MOHAMMAD HUSSIEN, NICHOLAS SPYROU & ANDY MA ANTHROPOMORPHIC VOXEL PHANTOM EFFECTIVE NEUTRON DOSES OUTSIDE A LINAC ROOM Department of Physics, University.

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

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

CURRENT STANDARDS

= HOMOGENEOUS

ELLIPSOIDS

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

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

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

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

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

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

6 MV 15 MV

UNWANTED HOT-SPOTS

SPARES HEALTHY TISSUES

MORE PENETRATING

FOR PELVIS / BIG PATIENTS

FOR HEAD&NECK /

SMALL PATIENTS

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

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

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

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

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

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

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

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?

• 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

• 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

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

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

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

• 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

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