Physics Aspects of SRS/SBRT

Luke Rock

Beacon Hospital

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Outline

• Programme Implementation

- equipment specification

- commissioning

- training

• Treatment Planning

- 4DCT

- treatment techniques

• Treatment Delivery

- respiratory management

- image guidance

- pre-treatment QA

• Quality Assurance

- end to end evaluation

- SOPs

- periodic QA

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AAPM TG 42 1995 – SRS AAPM TG 76 2006 – Respiratory Motion

AAPM TG 142 2009 – Linac QA IPEM Report 103 2010 – Small Field Photon Dosimetry

ACR 2006

ACR 2009

AAPM 2010

ASTRO 2011

Guidance Documents

SABR: A Resource 2014

• Surgical analogy

• Full MDT support during treatment delivery

Multidisciplinary Team

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TG 101 AAPM 2010; ASTRO 2011; ACR 2009

MDT

MDT

Radiation Oncologist

Medical Physicist

Radiation Therapist

Support

staff

• Standard Radiotherapy is generally safe

• May 2000 to Aug 2006: UK reported 181 incidents

40 per 100,000

{3 per 100,000 deemed clinically adverse}

• Introduction of a new technique challenges established

safety systems

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SBRTSkills

Intensive

Complex

Technology

SBRT Programme Implementation

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SBRT

Respiratory Motion

Treatment Planning

Treatment Delivery

Implementation – Equipment Acquisition

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Excluding the pre-implementation planning - Total = 16-26 weeks

Commissioning – How long will it take?

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• Treatment Planning System

- beam model creation and verification

- heterogeneity correction algorithm

• Image Guidance

- determine accuracy of IGRT system

• Respiratory management

- 4DCT commissioning

- gated treatment delivery

Commissioning – what’s needed?

• Small photon field data

• Very difficult measurement

- loss of lateral electronic equilibrium

- detector volume averaging

- detector positioning / orientation error

• High profile errors in Florida (77 pts),

France (145 pts), Minnesota (152 pts)

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TG 101 AAPM 2010

Commissioning – Beam Data Acquisition

• Use appropriate detectors

• Beware of diode detector energy dependence when moving

from large to small fields

• <2cm diode; 2-4cm diode + chamber; >4cm chamber

• Compare your data to other centres with same equipment

• Large differences may be observed (up to 30%)

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Measurement Detector

Depth Dose / TPR Diode /Ion chamber

Beam Profiles Diode /Ion chamber

Relative Output Diode /Ion chamber

Absolute Dose Ion chamber

Commissioning – Beam Data Acquisition

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0.4

0.6

0.8

1

0 1 2 3 4

Ou

tpu

t F

acto

r

Cone Diameter (cm)

Cone Output Factor Comparison

SRS Diode

SHY

GBD

8% difference

Commissioning – Data Comparison Example

• Need to account for motion due to respiration – 4DCT

• Two types of 4DCT:

• Regardless of 4DCT technique, breathing cycle needs to be

regular and reproducible

Respiratory Motion Management – 4DCT

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Prospective Retrospective

Image acquisition on selected part of

breathing cycle

Image acquisition across whole breathing

cycle

Standard size CT dataset Large CT dataset

No post-scan computation required CT data requires binning into phase bins

Limited breathing cycle information Full breathing cycle information

Prospective 4DCT

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Respiration Waveform from RPM Respiratory Gating System

CT Scan

Axial scan trigger,

1st couch position

Axial scan trigger,

2nd couch position

Exhalation

Inhalation

Scan Scan Scan

Axial scan trigger,

3rd couch position

Prospective 4DCT

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Conventional CT Image Gated CT Image

Images Courtesy Medical College of Virginia, Richmond VA

Tumor

Retrospective 4DCT

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X-ray on

Exhalation

Inhalation

1st couch

position

2nd

couch

position

3rd couch

position

“Image acquired”

signal to RPM

system

(Ford 2003, Vedam 2003)

Respiration Waveform from RPM Respiratory Gating System

Retrospective 4DCT

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4D Data and images courtesy

VUmc, Amsterdam, The Netherlands

80% isodose: volume 13 vs. 27 cc

20% isodose: volume 163 vs. 471 cc

4DCT Planning Volumes

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Korreman et al 2012

4DCT Planning Volumes

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Total Dose # Fractions Indications

25-34 Gy 1 Peripheral, small (<2cm) tumours,

>1cm from chest wall

45-60 Gy 3 Peripheral, >1cm from chest wall

48-50 Gy 4 Central, Peripheral, <4-5cm, <1cm

from chest wall

50-60 Gy 5 Central, Peripheral, <1cm from chest

wall

60-70 Gy 8-10 Central tumours

Treatment Planning – Fractionation Schemes

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• Multiple series for Lung SABR – no consensus

OAR 1 Fraction 3 Fractions 4 Fractions 5 Fractions

Spinal Cord 14 Gy 18 Gy

(6 Gy/fx)

26 Gy

(6.5 Gy/fx)

30 Gy

(6 Gy/fx)

Esophagus 15.4 Gy 30 Gy

(10 Gy/fx)

30 Gy

(7.5 Gy/fx)

32.5 Gy

(6.5 Gy/fx)

Brachial Plexus 17.5 Gy 21 Gy

(7 Gy/fx)

27.2 Gy

(6.8 Gy/fx)

30 Gy

(6 Gy/fx)

Heart 22 Gy 30 Gy

(10 Gy/fx)

34 Gy

(8.5 Gy/fx)

35 Gy

(7 Gy/fx)

Great Vessels 37 Gy 39 Gy

(13 Gy/fx)

49 Gy

(12.3 Gy/fx)

55 Gy

(11 Gy/fx)

Airways 20.2 Gy 30 Gy

(10 Gy/fx)

34.8 Gy

(8.7 Gy/fx)

32.5 Gy

(6.5 Gy/fx)

Chest Wall 30 Gy 30 Gy

(10 Gy/fx)

30 Gy

(7.5 Gy/fx)

32.5 Gy

(6.5 Gy/fx)

Skin 26 Gy 30 Gy

(10 Gy/fx)

36 Gy

(9 Gy/fx)

40 Gy

(8 Gy/fx)

Treatment Planning – OAR Dose Constraints

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NCCN Guidelines v3 (2012) based on RTOG 0618; 0813; 0915

Treatment Planning – Beam Arrangements

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• Use multiple beams from many directions

• Typically 8/9 fields for 3DCRT/IMRT

• 2/3 partial arcs for VMAT

• Skin dose <30% of prescribed dose TG 101 AAPM 2010

Treatment Planning – Coplanar or Non-coplanar?

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Non-coplanar Coplanar

Treatment Planning – Coplanar or Non-coplanar?

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• Advantages

- lower lung dose

- improved distribution in axial plane

• Disadvantages

- complicated treatment (increase risk of error)

- longer treatment time

- potential collisions (gantry/couch/patient)

Treatment Planning – Calculation Algorithm

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• Dose calculation in a challenging environment

– small field size

– target surrounded by low density tissue

• Heterogeneity correction is required- Pencil beam algorithms not sufficient

- Minimum requirement is an algorithm that accounts for 3D

scatter integration (e.g. convolution/superposition)

- Ideal algorithm will account for photon and electron transport

(e.g. Monte Carlo; LBTE)

TG 65 AAPM 2004

TG 101 AAPM 2010

TG 101 AAPM 2010

Treatment Planning – Calculation Algorithm

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• Reported Inaccuracies:

- 1D correction up to 30%

- 2D (e.g. Pencil Beam) up to 10-15%

- 3D (e.g. Convolution/Superposition) up to 5%

- Transport equations (e.g. Monte Carlo, LTBE) up to 2%

• Know limitations of your algorithm!

Treatment Planning - Interplay effect

Li et al JACMP 2013

• Effect increases with tumour motion

• Predominant in high dose gradients

• Use gating or sufficient margins to reduce effect

Chen et al 2009;

Li et al 2013

Treatment Delivery - Immobilisation

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BodyFix by Medical

Intelligence

Body Pro-Lok

by CIVCO

Stradivarius by

Q Fix

Treatment Delivery - Immobilisation

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• IGRT is essential for SBRT

Treatment Delivery - Localisation

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TG 101 AAPM 2010

TG 101 AAPM 2010; ASTRO 2011

• Determine the accuracy of your IGRT system

TG 101 AAPM 2010; ASTRO 2011; ACR 2009

• Physician led image review recommended

TG 101 AAPM 2010

• Patient localisation must be verified before each

treatment fraction

SRS/SBRT - Quality Assurance

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• Two types of QA

- QA of individual components of system (Linac, IGRT, respiratory

gating, MLC, etc); usually performed frequently (e.g. daily, weekly)

- QA of whole treatment system (End-to-End evaluation);

usually performed annually

Quality Assurance – Winston-Lutz Test

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• Measure of accelerator isocentre accuracy

• TG142 tolerance is avg 0.75mm (max 1mm)

• Commercial software available

Quality Assurance – Winston-Lutz Test

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• Can the isocentre move?

• Yes

• Noted drift in couch

isocentre over 6 month

period

• Adjust accelerator position

by ~1.0mm

Imaging

Planning

LocalisationTreatment Delivery

Analysis

Quality Assurance – End to End Evaluation

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Quality Assurance – End to End Evaluation

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• Annually

• Radiological Physics Center, MD Anderson

• External, independent audit of treatment chain

Quality Assurance – TG 142 Summary

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Conclusions

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• SRS/SBRT are well established techniques

• Both present significant technical challenges

• Requires multiple complex technologies

- Image Guided RT

- Respiratory management

- Immobilisation

- Treatment planning

- Precise treatment delivery

• QA programme requires more resources than standard RT

• Comprehensive practice guidelines available