SRS & SBRT - Unflattened Beam

11 October 2014

Outline

� SRS and SBRT

� History of SRS

� Recent advances in SRS and SBRT

� Advantage of Flattening Filter Free(FFF) beam

� Characteristic of Flattening Filter Free beam

� Recommendation of AERB & AAPM TG 101

Introduction

�Stereotactic Radiosurgery (SRS)

– SRS is a precise and focused delivery of a single, high dose

of irradiation to a small and critically located intracranial

volume while sparing normal structure

�Stereotactic Body Radiation Therapy (SBRT)

– SBRT is a treatment procedure similar to SRS, except that

it deals extra-cranial radiosurgery

SRS and SBRT Definition

1. High doses of radiation via multiple beams

2. Limited number of treatment session (1-5)

3. Image guided treatment (CT, PET, MRI)

4. Computer assisted robotic delivery4. Computer assisted robotic delivery

5. Real time respiratory motion accommodation

SRS and SBRT

The challenge for SRS and SBRT

is to accurately deliver is to accurately deliver

conformal high dose radiation

to the target and minimize

normal tissue damage.

Differences between Conventional and Stereotactic

Historical Background

� The first one to combine stereotactic methodology with

radiation therapy was the Swedish neurosurgeon Lars

Leksell. Leksel performed the first treatment in 1951, at the

Karolinska Institute, and called the new therapy approach

radiosurgery (RS)

� Leksel continued his work and built the first isotope � Leksel continued his work and built the first isotope

radiation machine, in 1968, the Gamma knife

� The stereotactic radiation therapy with LINAC started in the

early 1980s: the Swedish physicist Larsson proposed to use

the LINAC instead Co 60 or protons (Larsson et al. 1974)

Radiosurgery Machines

Gamma Knife Proton Therapy

Cyberknife Tomotherapy Brainlab Vero Varian-Truebeam

SBRT Work Flow

Immobilization &

Simulation

Motion Management PlanningSimulation

IGRTMotion VerificationDelivery

How LINAC Radiosurgery Works

� The gantry of the LINAC rotates around

the patient, producing an arc of radiation

focused on the target. The couch in

which the patient rests is then rotated in

the horizontal plane, and another arc is the horizontal plane, and another arc is

performed. In this manner, multiple non-

coplanar arcs of radiation intersect at the

target volume and produce a high target

dose, resulting in minimal radiation

affecting the surrounding brain and

normal tissue.

How Gamma Knife Radiosurgery Works

� The GammaKnife is used to treat brain

tumors. The procedure begins with the

patient receiving anesthesia and a frame

is attached to the head to hold it in place.

� The patient lays on their back and moved

head first into the machine, where 201

beams of cobalt – 60 radiation target the

diseased tissue, without damaging the

surrounding tissue.

Recent Advances in SBRT and SRS

�VMAT� Volumetric Modulated Arc Therapy (VMAT)

was first introduced in 2007 and described as a

novel radiation technique

� VMAT is the simultaneous variation of three

parameters during treatment

delivery, i.e. gantry rotation speed, treatment

aperture shape via movement of MLC leaves

and dose rate

Recent Advances in SBRT and SRS

� Flattening Filter Free (FFF) mode� FFF beam is produced without the use of

flattening Filter

� In the 1990s, several groups studied about FFF

high-energy photon beams. The main interesthigh-energy photon beams. The main interest

for that, is to increase the dose rate for

radiosurgery or the "physics interest”.

� Need of increase in dose rate from traditional

300-600 to 1400-2400MU/min to overcome

time-inefficiency and to improve patients

comfort specially in SRS/SBRT

Dosimetric advantages of FFF beams

� FFF has increased dose rate, e.g., 1400 MU/min for 6 MV,

2400 MU/min for 10 MV.

� FFF beams have less variation of off-axis beam hardening.

� FFF has less photon head scatter and thus less field size

dependence.

� FFF has less leakage outside of beam collimation

Potential advantages of FFF beams

� Fast treatment for Stereotactic Radiotherapy (SRT) and SRT

plans between FB and FFF beams should be similar for small

fields.

� FFF is especially useful for SBRT, where respiration controlled� FFF is especially useful for SBRT, where respiration controlled

treatment delivery is compromised by the large number of MU

to delivery high fraction doses.

� Patient beam on time can be reduced for IMRT

FFF Mode Machines

Cyberknife Tomotherapy BrainLab Vero

Varian -Truebeam Elekta – Versa HD

Two Different FFF machines

at RGCI

Varian – Truebeam

� Dose rate :

1400MU/min 6MV FFF &

2400MU/min 10MV FFF

Siemens – Artiste

� Dose rate:

• 2000MU/min - 6MV_FFF

� 120Leaf HD – MLC

Center - 2.5 mm width x 32 pairs

Peripheral 5.0 mm width x 28 pairs

� Modulation Area 22x40 cm 2

� Speed of MLC 2.5cm/sec

� 160Leaf MLC

Resolution 5.0 mm, 40cm wide

� Modulation Area 40x40 cm2

� Speed of MLC 4.0cm/sec

Comparison between 6X FB and FFF (Varian

TrueBeamTM) - Profiles

FB Profiles FFF Profiles

Comparison of PDD for FB&FFF for 10cmX10cm

10XFB

10XFFF

6XFFF

6XFB

1.0000

1.0100

1.0200

1.0300

1.0400

1.0500

He

ad

Sca

tte

r F

act

or

( S

c)

Variation of Output factor in air with field size

0.9400

0.9500

0.9600

0.9700

0.9800

0.9900

0 5 10 15 20 25 30 35 40

He

ad

Sca

tte

r F

act

or

( S

c)

Field Size in cm2

6MV-FB Varian True Beam

6MV-FFF True Beam

10MV-FB Varian True Beam

10MV-FFF True Beam

Dosimetry concern of FFF

• Due to the above changes, the Dosimetric parameters like field size

definition, beam quality, surface dose, off axis ratio (OAR), flatness,

symmetry, degree of un-flatness, penumbra and depth dose profiles differs

from standard Linac with Flat beam.

• There is no international standard/acceptance test protocol available for

FFF beam, AERB constituted a Task Group to evolve the acceptance

criteria for FFF beam

AERB Recommendations for FFF

Treatment should be

implemented with TPS

through Record & Verify

system, Manual system, Manual

planning and calculation

shall not be adopted in

clinical use of FFF

beam.


Measurements should cover

� Beam Energy:

TPR20/10 for 10 cm x 10 cm Field Size for all FFF energies

� Surface dose:� Surface dose:

10cm x 10cm and 20cm x 20cm compared with the corresponding

nominal flat beam energy

� OAR

At ±3 cm from central axis at the depth of 10 cm for 10 cm x 10 cm

collimator setting shall be measured for all available FFF energies

–

•


� Depth dose profiles

�Dose profile for field size 5cm x 5cm, 10cm x 10cm and 20cm x

20cm at depth of Dmax and 10cm shall be recorded for all available

energies

�FS ≤10 cm x 10 cm, the Dosimetric parameters such as field size,�FS ≤10 cm x 10 cm, the Dosimetric parameters such as field size,

penumbra, flatness, symmetry shall be measured and evaluated the

methods applied for flat beam

� If flatness is > ± 3%, the evaluation criteria of unflattend beam shall

be adopted



�Flatness: As per IEC

976 (IEC 60976), the

flat region for field

sizes less than 10cm x

10cm along major10cm along major

axes defined by

subtracting 1cm from

the beam profiles.

Eg. For F.S 5cm x 5cm

flat region is central

3cm.



� Inflection Point: Inflection

Point can be identified as per

its mathematical definition.

However, for practical

purposes it can be

approximated as the midapproximated as the mid

point on either side of the

high gradient region (sharply

descending part) of the beam

profile. IP is located at h/2.

� Penumbra:Lateral Separation

beween either side of profile

will be measured for the

penumbra


� Degree of un-flatness:

• To quantify the degree of un-

flatness, lateral distance from

the central axis at 90%, 75%

and 60% dose points onand 60% dose points on

either side of the beam

profile shall be recorded

along major axes.

Trubeam FB and FFF beam Stereotactic Plan

comparison – Liver

6 MV_FFF

1400MU/M

6 MV_FB

600MU/M

Trubeam FB and FFF beam Stereotactic Plan

comparison – Brain

6 MV_FB

600MU/M

6 MV_FFF

1400MU/M

Comparison of FFF and FB for SBRT

� Similar Dose distribution and DVH for FB and FFF

�Treatment plan strategies are similar between FB and

FFF beams since the beam profile are similar for field FFF beams since the beam profile are similar for field

size < 4 cm

AAPM TG 101 Recommendation for SBRT

� SBRT Patient Selection Criteria:

� When appropriate protocols are not available, clinicians must decide

whether they will treat patients in accordance with published guidelines

or develop new SBRT guidelines. At a minimum, an institutional

treatment protocol or set of guidelines should be developed by

radiation oncologists and physicists.radiation oncologists and physicists.

� Simulation imaging:

� The simulation study should cover the target and all organs at risk to

obtain geometric and Dosimetric information for the treatment setup

� Slice thickness: < 3 mm near clinically important organs


� Planning Recommendation:

� The adequacy of target margins i.e., GTV, CTV, ITV, in SBRT should

be based on from information in the current literature available

� Dose Calculation Algorithm:

� Algorithms that account for 3D scatter integration such as convolution/superposition� Algorithms that account for 3D scatter integration such as convolution/superposition

have been found to perform adequately in most clinical situations, including in many

cases circumstances where there is a loss of electronic equilibrium such as the lung tissue

interface or tumor margin in low-density medium.

� Calculation algorithms accounting for better photon and electron transport such as Monte

Carlo would be ideal for the most demanding circumstances, such as a small lesion

entirely surrounded by a low-density medium.


� Special Dosimetry Recommendation:

� Due to the small dimensions and steep dose gradients of photon beams

used in SRS/SBRT and IMRT, an appropriate dosimeter with a spatial

resolution of approximately 1 mm or better stereotactic detectors is

required to measure the basic dosimetry data, e.g., the total scatter

factor or relative output factor, tissue maximum ratio, and off-axisfactor or relative output factor, tissue maximum ratio, and off-axis

ratios.

Accuracy of SRS & SBRT depends on

� Linac Mechanical – Iso

� Accuracy of SRS frame & Immobilization

� Positional accuracy� Positional accuracy

� Dosimetry accuracy

Conclusions

� Advanced treatment techniques such as SBRT and IMRT have stimulated

interest in FFF beam, which provides higher dose rate, and reduced head

scatter, leaf transmission, and head radiation leakage.

� Clinical utilities of FFF beam include a treatment time reduction for SRT,

SBRT, and IMRT.SBRT, and IMRT.

� More studies are needed for FFF beam to specify and quantify the clinical

advantages, especially with respect to treatment plan quality and quality

assurance.

Conclusions

� Several aspects related to standardization, dosimetry, treatment planning,

and optimization need to be addressed in more detail in order to facilitate

the clinical implementation of FFF beams.

Our Publications & Abstracts of FFF beam

� “Comparison of Head Scatter Factor for 6MV and 10MV flattened (FB) and

Unflattened (FFF) Photon Beam using indigenously Designed Columnar Mini

Phantom”

– Journal of Medical Physics, July-September 2014

� A Comparison of Out-Of-Field Dose and Its Constituent Components for 6MV

Flattened and 7MV Unflattened

– AAPM 55th Annual Meeting - 2013– AAPM 55th Annual Meeting - 2013

� Comparison of the Depth Dose in the Build-Up Region and Surface Dose for 6MV

Flattened and 7MV

– AAPM 55th Annual Meeting - 2013

� Scatter Factors Comparison of 6MV Flattened and 7MV Unflattened Beams


� Effect of Surface Dose and Depth of Maximum Dose with Physical Wedge Filters

for 6MV Flattened and 7MV


�Our Medical Physics team

�Dr. Girigesh Yadav

�Mr. Manindra Mishra�Mr. Manindra Mishra

�Mr. T. Suresh

�Mr. Lalit Kumar

�Mr. Pavan Singh

SRS & SBRT - Unflattened Beam

Health & Medicine

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