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(Un-)Certainties in SABR

Johan Cuijpers PhD, VU University medical centre, Amsterdam.

Disclosures

No personal disclosures VUmc has a master research agreement with Varian

Medical Systems

Content

(Un-)Certainties in SABR for lung Pre-Treatment Imaging

4DCT

Treatment Planning & Dosimetry Dealing with motion Dose calculation accuracy Plan summation Multiple lesions

Image Guidance

Setup accuracy 6D corrections

Pre-treatment Imaging

4DCT One breathing cycle per table

position is acquired! Artefacts due to finite time

resolution CT-scanner Variability of breathing pattern

Imaging Artefacts 4DCT

0% bin

50% bin

20% bin

70% bin

PresentatorPresentatienotitiesImaging of programmable motion phantom.

The Mid Ventilation bin shows most artefacts

Amplitude variability during 4DCT

-0,5

0

0,5

1

1,5

2

0 20 40 60 80 100 120 140

Ampl

itude

RPM

(cm

)

Time (s)

smallseries

largeseries

VUmc 4DCBCT protocol

Acquire full length 4DCT

Acquire 2nd short 4DCT

Acquire 4D-CBCT

Is Quality of 4DCT OK? phase errors artefacts 4DCT

Send data to TPS

Treat Adapt plan

Quality of short 4DCT OK?

Tumor motion as in 4DCT?

yes no

yes no

yes no

Plan

8x2.5mm 10 bins At least 10 images per breathing cycle Time resolution 1/10 of breathing cycle

4D CBCT

8

PresentatorPresentatienotities10-15 min extra timeslotDefault protocol if breathing cycle < 8sec, otherwise extra slow protocolAbsence of online availability

Content

(Un-)Certainties in SABR for lung Pre-Treatment Imaging

4DCT Treatment Planning & Dosimetry

Motion management Dose calculation accuracy Plan summation Multiple lesions

Image Guidance Setup accuracy 6D corrections

Dosimetric margins for breathing motion

Respiratory motion leads to penumbra blurring Penumbra blurring can be compensated by increasing field size

asymmetric margins around mid-position symmetric margins around mean tumor position

Dosimetric margins are smaller than ITV margin (= App)

Uncertainties Blurring depends on

motion pattern the steepness of the penumbra PTV size

Dosimetric Margin versus amplitude

0

2

4

6

8

10

12

14

16

18

0 5 10 15

SD

Mar

ge

80 long95 long80 vherk95 vherk

Multi Institution assessment of accuracy of ITV

Hurkmans et al, Int J Radiat Oncol Biol Phys. 2010 Oct 13

ROSEL Trial

Multi Institution assessment of accuracy of Mid-V

Hurkmans et al, Int J Radiat Oncol Biol Phys. 2010 Oct 13

ROSEL Trial

Negative Margin Relative to ITV

Cuijpers et al: Radiother Oncol. 2010 Dec;97(3):443

Reduction ITV Expiration (cranial) side: -0.2 App + 1.3 (mm)

Inspiration (caudal) side: -0.3 App + 2.2 (mm)

-12,0

-10,0

-8,0

-6,0

-4,0

-2,0

0,0

0 5 10 15 20 25 30 35 40

mIT

V80

(mm

)

App (mm)

Expiration

Inspiration

PresentatorPresentatienotitiesIn order to reduce uncertainties in imaging (artefacts) and variability in breathing pattern and dose distribution a population averaged recipe can be used based on the ITV

Amplitude Monitored Treatment Delivery

Verification on amplitude using Varian RPM

Gated therapy with: Gating window is set to full amplitude range RPM system guards amplitude during treatment If breathing amplitude during irradiation is larger than during

CT, beam holds

CT and CBCT with similar breathing

CBCTs with different breathing

Treatment planning @ VUmc

Dose prescription SABR @ Vumc Prescription dose

3x18 Gy 5x11 Gy 8x7.5 Gy

Normalized on 80% Coverage PTV: V80% >99% Dmax PTV > 100% (>130% of prescription dose)

Use Average Intensity Projection for dose calculation RapidArc with 2 arcs (CW, CCW) 10 MV FFF beam Avoidance sector to spare the contralateral lung

Ave-IP good approximation

PresentatorPresentatienotitiesIdeal would be to use a true 4D calculation of dose, however this is not available in most commercial treatment planning systems

(Un-)Certainties due to small field dosimetry

1. Static Field 1 x 1 cm2 3 x 3 cm2

a) Dose Deviation in center

With courtesy to Wilko Verbakel

Absolute output Eclipse

Small lesion in low density lung tissue

4400 5400 6400 7400

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 74000

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 7400

Dose (cGy)

4400 5400 6400 7400

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 74000

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

9.3 cm3

7.1 cm3

1.6 cm3

11.4 cm3

5.5 cm3

2.6 cm3

3.9 cm3

9.3 cm3

PTVs 3x18

AAA versus Acuros

With courtesy to Miguel Palacios

4400 5400 6400 7400

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 74000

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 7400

Dose (cGy)

4400 5400 6400 7400

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 74000

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

9.3 cm3

7.1 cm3

1.6 cm3

11.4 cm3

5.5 cm3

2.6 cm3

3.9 cm3

9.3 cm3

PTVs 3x18

-865

2.76 cm

With courtesy to Miguel Palacios

AAA versus Acuros

PresentatorPresentatienotitiesIt is not only the size of the target that determines the deviations found with AAA, but also the distance to the thoracic wall

4400 5400 6400 7400

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 74000

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 7400

Dose (cGy)

4400 5400 6400 7400

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 7400Dose (cGy)

4400 5400 6400 74000

20

40

60

80

100

Dose (cGy)

Vol

ume

(%)

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

AAAAcuros

9.3 cm3

7.1 cm3

1.6 cm3

11.4 cm3

5.5 cm3

2.6 cm3

3.9 cm3

9.3 cm3

PTVs 3x18

-790

0.6 cm

With courtesy to Miguel Palacios

AAA versus Acuros

Acuros vs AAA

PresentatorPresentatienotitiesPrescribing dose to the mean of the ITV would reduce a lot of the uncertainty involved in dose prescription. For exceptional cases a density override of the ITV might be sensible.

How to deal with Multiple lesions?

Preferably in one optimization using single isocentre However: the human body is not a rigid body

Multiple isocentres necessary for independent setup on each lesion

Dose distributions per lesion have mutual influence

VUmc/London Ontario flow chart

Flowchart: H. Tekatli et al (submitted)

With courtesy to Hilal Tekatli

Single isocentre

With courtesy to Hilal Tekatli

Multiple iso-centres

With courtesy to Hilal Tekatli

(Un-)Certainty in previously delivered dose

Accounting for previously delivered dose @VUmc

Rigid 3D registration of old and new pCT Reconstruct plan on new pCT and recalculate dose Convert isodose lines to volumes for optimization

With pitch/roll Use 6D rigid registration (Velocity) Convert isodose lines to Volume Use this volume in optimization in Eclipse

Uncertainties

- Deformations - Changes in Anatomy

Including previous treatments

Including previous treatments

Including previous treatments

PresentatorPresentatie

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