a prototype calorimeter for hdr ir-192 brachytherapy sources
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A prototype calorimeter for HDR 192Ir brachytherapy sources
Thorsten Sander, Hugo Palmans, Simon Duane
National Physical Laboratory, Teddington, UK
LNE-LNHB / BIPM Workshop on “Absorbed Dose and Air Kerma Primary Standards”, Paris, 9 - 11 May 2007
Overview
• Absorbed dose measurements for brachytherapy • MC calculations for calorimeter design
parameters• MC calculated correction factors• Heat transfer simulations• Conclusions• Future work
Proposed measurement method:calorimetry
• Objective: investigate feasibility of calorimetry for brachytherapy
• Alternative method to current air kerma based approach (using reference air kerma rate (RAKR), dose rate constant Λ and TG-43 to derive absorbed dose)
• Direct measurement of absorbed dose
Tcm
ED prad Δ==point P
Problems in brachytherapy dosimetry
• High dose gradient over clinically relevant range (0.5 cm to 5 cm from source centre) resulting in heat flow down temperature gradients
• Self-heating of radioactive source
• Huge variety of brachytherapy source designs
Schematic drawing of prototype calorimeter(RZ-geometry)
R = 10 cm
Z =
14 c
mZ
R
1 mm vacuum gap
Absorber:Graphite ring, 2 mm x 5 mm, R = 2.5 cm
EGSnrc Monte Carlo simulations
• Various aspects of calorimeter modelled with DOSRZnrc
• Source (Nucletron microSelectron Classic):– Bare 192Ir spectrum used for 192Ir cylinder– AISI 316L stainless steel encapsulation
5.0 mm
3.50 mm
1.10 mm0.60 mm
Stainless steel cableAISI 316
Stainless steel plugAISI 316L
Stainless steel capsuleAISI 316L Active Iridium-192 core
0.40 mm
Build-up curves in water and graphite
Nucletron microSelectron Classic Ir-192 source in water and graphite
4.0E-145.0E-146.0E-147.0E-148.0E-149.0E-141.0E-131.1E-131.2E-131.3E-131.4E-13
0 1 2 3 4 5 6 7 8 9 10
Water equivalent radial distance R, cm
D x
r2 p
er e
mitt
ed p
hoto
n, G
y cm
2
(x m
uen_
w,g
for g
raph
ite)
WaterGraphite
• (μen/ρ)wg = 1.11 for mean 192Ir energy
• Energy dependent calculate fluence spectrum
5 cm4 cm3 cm2 cm
Scatter build-up along R-axis
ROI = 1 mm x 1 mm 1 cm
0 cm 20 cm
20 cm
Scatter build-up in graphite
Min. R required at ROI to get 99.9% and 99.5% of Dfull scatter
y = 4.0377x
0
5
10
15
20
25
30
35
40
45
50
0 1 2 3 4 5 6
Radial distance of ROI from source centre, cm
min
. R, c
m 99.9% of max.99.5% of max.Linear (99.9% of max.)Linear (99.5% of max.)
Variation of Daverage with core height
Daverage(core height) / Dfull scatter at various ROIs
97.0%
97.5%
98.0%
98.5%
99.0%
99.5%
100.0%
00.
10.
20.
30.
40.
50.
60.
70.
80.
9 11.
11.
21.
31.
41.
51.
61.
71.
81.
9 2
Core height, cm
Perc
enta
ge o
f Dav
erag
e /
Dfu
ll sc
atte
r
ROI: 0.5 cmROI: 1 cmROI: 2 cmROI: 3 cmROI: 4 cmPoly. (ROI: 4 cm)Poly. (ROI: 3 cm)Poly. (ROI: 2 cm)Poly. (ROI: 1 cm)
Summary of MC simulations
Source to core, cm (graphite)
Build-up
Min. R, cm
Min. Z/2, cm
Max. core height, cm
Dose gradient over 2 mm, %
1 X X
0.5
2.5 10.1 6.8 0.6 18.5 1.60E-02 2.69E-03
0.75
1.05
9.99E-02 1.68E-02
2 ( ) 8.1 5.4
X
23
15
11
2.52E-02 4.24E-03
3 12.1 8.1 1.12E-02 1.88E-03
4 16.2 10.9 6.21E-03 1.04E-03
X
Dose rate from 370 GBq source, Gy/s
ΔT in 120 s, K
5 X X X
Calorimeter correction factors
gap vacuumgraphite core,
graphite core,gap
+
=D
Dk
metalairgraphite core,
graphite core,inh
++
=D
Dk
0033.1core
pointvol ==
DD
k
‘point’ = 0.1 mm x 0.1 mm core = 2 mm x 5 mm Z
R
Inhomogeneity correction due to aluminium tube
Percentage change of dose to core due to absorption and scatter in aluminium tube
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
Thickness of aluminium tube, cm
Perc
enta
ge c
hang
e of
abs
orbe
d do
se, %
Inhomogeneity correction due to stainless steel tube
Percentage change of dose to core due to absorption and scatter in stainless steel tube
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0 0.01 0.02 0.03 0.04 0.05 0.06
Thickness of stainless steel tube, cm
Perc
enta
ge c
hang
e of
abs
orbe
d do
se, %
Gap correction factor
Percentage change of dose to core due to vacuum gap
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0 0.5 1 1.5Thickness of vacuum gap, mm
Perc
enta
ge c
hang
e of
ab
sorb
ed d
ose,
%
Displacement correction factor, Z-axis
0002.10.25mmZ displaced,
centralZdisp, ==
=DDk
Absorbed dose to core with respect to source position
1.795E-14
1.800E-14
1.805E-14
1.810E-14
1.815E-14
1.820E-14
1.825E-14
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
Source position relative to centre, cm
Abs
orbe
d do
se, G
y
Series1Poly. (Series1)
Displacement correction factor, R-axis
θε cosd=
d
r r + εθ
21)(r
rD ∝
)( ε+rD
∫∫ + )cos( θθ drdrDd
Taylor expansion
≈symmetric
displaced
DD
2
2
231
rd
+
0.1%With r = 25 mm
d = 0.64 mm
Self-heating of Nucletron microSelectron 192Ir source
• Maximum source activity = 550 GBq• Self-heating due to gamma radiation = 1.04E-2 W• Self-heating due to electrons = 1.59E-2 W• Total self-heating power = 2.63E-2 W
Stainless steel cable
Stainless steel plug
Stainless steel capsuleActive Iridium-192 core
Heat transfer simulations
• Main modes of heat transfer:• Conduction• Thermal
radiation• Heat equation:
solved by finite element modelling
• Stationary and transient solutions
Example: heat conduction from self-heating and radiative heating
Self-heating,Tinit= 10 KTinit= 0 K
Radiative heating
Self-heating and radiative heating, Tinit= 10 K
No conduction
0.0E+00
5.0E-04
1.0E-03
1.5E-03
2.0E-03
2.5E-03
3.0E-03
3.5E-03
4.0E-03
4.5E-03
5.0E-03
0 200 400 600 800 1000
Time, s
Cha
nge
of c
ore
tem
pera
ture
, K
Control of self-heating effect, stationary solution, thermostatic mode
Thermistor 1+2:
-45.55 W m-2
Modes of operation
• Adiabatic mode• Measurement of
temperature difference
• Thermostatic mode• Electrical substitution
method• Advantage: rapid repetition
of calorimeter runs is possible
Power
Prad
Ptr
Pelec
Time
Power of heat transfer to core = constant
0.00E+00
5.00E-04
1.00E-03
1.50E-03
2.00E-03
2.50E-03
3.00E-03
-200 -150 -100 -50 0 50 100 150 200
Time, s
Tem
pera
ture
cha
nge,
K
ΔT
Conclusions + future work
• Prototype calorimeter for HDR brachytherapy sources:– Suitable dimensions and materials derived from MC simulations– Vacuum gap around core to control self-heating effect of
radioactive source– Heat transfer simulations used to find optimum position for
thermistors and maximum heating power required per component
• Future work:– Build, test and characterise calorimeter + refine correction
factors for final design– Work out conversion from absorbed dose to graphite water– Measure absorbed dose and compare with RAKR approach and
TG43
Acknowledgements
• University supervisors: Ivan Rosenberg and Robert Speller
• UCLH: Derek D’Souza• NPL staff: Hugo Palmans, Simon Duane, Mark Bailey,
David Shipley, Nigel Lee, Chris Thomas, Gary Mapp, Edmund Kirby, Peter Bird, Keith Burgon, Geoff Stammers, Pravin Patel
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