The document outlines the history, advancements, and recommendations for stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT), highlighting the advantages of flattening filter free (FFF) beams, which allow for higher dose rates and reduced radiation exposure to surrounding tissues. It discusses the evolution of SRS/SBRT methods, including the use of linear accelerators and gamma knife technology, as well as dosimetric concerns associated with FFF beams. Additionally, the document provides recommendations from the AERB and AAPM Task Group regarding treatment protocols, simulation imaging, and effective dose calculation algorithms for implementing these advanced radiation therapies.

1. 11 October 2014

2. 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

3. 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

4. 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 delivery
5. Real time respiratory motion accommodation

5. SRS and SBRT
The challenge for SRS and SBRT
is to accurately deliver
conformal high dose radiation
to the target and minimize
normal tissue damage.

6. Differences between Conventional and Stereotactic

7. 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
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)

8. Gamma Knife Proton Therapy
Radiosurgery Machines
Cyberknife Tomotherapy Brainlab Vero Varian-Truebeam

9. SBRT Work Flow
Immobilization
Simulation
Motion Management Planning
Delivery Motion Verification IGRT

10. 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
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.

11. 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.

12. 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

13. 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 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

14. 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

15. 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
treatment delivery is compromised by the large number of MU
to delivery high fraction doses.
Patient beam on time can be reduced for IMRT

16. Cyberknife Tomotherapy BrainLab Vero
FFF Mode Machines
Varian -Truebeam Elekta – Versa HD

17. 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 cm2
Speed of MLC 2.5cm/sec
160Leaf MLC
Resolution 5.0 mm, 40cm wide
Modulation Area 40x40 cm2
Speed of MLC 4.0cm/sec

18. Comparison between 6X FB and FFF (Varian
TrueBeamTM) - Profiles
FB Profiles FFF Profiles

19. Comparison of PDD for FBFFF for 10cmX10cm
10XFB
10XFFF
6XFFF
6XFB

20. Variation of Output factor in air with field size
1.0500
1.0400
1.0300
1.0200
1.0100
1.0000
Head Scatter Factor ( Sc)
0.9900
0.9800
0.9700
0.9600
0.9500
0.9400
0 5 10 15 20 25 30 35 40
Field Size in cm2
6MV-FB Varian True Beam
6MV-FFF True Beam
10MV-FB Varian True Beam
10MV-FFF True Beam

21. 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

22. AERB Recommendations for FFF
Treatment should be
implemented with TPS
through Record Verify
system, Manual
planning and calculation
shall not be adopted in
clinical use of FFF
beam.

23. AERB Recommendations for FFF
Measurements should cover
Beam Energy:
TPR20/10 for 10 cm x 10 cm Field Size for all FFF energies
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
–
•

24. AERB Recommendations for FFF
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,
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

25. AERB Recommendations for FFF
Depth dose profiles
Flatness: As per IEC
976 (IEC 60976), the
flat region for field
sizes less than 10cm x
10cm along major
axes defined by
subtracting 1cm from
the beam profiles.
Eg. For F.S 5cm x 5cm
flat region is central
3cm.

26. AERB Recommendations for FFF
Depth dose profiles
Inflection Point: Inflection
Point can be identified as per
its mathematical definition.
However, for practical
purposes it can be
approximated 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

27. AERB Recommendations for FFF
Degree of un-flatness:
• To quantify the degree of un-flatness,
lateral distance from
the central axis at 90%, 75%
and 60% dose points on
either side of the beam
profile shall be recorded
along major axes.

28. Trubeam FB and FFF beam Stereotactic Plan
comparison – Liver
6 MV_FFF
1400MU/M
6 MV_FB
600MU/M

29. Trubeam FB and FFF beam Stereotactic Plan
comparison – Brain
6 MV_FB
600MU/M
6 MV_FFF
1400MU/M

30. 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
size 4 cm

31. 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.
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

32. AAPM TG 101 Recommendation for SBRT
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
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.

33. AAPM TG 101 Recommendation for SBRT
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-axis
ratios.

34. Accuracy of SRS SBRT depends on
Linac Mechanical – Iso
Accuracy of SRS frame Immobilization
Positional accuracy
Dosimetry accuracy

35. 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.
More studies are needed for FFF beam to specify and quantify the clinical
advantages, especially with respect to treatment plan quality and quality
assurance.

36. 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.

37. 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
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
– AAPM 54th Annual Meeting - 2012
Effect of Surface Dose and Depth of Maximum Dose with Physical Wedge Filters
for 6MV Flattened and 7MV
– AAPM 54th Annual Meeting - 2012

38. Our Medical Physics team
Dr. Girigesh Yadav
Mr. Manindra Mishra
Mr. T. Suresh
Mr. Lalit Kumar
Mr. Pavan Singh