Download - (Un-)Certainties in SABR · Dose (cGy) Volume (%) AAA Acuros AAA Acuros AAA Acuros AAA Acuros AAA Acuros AAA Acuros AAA Acuros AAA Acuros 9.3 cm3 7.1 cm3 1.6 cm3 11.4 cm 3 5.5 cm3

(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

Presentator

Presentatienotities

Imaging of programmable motion phantom. The Mid Ventilation bin shows most artefacts

Amplitude variability during 4DCT

-0,5

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Ampl

itude

RPM

(cm

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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

Presentator

Presentatienotities

10-15 min extra timeslot Default protocol if breathing cycle < 8sec, otherwise extra slow protocol Absence of online availability

Content


– 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 (=½ App)

Uncertainties – Blurring depends on

– motion pattern – the steepness of the penumbra – PTV size

Dosimetric Margin versus amplitude

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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

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mIT

V80

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App (mm)

Expiration

Inspiration

Presentator

Presentatienotities

In 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

Presentator

Presentatienotities

Ideal 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

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Dose (cGy)

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Dose (cGy)

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Dose (cGy)

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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

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Dose (cGy)

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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


AAA versus Acuros

Presentator

Presentatienotities

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

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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


AAA versus Acuros

Acuros vs AAA

Presentator

Presentatienotities

Prescribing 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


Multiple iso-centres


(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

Presentator

Presentatienotities

Many uncertainties exist here: Deformations Changes in anatomy (scar tissue) However, we think this is better than having a side by side look of previous and current treatment plan and visually assess the previously devliverd dose

Content


– 4DCT – Treatment Planning & Dosimetry

– Dealing with motion – Small field dosimetry – Inhomogeneities – Plan summation – Multiple lesions

– Image Guidance – Setup accuracy – PTV margins

Setup Uncertainties

State-of art practice in SABR is on-line soft tissue matching using CBCT

Treatment Margins determined by: On-line setup accuracy CBCT Roll / Pitch / Yaw errors Deformations Intrafraction variability

Intra-fraction motion: VUmc data •Intra-fraction motion measured by repeat CBCT

–Supine position

–Two (or one) arms above the head

–Mattress for patient comfort

–Knee cushion

–No further immobilization

–Fast RapidArc™ treatment

AP (mm) SI (mm) LR (mm)

Novalis Tx

(6D)

39 pats,193 fractions

Spine 0.8 0.8 1.0

ITV 1.7 1.4 1.1

True Beam (FFF)

32 pats,140 fractions

ITV 1.4 1.3 1.2

1 SD of intra-fraction motion

Presentator

Presentatienotities

Intrafraction tumor motion (lung) is largely determined by internal ITV shift

Total variance

Components of random variation in ITV positioning (1 SD).

AP (mm) SI (mm) LR (mm)

Intra-fraction variability 1.7 1.4 1.1

CBCT+couch accuracy 0.5 0.5 0.5

Inter-observer variability 0.5 0.5 0.5

Total variance 1.8 1.6 1.3

Presentator

Presentatienotities

Including other uncertainties (Image Guidance, Inter-observer variability in matching images)

Correction of Pitch & Roll positioning error

Pitch Roll ExacTrac

X-ray correction

mean 0.97 -0.07

SD 1.12 0.88

Residual error on CBCT

mean -0.13 0.07

SD 0.51 0.55

Before and after Exactrac robotics couch corrections 39 patients, 193 Treatment fractions

y = -0.36 xR2 = 0.28

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roll ET

roll

CB

CT

y = -0.074 xR2 = -0.022

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Residual Pitch error Residual Roll error

Presentator

Presentatienotities

This table shows the mean and the SD of the pitch and roll setup error as determined by the ExacTrac system. Secondly, the mean and SD of the residual pitch and roll setup error as measured using the Cone Beam CT is displayed. ExacTrac shows a mean pitch correction of 1 degree. This systematic setup correction compared to the plannings-CT is due to table sag. There is a significant reduction of the SD for pitch setup corrections. However, the SD of the roll was reduced with 38% only. A further analysis of the roll setup correction follows in the next slide.

Correction for roll&pitch

PTV margin in case of hypo fractionation

• For hypofractionated treatments setup errors are to be treated as systematic errors

– ‘Random part’ vHerk margin recipe 2.5Σ+0.7σ not applicable

• Statistical probability of getting prescription dose in GTV

PTV margin

Σ=1.5mm 1 3 5 8

2.5*Σ 3.8 mm 0.9 (0.9)3 (0.9)5 (0.9)8

2.8*Σ 4.2 mm 0.95 (0.95)3 (0.95)5 (0.95)8

3.4*Σ 5.1 mm 0.99 (0.99)3 (0.99)5 (0.99)8

X

PTV margin

Σ=1.5mm 1 3 5 8

2.5*Σ 3.8 mm 0.9 0.73 0.59 0.43

2.8*Σ 4.2 mm 0.95 0.86 0.77 0.66

3.4*Σ 5.1 mm 0.99 0.97 0.95 0.92

Presentator

Presentatienotities

- For 95% probability for 5 fractions we need 3.4Σ Typical dose gradients for our RapidArc treatment technique are 5% per mm in AP and lateral direction and 10% per mm in the SI direction. The volume encompassed by the 70% isodose is therefore (with a PTV margin of 5mm) at a distance of 7mm from the ITV in the AP and LR direction, and 6mm in the SI direction.

Radiobiological margin

However: -Dose outside PTV is not equal to zero -Small underdosage PTV is not very detrimental for TCP Selvaraj et al, Med. Phys. 40 (2013) - MC simulation of setup errors - less than 1% TCP loss calculation

Σ=1 mm, σ=3 mm

Presentator

Presentatienotities

From a radiobiological point of view the ‘statistical’ PTV margin is too large.

In Conclusion

• Uncertainties exist in all parts of the SABR process – 4DCT – Motion management & Target Volume definition – Treatment planning – Treatment delivery

• An appropriate level of certainty can be obtained by

– Appropriate 4DCT protocols – Appropriate motion management margins & verification – Modern calculation algorithms (with special attention to small

field dosimetry) – Properly chosen PTV-margins

Thank you for your attention

Q4 2017: VUmc SABR/SMART symposium www.sbrt.eu

Download - (Un-)Certainties in SABR · Dose (cGy) Volume (%) AAA Acuros AAA Acuros AAA Acuros AAA Acuros AAA Acuros AAA Acuros AAA Acuros AAA Acuros 9.3 cm3 7.1 cm3 1.6 cm3 11.4 cm 3 5.5 cm3

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