Yue YanProf. Bhudatt Paliwal 10/24/2014

Background Motivation Preliminary studies

• Clinical comparison• Monte-Carlo (MC) simulation• Measurement test

Conclusion Questions and answers

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Flattening filter (FF) has been applied as an essential part of Linac

system over decades.

The development of modern RT makes the FF no longer an

indispensable part of Linac system.

The potential clinical application of the FFF bremsstrahlung photon

beams generated by modern Linacs is becoming much more

apparent[1-12].

However, limitations still exist for the FFF beam.

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Pros of FFF• Greatly increased dose rate (approx. 2-4 times compared with the

flattened beam)[2-5,9].

• Large decrease of external scatter from the gantry head[6].

• Greatly reduced neutron contamination for high energy photon beams[7].

• Reduced uncertainty in dose calculation due to scatter from the FF[8]. Cons of FFF

• High dose rate of the FFF beam is more likely to lead to unacceptable

dose deviation caused by intrafraction patient motion during treatment

delivery[1].

• Soft spectrum of the FFF beam tends to increase the internal patient

scatter to peripheral organs[9-12].

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Internal patient scatter is the predominate factor

contributing to peripheral organ dose[12].

External head scatter and dose leakage from the gantry

head also contribute smaller amounts to peripheral

organ dose.

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The hypothesis: Dose to OARs can be further reduced by attenuating the soft

photons in the FFF beam spectrum.

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FFF Beam Flattened BeamLow head scatter High head scatter from the flattening filter

Soft spectrum Hard spectrum

High dose rate Low dose rate

Increased internal scatter due to soft photons Increased internal scatter due to dose normalization

Lower tail dose in the beam profile Higher tail dose in the beam profile

SPECTER

In general, lower mean dose to the OARs In general, higher mean dose to the OARs

Adjusting both the beam spectra and the profile to yield a more hardened SPECTER beam could reduce the internal patient scatter and reduce the dose to OARs for the FFF beam.

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Clinical comparison (cont…):• Treatment plans study summary:

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Category Beam ModalityDelivery

TechniqueTreatment Schedule OARs

Head and Neck

6 MV and 10 MVFlattened beam &

FFF Beam

Static IMRT&

VMAT

60 Gy/30 frac. (2 pts.)30 Gy/15 frac. (1 pt.)70 Gy/28 frac. (1 pt.)

spinal cord, brainstem,

brain, larynx, pharynx, parotid

Lung45 Gy/30 frac. (1 pt.)66 Gy/33 frac. (2 pts.)60 Gy/30 frac. (1 pt.)

lungs, spinal cord, heart,

larynx, thyroid, esophagus

Prostate 70 Gy/28 frac. (3 pts.)78 Gy/39 frac. (1 pt.)

rectum, bladder, hips

Breast(RTOG 1005)[13]

Static IMRT50 Gy/25 frac.

12 Gy/6 frac. (1 pt.) (sequential boost)

lungs, heart, thyroid, skin

Clinical comparison (cont…):• Normalization: 95% @95% to exclude lower-

mean-dose effect to the target of the FFF beam compared with the

flattened beam.

• TrueBeam (Varian, CA) system was commissioned on Eclipse®

(Varian, Palo Alto, Medical Systems) TPS workstation.

• Anisotropic Analytical Algorithm (AAA) was used to calculate the

dose.

• Treatment parameters (e.g. field size, arc number, beam angle)

were kept to be identical to exclude differences from these factors.

• Clinical objectives were used to simulate clinical application.

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Clinical comparison (cont…): head and neck (case 7):o Dashed line: FFF beam; solid line: flattened beam

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VMAT Plans

IMRT Plans

Clinical comparison (cont…):

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Dose and NTCP Ratios(FFF/flattened) for Head and Neck Cancer CasesVMAT

6 MV 10 MV

Organs' name Mean dose ratio

Max dose ratio

Mean BED ratio NTCP ratio Mean dose

ratioMax dose

ratioMean BED

ratio NTCP ratio

Larynx 0.98±0.02 0.99±0.02 0.97±0.02 0.84±0.20 0.97±0.04 1.00±0.01 0.97±0.05 0.87±0.19R parotid 0.94±0.03 0.96±0.02 0.93±0.03 0.31±0.20 0.91±0.04 0.96±0.05 0.91±0.04 0.24±0.27

R submandibular 0.97±0.02 0.98±0.00 0.96±0.02 0.64±0.03 0.92±0.04 0.96±0.04 0.91±0.04 0.28±0.14L cochlea 0.97±0.04 0.98±0.04 0.97±0.05 0.71±0.48 0.94±0.06 0.95±0.06 0.93±0.07 0.43±0.40

IMRT

6 MV 10 MV

Organs’ name Mean dose ratio

Max dose ratio

Mean BED ratio NTCP ratio Mean dose

ratioMax dose

ratioMean BED

ratio NTCP ratio

Larynx 1.02±0.02 1.02±0.01 1.03±0.02 1.57±0.39 1.02±0.01 1.02±0.01 1.02±0.01 1.42±0.25R parotid 1.00±0.02 1.01±0.01 1.00±0.02 0.95±0.47 0.99±0.04 1.00±0.02 0.99±0.04 0.89±0.72

R submandibular 1.00±0.04 1.00±0.01 1.00±0.05 1.21±0.75 1.04±0.02 1.02±0.02 1.05±0.02 1.74±0.15L cochlea 0.98±0.03 1.00±0.07 0.98±0.04 1.32±1.29 1.00±0.03 1.02±0.08 1.00±0.05 2.18±2.25

Clinical comparison (cont…):

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MU for Head and Neck Cancer CasesEnergy 6 MV 10 MV

Delivery Method VMAT IMRT VMAT IMRT

Beam Modality Flattened FFF Flattened FFF Flattened FFF Flattened FFF

Total MUs

Case 5 559 640 2117 3072 545 674 2001 3633

Case 7 812 959 1726 2647 780 1011 1655 3128

Case 8 605 757 965 1292 581 801 883 1380

Case 10 390 459 916 1323 373 483 873 1773

Case 24 389 430 803 1254 370 427 768 1758MU Ratio

(FFF/Flattened) 1.17±0.05 1.47±0.09 1.27±0.08 1.92±0.27

Clinical comparison (cont…):• Summary and discussion:

The differences between the FFF beam and the flattened beam are more

obvious in VMAT plans compared with the static IMRT plans.

Maximum dose rates may not be achievable for the FFF beam plans in

general due to MLC speed limitation.

In certain clinical cases with large field sizes (~16 20 ), the FFF

beam tends to provide higher dose to OARs compared with the flattened

beam in the static IMRT plans.

The FFF beam plans uses higher MU to provide the same target

coverage compare with the flatted beam plans.

In general, FFF beam provides lower mean dose to OARs compared with

the flattened beam.

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Monte-Carlo study:• Detailed geometry information is necessary to accurately simulate

the TrueBeamTM system (Varian, CA).

• Varian provided the IAEA (International Atomic Energy

Agency)phase space data above the jaws inside the gantry head.

14(http://sudentas.com/tag/truebeam) (TrueBeam Monte Carlo Data Package, Varian Medical System)

Monte-Carlo study (cont…):• VirtuaLinac web interface based on Amazon Web Services (AWS) system enables

us to consider the geometry of the jaws inside the gantry head[17,18].

• Based on the new IAEA phase space data, BEAMnrc MC software package was

used to simulate the TrueBeam system.

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Monte-Carlo Study (cont…):• Excellent agreements were obtained between the MC simulation

and the standard measurement data.

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Monte-Carlo study (cont…):• Dose profile normalization: 6 MV beams

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Monte-Carlo study (cont…):• Dose normalization: 6 MV and 10 MV beams

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6 MV

Monte-Carlo study (cont…):• A soft-spectrum-filter (SPECTER) is proposed to attenuate the soft

photons in the FFF spectrum.

• The low density of the SEPCTER in the central axis (filled with air)

enables us to keep the high dose rate of the FFF beam.

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Monte-Carlo study (cont…):• Different designs of the SPECTER: (gantry head z=58 cm.)

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21 cm18.3 cm

15.4 cm

12.5 cm

9.6 cm

6.7 cm

21 cm

18.3 cm

15.4 cm

12.5 cm

9.6 cm

6.7 cm

18.3 cm

15.4 cm

12.5 cm

9.6 cm

6.7 cm

18.3 cm

15.4 cm

12.5 cm

9.6 cm

6.7 cm

Monte-Carlo study (cont…): • Steps to calculate the internal scatter[12,19]:

.

• Boundary of the central region was slightly larger than the field

size.

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Monte-Carlo study (cont…): beam fluence

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All MC simulations after this slice were based on the lead circularcross section SPECTER.

Monte-Carlo study (cont…):• The SPECTER beam reduced the total dose and the internal scatter compared

with the FFF beam.

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25 25 , 10 40 40 , 10

Measurement results:• Two SPECTERs were made using lead and tin.

• Cross sections were square.

• The heights were chosen to be 6 for lead and 1.5 for tin.

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Measurement results (cont…):• Lead SPECTER provides better dose reduction effect compared

with tin, due to higher attenuation of the soft photons of lead.

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Field sizes from left to right: 3 3, 5 5, 10 10, 15 15,20 20, 25 25 . 10 . Solid line: FFF beam,dashed lines with and without dots: SPECTER beamswith square cross section. Dashed line without dots: leadSPECTER; dashed line with dots: tin SPECTER.

Measurement Results

Field size is chosen to be 25 25 . Solid line: FFF beam,dashed lines with and without dots: SPECTER beamswith square cross section. Dashed line without dots: leadSPECTER; dashed line with dots: tin SPECTER.

Measurement results (cont…):• However, the MC simulation did not agree with the measurement

data in the tail region.

• It can be explained by the over-responding of the Profiler(Sun

Nuclear, Melbourne, FL)[21].

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Conclusion:• The SPECTER beam has potential to reduce the internal scatter

and improve the dose sparing effect to OARs.

• The low density in the central axis of SPECTER provides high

dose rate compared with the flattened beam.

• Attenuating the soft photons in the FFF beam spectrum may not

lead to prolonged treatment time compared with the FFF beam due

to the speed limitation of the MLCs.

• Geometry and material are important for the design of the

SPECTER.

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