issue for some delivery systems)

MLC physical characteristics MLC physical characteristics (leaf transmission, leaf(leaf transmission, leaf--end end leakage, and interleakage, and inter--leaf leakage can be an issue)leaf leakage can be an issue)

Leaf sequencing algorithms Leaf sequencing algorithms ( need to be optimized for ( need to be optimized for smoothness of intensity distribution, number of segments, mechansmoothness of intensity distribution, number of segments, mechanical ical limits of MLC, MU efficiency, leaf travel, and delivery time)limits of MLC, MU efficiency, leaf travel, and delivery time)

IMRT Delivery System Issues Mechanical Alignment of MLC (SMLC Delivery Issue)

No offset 0.6 mm offset 1.0 mm offset

Leaf Speed Accuracy(DMLC Delivery Issue)

MLC Control System Issues

Dose Rate: 300 MU/min

Dose Rate: 600 MU/min

1 MU per stripLow MU and high dose rate can be a problem for some Linac control systems

Data measured on Varian 2100C

Sub-field MU Distribution

(Data from first 100 Head and Neck IMRT patients treated at University of Florida)

• Most sub-fields have less than 3 MU

Patient Plan Phantom Geometry

Patient Specific Quality Assurance

Level 1 QA

6

Corvus Calculation

Film Measurement

Treatment Plan Verification: 2-D Dose Distribution

Level 2 QACompare isodoses (film) and absolute dose (chamber)

Level 3 QA

Establishing a Rationale for Establishing a Rationale for Tolerance Limits and Action Tolerance Limits and Action

LevelsLevels

IMRT Process Uncertainty IMRT Process Uncertainty (Prostate)(Prostate)

Overall Uncertainty (mm) 7.60

1.44

7.35

1.031.00

2.005.005.00

0.50 0.50 0.75

Delivery Machine

IsocenterMLC

SetupInter-fractionIntra-fraction

Organ motion

0.87

1.22

0.87

1.000.500.50

0.47 0.47 1.500.94 0.94 3.00X Y ZX Y ZImaging CT

Planning RTPData Input

CalculationNon-dosimetric

Process Uncertainty (mm)

ItemUncertainty(mm)

MeanDisplacement(mm)

Voxel Size (mm)

IMRT Process Uncertainty IMRT Process Uncertainty (Head and Neck/CNS)(Head and Neck/CNS)

Overall Uncertainty (mm) 2.85

1.44

1.12

1.031.00

0.501.000.00

0.50 0.50 0.75

Delivery Machine

IsocenterMLC

SetupInter-fractionIntra-fraction

Organ motion

0.55

1.22

0.55

1.000.500.50

0.47 0.47 1.500.94 0.94 1.50X Y ZX Y ZImaging CT

Planning RTPData Input

CalculationNon-dosimetric

Process Uncertainty (mm)

ItemUncertainty(mm)

MeanDisplacement(mm)

Voxel Size (mm)

Comparison of Measured and Calculated Cross Plot

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Comparison of Measured and Calculated Cross Plot

Comparison of Measured and Calculated Cross Plot

10 mm DTA difference

Data analysis

Measured with a diode-array (Map Check; Sun Nuclear Corp.)

Credentialing: IMRT

RPC IMRT Head and Neck PhantomTLD in the Target and Organ-at-risk volumesOrthogonal Radiochromic films

Target

Cord

TLD

Anterior Posterior Profile

0

2

4

6

8

-4 -3 -2 -1 0 1 2 3 4

Distance (cm)

Dos

e (G

y)

RPC Film Institution Values

Organat Risk

AnteriorPosterior

Primary PTV

RPC IMRT Phantom Results

Institution A

Posterior-Anterior Profile

0

2

4

6

8

-4 -3 -2 -1 0 1 2 3 4

Distance (cm)

Dos

e (G

y)

RPC Film Institution Values

Posterior Anterior

Organat Risk

PrimaryPTV

RPC IMRT Phantom Results

Institution B

Posterior-Anterior Profile

0

1

2

3

4

5

6

7

8

9

-3 -2.5 -2 -1.5 -1 -0.5 0Distance (cm)

dose

(Gy)

RPC Film

InstitutionValues

RegressionFilm

RegressionInst.

Posterior Central

PrimaryPTV

CriticalStructure

DMax

DminD=DMax-Dmin

75%=Dmin+0.75 D50%=Dmin+0.50 D25%=Dmin+0.25 D

distance=d

d=(d75+d50+d25)/3

75%

50%

25%

d75

d50

d25

RPC Criteria for IMRT Credentialing

8

Posterior-Anterior Profile

0

1

2

3

4

5

6

7

8

9

-4 -3 -2 -1 0 1 2 3 4

Distance (cm)

Dose

(Gy)

RPC Film

InstitutionValues

Regression Film

Regression Inst.

PrimaryPTV

CriticalStructure

Posterior Anterior

7.36Gy+/-2%

2.49Gy+/-3%

RPC Criteria for IMRT Credentialing

Inst. Diff (mm)1 -1.32 2.03 -1.04 0.05 -3.36 -3.77 -2.38 0.09 -0.710 -4.0

0.50

1.00

1.50

-6.0 -4.0 -2.0 0.0 2.0 4.0

Plan - Film (mm)

TLD

(RP

C/In

st)

7% for Planning Target Volume4 mm DTA for the Organ-at-Risk

Suggested Confidence Limit and Action Level Values for IMRT Planning

3 mm DTA2 mm DTAδ90-50% (dose fall off)

7%4%δ1 (low dose, small dose gradient)

15% or 3 mm DTA10% or 2 mm DTAδ1 (high dose, large dose gradient)

5%3%δ1 (high dose, small dose gradient)

Action LevelConfidence Limit* (P=0.05)

Region

* Mean deviation used in the calculation of confidence limit for all regions is expressedAs a percentage of the prescribed dose according to the formula,δi = 100% X (Dcalc – Dmeas./D prescribed)

Segmental Multileaf Collimator (SMLC) Delivery System

3%3%

2%2%

Beam StabilityLow MU Output (<2MU)Low MU Symmetry (<2MU)

1.00 mm radius

0.75 mm radius

Gantry, MLC, and Table Isocenter

2 mm0.5 mm0.5 mm

1 mm0.2 mm0.2 mm

MLC*Leaf position accuracyLeaf position reproducibilityGap width reproducibility

Action Level

Tolerance Limit

* Measured at all four cardinal gantry angles

Dynamic Multileaf Collimator (DMLC) Delivery System

5%3%

3%2%

Beam StabilityLow MU Output (<2MU)Low MU Symmetry (<2MU)

1.00 mm radius

0.75 mm radius

Gantry, MLC, and Table Isocenter

1 mm0.5 mm0.5 mm

±0.2 mm/s

0.5 mm0.2 mm0.2 mm

±0.1 mm/s

MLC*Leaf position accuracyLeaf position reproducibilityGap width reproducibilityLeaf speed

Action Level

Tolerance Limit

Resource Requirements for IMRT

IMRT Treatment Planning(per patient treatment planning requirements for IMRT are challenging

Additional Planning Time/Patient

0

2

4

6

8

10

12

1

Type of IMRT Facility

Addi

tiona

l Hou

rs/IM

RT P

lan

800-bed AMC(2001)400-bed AMC(1998)600-bed Community(2001)300-bed Community(1999)

IMRT requires significantly more time for planning than conventional 3DCRT. It also turns out that large academic medical centers invest more hours per patient in IMRT planning than community centers. Driving the disparity is the variation in:

Initial goals, constraints and target definitions

Number of plans prepared for physician review

Time spent reviewing plans (physician and physicists, jointly)

Data from Oncology Roundtable

9

Patient-Specific Quality Assurance

Additional QA Time/Patient

0

1

2

3

4

5

6

1

Type of IMRT Facility

Hour

s

800-bed AMC(2001)600-bed AMC(1997)400-bed AMC(1998)600-bed Community(2001)300-bed Community(1999)

A consensus of approximately four additional hours per patient at program inception. However, the QA time is likely to go down as the program matures.

•Check dosimetry (absolute dose)

Ion chamber measurements in a phantom

•Verify spatial dose distribution

Film in a phantom

•Check pretreatment position

Port films taken with initial MLC jaw position and matched to correspondingDRRs

Data from Oncology Roundtable

Head & Neck IMRT Treatment Planning Effort/Patient @ UF

Experience

Physics staff

Radiation Oncologist

Effo

rt/P

atie

nt

IMRT program startup effort (approximately 20 hrs/patient) can be significant for the physics staff, but it comes down rapidly.

(Months)3 6 9 12

(Hou

r)

10

20

Resource Requirement Analysis(A facility treating a total of 300 RT patients that includes 40 IMRT patients)

Additional Physics/Dosimetry time (.4 FTE)Additional machine QA (100hr)Additional treatment planning QA (50 hr)Patient specific QA (160 hr)Additional time for treatment planning (200 hr)

Maximum machine workload (per 8 hr shift)Expected to go down from 32 patients to 24

Machine uptimeExpected to go down from 99% to 95% due to more wear and tear of delivery equipment with IMRT