imrt: treatment planning and dosimetry nesrin dogan, ph.d department of radiation oncology virginia...

IMRT: Treatment Planning and Dosimetry

Nesrin Dogan, Ph.DDepartment of Radiation OncologyVirginia Commonwealth University

Medical College of Virginia HospitalsRichmond, VA, USA

N. Dogan / Nov 2007

Fundamental Issues

• Beam Modeling

• Dose Calculation

• Inverse Planning

• IMRT QA

N. Dogan / Nov 2007

Beam Modeling• For small fields, minor uncertainties due to approximations

in dose calculation models, methods for determining MLC leaf sequences and other factors may form a large fraction of dose delivered, and lead to inaccuracies in delivered dose.

10 cm1 cm1 cm1 cm1 cm1 cm1 cm1 cm1 cm1 cm1 cm

N. Dogan / Nov 2007

Beam Modeling, cont.

• Dosimetric accuracy of the IMRT plan delivery depends on the accurate representation of Accurate Beam Penumbra representation – MLC /

collimator jaws. Adequate characterization and accounting of

transmission and scattering properties of MLC leaves. Output factor for small field size. Accuracy of dose calculation algorithm. Approximations of leaf sequence generation algorithm. Leaf positioning accuracy.

N. Dogan / Nov 2007

Penumbra• Need to be measured with microchamber, film or

diode.• Subtle effects make a difference in IMRT.

Beam model based on penumbra measured with 6 mm diameter chamber

Beam model based on penumbra measured with film

Courtesy of G. Ezzel, Ph.D., Mayo Clinic

N. Dogan / Nov 2007

MLC Leaf Characteristics

• Inter- and intra-leaf transmission

• Tongue-and-groove – can lead to under-dosages ~30% in a 2 mm wide region

• Rounded tip

~12% ~1% ~1.5% ~2.5%

N. Dogan / Nov 2007

Radiation Field Offset for Rounded Leaf Ends

• Offset for between the light and radiation field edge = ~0.6 mm

Measuring the offset

0.6 is best, i.e. subtract 0.6 mm from MLC settings -> TPS should take care of this


N. Dogan / Nov 2007

MLC Leaf Transmission and Scattering

• Leaf leakagetransmissionrounded leaf tip

transmissionMLC scatter

• Collimator scatter upstream from the MLC.

Leakage through leaf ~2%

Leakage between neighboring leaf ~5%

• Leakage through closed opposing leaf pair for rounded ends ~20% - leaves should be parked under the jaw

•Minimum gap between opposed leaves = 0.5 -0.6 mm to avoid collisions


N. Dogan / Nov 2007

Output Factor Small Fields

D.A. Low et al. “Ionization chamber volume averaging effects in dynamic intensity modulated radiation therapy beams, Med.Phys.30(7): 1706-1711 (2003).

Micro cham: 0.009cc

PTW: 0.125cc

Farmer:0.65cc

N. Dogan / Nov 2007


MLC and Small Fields

• Output for small fields very dependent on MLC accuracy.

• 10%/mm for 1 cm segment.

N. Dogan / Nov 2007

Which Detectors to Use?• Need to determine energy dependence

and angular response.• Small field detectors required for small

field characterization. Sensitive to position Detector should be smaller than

homogeneous region of dose to be measured

• Assess electrometer response.

N. Dogan / Nov 2007

DetectorVolume

(cm3)Diameter

(cm) Disadvantages

Micro-chamber

0.009 0.6 Poorer resolution than diodes

Pinpoint chamber

0.015 0.2Over-respond to low energy

photonsMartens et al. 2000p-type Si

diode0.3 0.4

Stereotactic diode

NA 0.45

MOSFET NA NA Non-linear dose response for <30 cGy

Diamond 0.0019 0.73 < resolution than diodes, dose rate dependence, expensive

Courtesy of Jean Moran, Ph.D, UofMichigan

Small 1-D Detectors

N. Dogan / Nov 2007

Pasma Med Phys 26: 2373-2378 (2376) 1999

PredictedEPIDIon Chamber+

Discrepancies in the penumbra region (up to 10%)

Overall: Good agreement

10 MV 25 MV

EPID: DMLC measurements

Courtesy of Jean Moran, UofMichigan

N. Dogan / Nov 2007

Dose Calculation• Current IMRT systems use simplified

dose calculations during plan optimization: e.g., pencil beam -> uses very simple heterogeneity corrections, causing significant dose errors (10% or more non-IMRT cases)

• Final dose calculation is performed using a separate independent dose calculation that incorporate the influence of the MLC: e.g, convolution / superposition; more accurate than Pencil beam; however, inaccuracies persist under certain circumstances

N. Dogan / Nov 2007

Conventional dose algorithms can be inaccurate for • Small fields• Regions of dose gradients (radiation

disequilibrium)• Heterogeneous conditionsIMRT is typically delivered through a sequence of small static fields or with a dynamically moving aperture with a small width. Dose gradients are common place in IMRT fields. For such fields, assumptions used in conventional algorithms regarding scatter equilibrium and output factor variation with field size typically break down.

N. Dogan / Nov 2007

• Significant fraction of the dose within targets and organs at risk is due to scattered or leakage radiation calculated dose distributions have the greatest

uncertainties due to approximations inherent in conventional methods of transforming intensities into MLC leaf sequences

• Experimental checks of IMRT fields routinely shows discrepancies between the planned (desired) and actual.

For IMRT

N. Dogan / Nov 2007

Dose Calculation AlgorithmsC

alc

ula

tio

n S

pe

ed

Calculation Accuracy

Pencil Beam

Monte Carlo

Superposition/Convolution

Courtesy: Jeff Siebers, VCU

N. Dogan / Nov 2007

Comparison of SC and MC

Comparison of a) Superposition-Convolution (SC) and b) MC dose calculations

N. Dogan / Nov 2007

Target

RT Lung

0 5 10 15 20

Dose (Gy)

0

20

40

60

80

100

Vol

um

e (%

)

Cord

Monte Carlo

Pencil Beam

Pawlicki et al., Med Dosim, 26 157 (2001)

Comparison of PB and MC

Pencil BeamMonte Carlo

N. Dogan / Nov 2007

Superposition

Monte Carlo

Slice 45

Monte Carlo

Slice 55

Monte Carlo

Slice 64

Comparison of SC and MCSuperposition Superposition

N. Dogan / Nov 2007

Consequences of Inaccuracy

• Dose Prediction Error (DPE) For a given intensity distribution, dose predicted differs

from that actually delivered to the patient/phantom Can be avoided by performing final calculation with

accurate algorithm

• Optimization Convergence Error (OCE) Consequence of systematic error during optimization Optimization with an inaccurate algorithm results in

different intensities than those predicted by an accurate algorithm

Actual dose is not optimal, a better solution exists Can be avoided by optimization with an accurate

algorithm

N. Dogan / Nov 2007

DPE(same intensities)

PB computed SC computed

Make sure your final dose calculation is with an accurate algorithm

68 Gy 64 Gy 60 Gy 50 Gy 40 Gy 30 Gy

N. Dogan / Nov 2007

OCE(different intensities)

SC optimizationSC calc

PB optimization SC calc

68 Gy 64 Gy 60 Gy 50 Gy 40 Gy 30 Gy

N. Dogan / Nov 2007

Conventional IMRT Optimization Process

Create Leaf Sequence

“Deliverable” DoseCalculation

Create Deliverable Intensities

Optimization

Leaf Sequencer

Leaf positions do not exist

Deliverable Plan

N. Dogan / Nov 2007

Problems with Conventional IMRT process

• Optimized plans are converted to deliverable plans through leaf-sequencing process that takes into account the limitations and effects (leakage/scatter) of the MLC

• The idealized optimal plan is replaced with “deliverable” plan

• Optimized and deliverable IMRT plans differ Different intensity distributions More complex the intensity distribution, the

greater the deviation

N. Dogan / Nov 2007

Comparison of Isodoses

a) An optimized intensity distribution b) A deliverable distribution using DMLC calculated using Convolution/Superposition algorithm

N. Dogan / Nov 2007

InitialIntensity (II(x,y))

EvaluatePlan Objective

Converged?

Ad

just

I(x,

y)

ComputeDose (DO)

Optimized Intensity (IO(x,y)) and Dose DO = DD

NoYes

1

3

4

5

2

Create Leaf Sequence 7

Create Deliverable Intensities(ID(x,y)) 8

6

Final dose is deliverable

Deliverable IMRT Optimization Process

combine optimization and

delivery into one process

Leaf Sequencing

N. Dogan / Nov 2007

Deliverable Optimization

Deliverable optimization can restore

original optimized plan

Original SCopt

Deliverable Plan SC

MC of Deliverable

MCopt

(deliverable)

Head and Neck IMRT plan

N. Dogan / Nov 2007

Heterogeneity Corrections• More important for IMRT than

conventional treatments.• Heterogeneities may effect some

beamlets more than others -> causing different localized dose differences.

• The reliability of clinical experience with DVH prescriptions and results may be significantly compromised if heterogeneity corrections are not used (e.g., Lung).

• Use AAPM Report No:85 Tissue Inhomogeneity Corrections for Megavoltage Photon Beams.

• 4% - 10% error in relative e- density result in ~2% error in dose.

w /Hetero

w/o Hetero

N. Dogan / Nov 2007

• Size of the OARs.• Dose gradients near the OARs.• Finer dose grids are necessary for cases in which high gradients are needed.• Dose grid should be finer than the size of the beamlets or incident fluence map so that the effects of modulation are adequately sampled.

Dose Grid

N. Dogan / Nov 2007

What do we do about differences?

• May need to adjust the beam model.• May need to live with it.

Take known deficiencies into account when evaluating plans

• May be important for OARs.

N. Dogan / Nov 2007

Buildup Region• Important when target regions (PTV)

extend into the buildup region.• Calculated doses are often

inaccurate and lower than delivered doses.

• Likely to cause hot spots in the target and elsewhere as a result of inverse planning engine fighting with the buildup effect – may cause excessive skin reactions and compromise the plan quality.

• Bolus needs to be added if the target is in the buildup region: needs to be included during the scanning of patient.

N. Dogan / Nov 2007

• Definition of Target Volumes GTV, CTV and PTV need to be explicitly definedConsistent with the ICRU definitions (ICRU 50)

• Definition of OARsPlanning OAR Volume (ICRU 62)

• Need to • Use contrast-enhanced CTs• Image fusion (PET, MRI, preopt CTs, etc..)

Target and OARs

N. Dogan / Nov 2007

Margins for Targets• IMRT does not inherently demand for

tight target margins.• CTV to PTV margins depends on each

individual patient and the patient immobilization / location techniques used.

• Tight target margins can be achieved by improved imaging for planning, immobilization and image-guided verification.

N. Dogan / Nov 2007

Realistic for CTV?


• Automatic CTV expansions may unrealistically cross tissue boundaries.

Automatic CTV expansions

N. Dogan / Nov 2007

Margins for OARs

• ICRU 62 recommendations suggest the use of margins for OARs.

• Generate expanded OARs if it is possible. e.g.;CordExpand = Cord + 5 mm

BrainstemExpand = Brainstem + 5 mm

• Create “pseudo” structures to achieve sparing at the desired areas.

N. Dogan / Nov 2007

Oral mucosa - avoid


Defining Normal Tissues• Tissues to be spared need to be explicitly defined; e.g., oral mucosa when changing from parallel-opposed to IMRT.

N. Dogan / Nov 2007

Target

Nodes

Spinal cord

Avoidance tissue

Avoidance tissue

Gy

60

50

45

30

Gy

60

50

45

30


N. Dogan / Nov 2007

Hot Spots Outside of Target Regions

• Occurs in regions that are not contoured.

Work-around• Create “Unspecified Tissue”

Region and include in the optimization.

N. Dogan / Nov 2007

Defining OARs for Optimization

• Create nonPTVOARs for organs overlapping with PTVs:

NonPTVSmallBowell, NonPTVRectum,

NonPTVBladder, etc.

N. Dogan / Nov 2007

Guidelines for Target Expansions Prostate CTV: Expand prostate by 0.5cm in all directions except posteriorly then + seminal vesicles (no expansion for seminal vesicles) Prostate PTV: Expand Prostate CTV by 0.5cm in all directions (3D expansion) Lymph Nodes CTV: Expand lymph nodes by 1.0 cm in anterior, posterior, right and left (2D expansion) with small bowel, bladder, rectum, bones, muscle,

skin1cm and prostate PTV tissues being the limiting organs Lymph Nodes PTV: Expand Lymph Nodes CTV 0.5 cm in all directions (3D expansion) with only skin1cm and Prostate PTV as the limiting structures

N. Dogan / Nov 2007

• Required by the inverse planning process – dose or dose-volume constraints for all structures

• A trial and error process to come up with the proper dose or dose-volume constraints.

• Don’t ask the impossible – set realistic goals – improperly specified constraints will result in inferior plans.

• Create site-specific protocols which can be used for similar cases.

Constraints

N. Dogan / Nov 2007

H&N IMRT Treatment Planning Instruction FormDepartment of Radiation Oncology, VCU Health SystemsStructures Limiting Dose(Gy) Volume (%) / cc Fraction

size

PTV1 7077

97< 20

35

PTV2 56 95 35

PTV3 ----- -----

Brainstem + 0.5 cm 50 0

Cord + 0.5 cm 48 ≤ 0.03 cc

Mandible 60 30

Oral Cavity 45 50

Parotids (L & R) – at least one of them

30 ≤ 50

Larynx – if feasible 45 0

Esophagus 45 30

Brachial Plexus 60 0

Unspecified Tissue 70 ≤ 5

CTV1= GTVt + GTVn + 1cm margin for subclinical disease

PTV1 = GTVt + GTVn + 0.5 setup uncertainty

PTV2 = CTV1 + 0.5 setup uncertainty

PTV3 = CTVnodes + 0.5 setup uncertainty

N. Dogan / Nov 2007

Prostate IMRT Treatment Planning Instruction FormDepartment of Radiation Oncology, VCU Health SystemsStructures Limiting Dose(Gy) Volume (%) Fraction Size

PTV 63 97 28

PTVNodes50.4 95 28

Femurs (L&R) 354045

50 10 2

Periprostatic rectum 556365

5010 2

Remaining rectum 455560

5010 2

Bladder 456065

5010 2

Small Bowel 254550

5010 2

Skin1cm 303545

5010 2

Unspecified Tissue 50 10

N. Dogan / Nov 2007

General Principles for Beam Angle Selection

N. Dogan / Nov 2007

Beam ConfigurationsGeneral Principles

• It is useful to minimize the number of beams for practical reasons

• The minimum number of beams depends upon a complex combination of factors: Shape and size of target volume Locations, tolerances and tissue architecture of normal

tissues Prescription dose (higher doses would normally

require more beams)

• The optimum number may be determined for each class of radiotherapy problems by trial-and-error

N. Dogan / Nov 2007


• If sufficient number of beams are used, the IMRT plan quality is relatively insensitive to beam angles The computer should be able to adjust the weights of

rays to make up for modest imperfections in beam placement

Beams may be placed at equiangular steps

• The larger the number of beams, the better the IMRT plan Rotational IMRT should be better

• Non-coplanar beams should provide additional benefit

N. Dogan / Nov 2007


• In general, if beam angles are optimized the plan optimality should improve the number of beams required for equivalent

dose distribution is smaller than if beams are placed at equi-angular steps

• Computer-aided optimization of the beam angles is a difficult and as yet inadequately solved problem Extremely large number of plans need to be

compared

N. Dogan / Nov 2007

• Choose shortest path to irradiate target(s)• Avoid OARs• Keep large beam separation if it is

possible• Beam angle may become important for

tumors that are not centrally located.• It depends on the optimizer


N. Dogan / Nov 2007

H&N:5 Beam

9 Beam

7 Beam

15 Beam

N. Dogan / Nov 2007

Isocenter Placement

• Better plans can be achieved by selective isocenter placement.

• Desirable to shift isocenter to provide best separation between target and tissues.

• Desirable to have isocenter in region of reliable bony anatomy.

Center of All Targets

Center of PTV

N. Dogan / Nov 2007

7 rows to cover target

One row hits target and structure

6 rows to cover target

Split between target and structure

Spatial quantization effects• Shift isocenter to provide best

separation between target and tissues.


N. Dogan / Nov 2007

Dose-Volume Based vs. EUD-Based Optimization – H&N

ExampleEUD + tumor as

“virtual normal tissue”

3045

50(c)

70

60

EUD unconstrained

(b)3045

50

80

60

70

45

Dose-Volume

(a)

30

5060

70

45

N. Dogan / Nov 2007

N. Dogan / Nov 2007

60

50

70

40

(a)

80

40

80

6070

50

(c)

9080

4090

6050

70

(b)

Bladder

Target

Bladder

Target

BladderRectum

Dose-Volume EUD unconstrainedEUD + tumor as

“virtual normal tissue”

Dose-Volume Based vs. EUD-Based Optimization – Prostate

Example

N. Dogan / Nov 2007

N. Dogan / Nov 2007

SIB IMRTTwo-Phase IMRT

70

5045

60

3540

45

50

35

40

45

45

50

60

70

50Mean

dose to nodes 59 Gy

Mean dose to nodes 51 Gy

Sequential vs. SIB

N. Dogan / Nov 2007

Non-Target Tissue Volumes Receiving Specified Dose

Dose Level (cGy)

Two-phase IMRT

Simultaneous integrated boost

IMRT

1000 2183 2169 0.6

2000 1975 1941 1.8

3000 1557 1459 6.7

4000 1096 1016 7.9

5000 732 604 21.2

6000 388 238 63.0

7000 83 62 33.9

% Difference between SIB IMRT and 2-phase IMRT

Volume (cc) outside the target (tumor and nodes) at specified

dose level or higher

N. Dogan / Nov 2007

Minimize Number of Segments

N. Dogan / Nov 2007

Minimize Number of Segments

50 Segments

75 Segments

100 Segments

150 Segments

200 Segments

Segments MU

50 550

75 582

100 604

150 619

200 631

N. Dogan / Nov 2007

Impact of Degree of Fluctuations (“Complexity”) in Intensity Patterns

on MUs for IMRT

100

Total MUs= 100

100

Total MUs = 300

100 100

10 cm

10 cm

100 100

Total MUs = 200

10 cm

Total MUs = 150

50

50 50

10 cm

N. Dogan / Nov 2007

Attention to Objective Function

N. Dogan / Nov 2007

Target Volumes

Critical Structure

TLDs in Target VolumesRadiochromic film through multiple plansDelivery is required by RTOG for participation in IMRT trials

Removable DryInsert

Water

Water

Anthropomorphic: RPC Head Phantom

Courtesy of Jean Moran, UofM

N. Dogan / Nov 2007

Anthropomorphic: RPC Head Phantom

N. Dogan / Nov 2007

Examples

N. Dogan / Nov 2007

HN IMRT w / Supraclavicular NodesTreat Nodes with

AP ScV field• Requires matching of

IMRT fields w/AP field

• May cause hot or cold spots

• ScV field needs to be included in the IMRT optimization

• Feathering

• Watch out for overlaps if the IMRT plan wants to open the jaws into the ScV area -> may need to adjust the jaw

ScV field

IMRT field

N. Dogan / Nov 2007

This row might be used by the IMRT plan if the target is drawn too close to the isocenter plane


Human planners sometimes have to trim IMRT beams…..

N. Dogan / Nov 2007

HN IMRT w / Supraclavicular Nodes

Treat Nodes with

IMRT• No matching –

eliminates junction issues.

• Needs extra care treating shoulders – avoid hot spots.

• Immobilization.

N. Dogan / Nov 2007

Nasopharynx

Nasopharynx field

arrangement

7 posterior fields

N. Dogan / Nov 2007

SIB IMRT

PTV1: 54 Gy/30 fx’s

(1.8Gy/fx)

PTV2: 60 Gy/30 fx’s

(2.0Gy/fx)

PTV3: 67.5 Gy/30 fx’s

(2.25Gy/fx)

• The field length on the 2 lateral fields is stopped at the top of the shoulder.

• The s/clav portion of the target volume is treated with the remaining 5 fields.

Nasopharynx

N. Dogan / Nov 2007

Sinus

• Single Tx Volume• 60 Gy/30fx’s• Challenging due to

the proximity and sometimes overlap of tumor volume and critical optical structures and large air cavity.

N. Dogan / Nov 2007

7 beam field arrangement

N. Dogan / Nov 2007

• A tunnel was carved out in the dose cloud around the optic structures.

• Achieved with the use of expanded structures and the ability to manipulate both the imp. Weighting and overlap priorities.

N. Dogan / Nov 2007

Sinus DVH

N. Dogan / Nov 2007

Nasal Cavity and

lymph nodes

• 2 Tx volumes• PTV1 46 Gy/ 23 fx’s

nasal cavity & nodes• PTV2 24 Gy/12 fx’s

nasal cavity boost• Sequential IMRT

plans• 0.5 cm Bolus over

nose

N. Dogan / Nov 2007

PTV1• 7 beams• Gantry angles

205, 285, 315, 0, 20, 75, and 153

N. Dogan / Nov 2007

PTV2

• Sequential IMRT plan

• 4 beams• Gantry angles

285, 315, 20, and

75

N. Dogan / Nov 2007

Nasal cavity isodose lines

N. Dogan / Nov 2007

Prostate and Lymph Nodes

1. Prostate PTV+ Lymph Nodes PTV: 50.4 Gy / 28 fx

Prostate PTV : 63 Gy (BED = 70 Gy) / 28 fx

2. Sequential 9 Gy IMRT Boost to Prostate PTV : 72 Gy (BED = 80 Gy)

or

Upfront 6 Gy HDR boost

SIB-IMRT

N. Dogan / Nov 2007

Heart

IM nodes

Heart

IM nodes

Breast IMRT w/IM nodes

N. Dogan / Nov 2007

Breast IMRT W/regional Nodes

N. Dogan / Nov 2007

Lung IMRT

N. Dogan / Nov 2007

Boost1

Primary Tumor

Bladder

RectumCTV

Brachytherapy + IMRT of Cervix

• Stage IIIB cervix cancer

• Patient receives both brachytherapy (BRT) and external radiotherapy (XRT)

Boost2(Involved Node)

N. Dogan / Nov 2007

GYN Brachytherapy + IMRTIMRT+BRTConventional 3DCRT+BRT

Primary Tumor

N. Dogan / Nov 2007

0

20

40

60

80

100

0 20 40 60 80

Rectum

XRT + BRTIMRT + BRT

Dose (Gy)

0

20

40

60

80

100

0 20 40 60 80

CTV

XRT + BRTIMRT + BRT

Dose (Gy)

0

20

40

60

80

100

0 20 40 60 80

Bladder

XRT + BRTIMRT +BRT

Dose (Gy)

0

20

40

60

80

100

0 20 40 60 80

Boosts

Boost 1 XRT + BRTBoost 1 IMRT + BRTBoost 2 XRT + BRTBoost 2 IMRT + BRT

Dose (Gy)

Conventional 3DCRT+BRT vs. IMRT+BRT

N. Dogan / Nov 2007

QA tasks for IMRT

• Machine QA- Acceptance and routine QA of the MLC for IMRT delivery - dosimetric and geometric characteristics

• Algorithm QA for IMRT - QA of planning system and data consistency with machine

• Patient Specific QA – prove plan works 1D and 2D dosimetry of treatment components such

IM beams and segments 3D dosimetry of entire treatment delivery

• Post Treatment QA• Log-file analysis

N. Dogan / Nov 2007

Phantom Dose Verification

Beams on Patient Beams on Phantom

N. Dogan / Nov 2007

SampleFilm Dosimetry Results

Other Analysis Distance to Agreement Gamma …

Isodose Comparison

Profiles

Dose Differences

N. Dogan / Nov 2007

Compare isodoses (film) and absolute dose (chamber)

Current Practice


N. Dogan / Nov 2007

Gamma Analysis

Measured Film

Adaptive Convolution

Monte Carlo

N. Dogan / Nov 2007

Calculation to Measurement Comparison

(b)

Measured Calculated

54% of points have a dose difference <2% or a DTA <2 mm

N. Dogan / Nov 2007

MC to Measurement Comparison

(b) (c)

Measured Calculated

Measurement and MC w transport through MLC 97% within 2%,2 mm

Measurement and MC using Tx planning systems Intensity Matrix

N. Dogan / Nov 2007

=10%Superposition Monte Carlo

MC Verification

N. Dogan / Nov 2007

The percentage of points, averaged over all of the plan’s treatment fields for each patient with ≥1 with 2% tolerance and 2 mm DTA.

MC (8.1% ± 3.7% points failed; range = 4.9% – 18.4%) SC (16.7% ± 5.6% points failed; range = 11.3% – 30.7%)

MC Verification of Prostate IMRT Plans

N. Dogan / Nov 2007 Courtesy of G. Ezzel, Ph.D., Mayo Clinic

Check standard patterns for constancy

N. Dogan / Nov 2007

DMLC field 14x14 cm2

at SSD =100 cm, 2 cm separated strips

• Using radiographic films Intensity-modulated

pattern field Check leaf position,

acceleration, motion stability

Check for hot and cold density

Visual check

Routine DMLC QA

Courtesy of Jean Moran, Ph.D, UofMichigan

N. Dogan / Nov 2007

Summary

• Inverse IMRT Planning is not intuitive – however, easy to establish protocols and class solutions for each specific site.

• Necessary to define realistic goals and constraints.

• Simplify IMRT plan as much as possible once you have acceptable solution.

• Know the limitations of your inverse treatment planning system.

N. Dogan / Nov 2007

Summary• Need to characterize the MLC system for

IMRT with special emphasis on penumbra, leaf leakage and transmission.

• Need to know the limits of the mechanical systems and interactions with controller and accelerator software for delivery.

• Continued need for improvements to software for delivery system, measurement devices, phantoms, and dose analysis tools.

N. Dogan / Nov 2007

Acknowledgements

Jeffrey Siebers – VCUGary Ezzel – Mayo ClinicMark Oldham – Duke UniversityJean Moran – U of MichiganIvaylo Mihalov - UofArkansas

imrt: treatment planning and dosimetry nesrin dogan, ph.d department of radiation oncology virginia...

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