mohammad hussien, nicholas spyrou & andy ma anthropomorphic voxel phantom effective neutron...
TRANSCRIPT
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MOHAMMAD HUSSIEN, NICHOLAS SPYROU & ANDY MAMOHAMMAD HUSSIEN, NICHOLAS SPYROU & ANDY MA
ANTHROPOMORPHIC ANTHROPOMORPHIC
VOXEL PHANTOM VOXEL PHANTOM
EFFECTIVE NEUTRON DOSES OUTSIDE A EFFECTIVE NEUTRON DOSES OUTSIDE A
LINAC ROOMLINAC ROOM
Department of Physics, University of Surrey, Guildford GU2 7XH, United KingdomDepartment of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom
presented on behalf ofpresented on behalf of
by Mary Chinby Mary Chin
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reproduced from Chin MPW 1999 MSc Dissertation, University of Surrey
PROBLEM STATEMENT
RADIOTHERAPY
DELIVERED IN THE ROOM
STAFF WORK H
ERE
POSSIBLY P
UBLIC A
REA
WE N
EED TO E
STIMATE
THE DOSE
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CURRENT STANDARDS
= HOMOGENEOUS
ELLIPSOIDS
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THIS WORK
= ANTHROPOMORPHIC PHANTOM
Zubal G et al 1994 Med. Phys. 21 299Alghamdi A A et al 2005 Radiat. Prot. Dosimetry 115 606
WE GET TO KNOW NOT ONLY THE
EFFECTIVE DOSE, BUT ALSO THE
ABSORBED DOSE ORGAN-BY-ORGAN
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Chin MPW 1999 MSc Dissertation, University of Surrey
*252Cf neutron source
METHOD: A 2-STAGE SIMULATION
STAGE #1
FIND NEUTRON
SPECTRUM AT
EACH OF THESE
LOCATIONS
ACTUAL LINAC & PHOTONEUTRON PRODUCTION
NOT MODELLED
JUSTIFIED IN Chin, Spyrou & Tomlinson 2007
Trans ANS
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METHOD: A 2-STAGE SIMULATION
STAGE #2
FOUND NEUTRON
SPECTRUM
SURFACE SOURCE ORIENTATION REPEATED FOR:
• LEFT-TO-RIGHT
• RIGHT-TO-LEFT
• FRONT (ANTERIOR-TO-POSTERIOR)
• BACK (POSTERIOR-TO-ANTERIOR)
1 2345
DOSE ESTIMATES AT 5 POSITIONS
REPEATED FOR:
• ICRP PUBLICATION 60 (1991)
• ICRP RECOMMENDATIONS 2005
• ICRP RECOMMENDATIONS 2007
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E = EFFECTIVE DOSE
WT = TISSUE WEIGHTING FACTOR
WR = RADIATION WEIGHTING FACTOR
DT, R = AVERAGE ABSORBED DOSE
Most significant changes in wT between 1990 and 2007
Gonads from 0.2 to 0.08
Breast from 0.05 to 0.12
Liver, thyroid, bladder & oesophagus from 0.05 to 0.04
Salivary glands introduced at 0.01
Prostate, lymph nodes & gall bladder added to the ‘remainder’ section
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E = EFFECTIVE DOSE
WT = TISSUE WEIGHTING FACTOR
WR = RADIATION WEIGHTING FACTOR
DT, R = AVERAGE ABSORBED DOSE
DOSE ESTIMATES AT 5 POSITIONS
REPEATED FOR:
• ICRP PUBLICATION 60 (1991)
• ICRP RECOMMENDATIONS 2005
• ICRP RECOMMENDATIONS 2007
FOR THERMAL NEUTRONS
WR = 5
WR = 1
WR = 2.5
THE NEED TO REDUCE WR=5
WHEN NEUTRONS
INTERACT, CONTRIBUTION
FROM SECONDARY
PHOTONS INCREASES WITH
DECREASING NEUTRON
ENERGY
Alghamdi A 2006 PhD Thesis
University of Surrey
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ADDITIONAL SHIELDING CONCERNS
RADIATION
BU
NK
ER
W
AL
L
PHOTONUCLEAR REACTION
1. PHOTON (TREATMENT) BEAM
2. NEUTRON CONTAMINATION
NEUTRON REACTIONS
3.NEUTRON- INDUCED PROMPT RADIATION
4.NEUTRON- INDUCED DELAYED RADIATION
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6 MV 15 MV
UNWANTED HOT-SPOTS
SPARES HEALTHY TISSUES
MORE PENETRATING
FOR PELVIS / BIG PATIENTS
FOR HEAD&NECK /
SMALL PATIENTS
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15 MV
MORE PENETRATING
IN BODY TISSUES
CO
NC
RE
TE
W
AL
L
ALSO MORE PENETRATING IN BUNKER WALLS
RADIATION
PATIENT
WE WANT TO TREAT
HENCE ADDITIONAL SHIELDING CONCERNS
STAFF /
PUBLIC WE MUST
PROTECT
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Organ Absorbed Dose
0
0.5
1
1.5
2
2.5
3
3.5
4
Abso
rbed
Dos
e (x
10-1
6Gy
/ so
urce
neu
tron
) Anterior-Posterior
Posterior-Anterior
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0
0.5
1
1.5
2
2.5
3
3.5
Abso
rbed
Dos
e (x
10-1
6G
y /
sour
ce n
eutr
on)
Right Lateral
Left Lateral
Organ Absorbed Dose
SHIELDED BY
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ORGAN SHIELDING EFFECTS BY ANOTHER ORGAN
ORGANS ON THE LEFT vs ORGANS ON THE RIGHT
ORGANS ON THE ANTERIOR vs ORGANS ON THE POSTERIOR
RESULTS CONSISTENT WITH
EARLIER FINDINGS
Tzortzis M. 2003 MSc Thesis University of Surrey
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Effective Doses
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
1 2 3 4 5
Position
E(µS
v G
y-1)
Anterior Posterior ICPR 1990
Anterior Posterior ICRP 2007 draft
Posterior Anterior ICPR 1990
Posterior Anterior ICRP 2007 draft
1 2345
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0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4 5Position
E(µS
v G
y-1)
RLAT ICRP 1990
RLAT ICRP 2007 draft
LLAT ICRP 1990
LLAT ICRP 2007 draft
1 2345
Effective Doses
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Comparison between ICRP 1990 and 2007 draft using wR = 1 but different wT
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1 2 3 4 5
Position
E(µS
v G
y-1)
ICRP 1990
ICRP 2007 draft
1 2345
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Comparison between ICRP 1990, 2005 draft and 2007 draft
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
1 2 3 4 5
E(µS
v G
y-1)
Position
ICRP 2007 draft
ICRP 1990
ICRP 2005 draft
1 2345
WR = 5
WR =
1
WR =
2.5
FROM 1990 TO 2005 ICRP
CHANGED ITS MIND BY 500%
FROM 2005 TO 2007 ICRP
CHANGED ITS MIND BY 250%
QUESTION
DO THE REVISIONS MAKE US
MORE, OR LESS, CONFIDENT?
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• linac operates at 4 Gy / min
• typical prostate patient receives 2 Gy per visit
• 10 patients treated per day on a linac
• radiographer works 230 days per year
• effective dose per year = 19.17 mSv using current ICRP 1990 (6.78 mSv using 2007 draft) due to neutrons only.
WORST-CASE SCENARIO
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• no breasts, salivary glands & thymus in the phantom
• only one adrenal gland is included in the phantom
• a male phantom was used
• UK statistics: 9/10 radiotherapy nurses are female
• wT for breast increased from 0.05 to 0.12 in 2007 draft
POINTS TO NOTE
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MCNPX 2.4.0 DOES NOT SIMULATE DELAYED GAMMAS
SIMULATION WAS DONE USING MCNPX 2.4.0
ALTHOUGH IN SOME CASES ARTIFICIAL LINES HAVE BEEN
INCLUDED IN THE CROSS SECTIONS
[MCNPX manual]
BETA VERSIONS 26C/D DO SIMULATE DELAYED GAMMAS
HOWEVER, DUE TO MULTIPLE GLITCHES WE FOUND THE
CAPABILITY NOT READY FOR OUR USE [Chin & Spyrou 2007. Monte
Carlo simulation of (γ, n) and (n, γ) activations: a multi-code comparison with
theory. 12th Int Conf Modern Trends in Activation Analysis. Tokyo, ]
Note RESULTS SHOWN HERE EXCLUDE DELAYED GAMMAS
ATOMIC NUMBER
MASS NUMBER
LIBRARY SPECIFIER
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eg
. Z
UB
AL‘
s
VERY DETAILED INDEED … BUT
MONTE CARLO CODES AREN’T
GOING TO MAKE MUCH SENSE
OUT OF IT
EACH TISSUE/ORGAN TYPE
MUST BE MAPPED INTO
ELEMENTAL (IF NOT ISOTOPIC)
COMPOSITION
TEDIOUS SEGMENTATION
WORK… ASK KRAMER et al
ANTHROPOMORPHIC VOXEL PHANTOMS
WE KNOW SOME (eg Spyrou et
al papers) BUT NOT ALL
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IN FACT, VOXEL
PHANTOMS ARE
ROUTINELY USED FOR
RADIOTHERAPY
PLANNING /
VERIFICATION
CHIN MPW et al 2006 Scientific Computing
• INDIVIDUAL PATIENT INSTEAD OF CADAVER / VOLUNTEER
• DOSE TO THE INDIVIDUAL PATIENT INSTEAD OF PUBLIC RISK ASSESSMENT
• VOXELS ATTRIBUTES: HOUNSFIELD UNITS INSTEAD OF TISSUE / ORGAN TYPES
THE ISSUE OF MAPPING
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• VOXELS ATTRIBUTES: HOUNSFIELD UNITS INSTEAD OF TISSUE / ORGAN TYPES
MORE MEANINGFUL IN TERMS OF RADIATION PROPERTIES
BUT NOT AS STRAIGHT-
FORWARD AS WE
WISHED …
BELGIUM
USAUK
GermanySwitzerland
ItalySpain
Portugal
THE ISSUE OF MAPPING
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MATHEMATICAL /
MIRD–TYPE PHANTOMS
Dow
nloa
ded
from
http
://w
ww
.inte
rnal
dosi
met
ry.c
om
ZUBAL / FAX / MAX / …
VOXEL PHANTOMS
WE STILL HAVE A LONG WAY TO
GO
FAITHFUL
REPRESENTATION
OF THE INDIVIDUAL /
VOLUNTEER /
CADAVER
BEFORE DEBATING ON
GENDER / SIZE / ETHNIC
DIVERSITIES
WE HAVE GONE A LONG WAY