yue yan prof. bhudatt paliwal - aapm...
TRANSCRIPT
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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