dose to water vs. muscle: rationalerpc.mdanderson.org/rpc/publications/rpc...stephen f. kry*,...
TRANSCRIPT
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Dose to water vs. muscle:
Rationale
Stephen F. Kry*, Vladimir Feygelman,
Peter Balter, Tommy Knoos, Charlie Ma,
Michael Snyder, Brian Tonner, Oleg Vassiliev
AAPM Annual Meeting, July 2017
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Introduction
•Clinical reference dosimetry (i.e., TG-51 calibration)
is done in water
•Dose delivered in the patient is in tissue
•These are not the same!
≠
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Differences
Water Tissue Muscle
• Density (g/cm3) 1.00 1.025 1.050
• Relative e- density 1.00 1.019 1.042
• Cost at steak house ($) 0 ~40
• Composition O H O C H N Na,P,S,Cl,K
(% mass) 89 11 57/71 29/14 10 3 Trace
ICRU 46
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Problem
• We want to know dose to tissue/muscle
–This is what patients are made of
–This is what clinical experience is based on (clinical trials)
–This is where dose calculation algorithms are headed
• How do we manage this in terms of calibration
–calibration (water) vs. calculation (muscle)?
• How do we move between these two media?
–Not talking about Dm vs Dw, just talking about the specific
issue of how do we move between these during calibration
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Ideal solution
• Calibration is done in water
• TPS recognizes that the patient is not water and
inherently accounts for this difference
• Then moving between media is implicitly handled by
the underlying physics (as it should be!)
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Historical management
• TPS (or hand calcs) didn’t handle the non-water nature
of the patient
• Calibrate in water
• Apply a conversion 0.99 during the calibration
–μen/ρ or S/ρ
–Accounts for difference in chemical composition
• This yielded “dose-to-muscle”
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Question 1:
• How well did this work?
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Dose deposition in water vs tissue
• Dose difference between tissue/muscle and water with the
same electron density (high density water)
• Photon Beams Electron Beams
6 M V IC R U -4 4 M u s c le
6 M V IC R U -4 4 S o ft T is s u e
1 8 M V IC R U -4 4 S o ft T is s u e
0 1 0 2 0 3 0
-0 .5
0 .0
0 .5
1 .0
1 .5
2 .0
D e p th (c m )
(DW
- D
T)/
DT
(%
)
6 M e V
2 2 M e V
0 2 4 6 8
0 .0
0 .5
1 .0
1 .5
2 .0
D e p th (c m )
(DW
- D
T)/
DT
(%
)
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Question 2:
• Is this still appropriate?
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What is the current status?
• Mixed result
• From IROC reporting of output verification
–75%: Dose to water
–25%: Dose to muscle (via 0.99 correction)
• No consistency
• No dependence on planning system or algorithm
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Why does the TPS matter?
• Cleanest situation:
–Calibrate dose to water, TPS inherently maps to the medium (i.e., muscle)
–No error (dose calculated correctly)
• If TPS inherently maps from water (calibration) to muscle (patient
calculation) and we apply a 0.99 correction
–We have a 1% error (calculated dose too low)
• If TPS does not map from water to muscle and we don’t apply a
0.99 correction
–We have a 1% error (calculated dose too high)
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Motivation
• Provide clarity for the link between calibration (water)
and dose to the patient (muscle)
–For a given algorithm how do we manage water vs. muscle
calibration so that results are as consistent as possible.
• Increase accuracy – everyone is getting the same
answer under the same conditions
• Yes 1% is small
–Half the uncertainty budget
–Not small in calibration terms – larger than kQ, Pion, Ppol,….
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AAPM Group
AAPM report on clinical reference calibration:
dose to water or dose to muscle?• Stephen Kry (co-chair)
• Vladimir Feygelman (co-chair)
• Peter Balter
• Tommy Knoos
• CM Charlie Ma
• Michael Snyder
• Brian Tonner
• Oleg Vassiliev
Report is under review by the AAPM
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Now on to part 2
• Hopefully this has provided a clear framework
• How do these results look
• How should one incorporate this into their clinical
practice
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End
Thank you