A Monte Carlo study of detector dose response
in small megavoltage photon beams
Hugo Bouchard1, Jan Seuntjens2, Simon Duane1, Yuji Kamio3,
Hugo Palmans1,4
1. Acoustics and Ionising Radiation Team, National Physical Laboratory, Hampton Road, Teddington TW11
0LW, United Kingdom
2. Medical Physics Unit, McGill University, Montreal, Quebec H3G 1A4, Canada
3. Centre hospitalier de l'Universite de Montreal (CHUM), 1560 Sherbrooke est, Montreal, Quebec H2L 4M1,
Canada
4. Medical Physics, EBG MedAustron GmbH, A-2700 Wiener Neustadt, Austria
Correction factors in small fields
• In small photon fields, radiation detectors cannot be used in
the same way as in reference beams
• A new CoP for small fields is underway to allow small field
dosimetry
New dosimetry
technique
Machine
MSR
SSD/SAD
Machine
SSD/SAD
dref
dref
H2O
Alfonso et al. 2008
f clin , f msr
Qclin ,Qmsr
Ω
CLIN
=
Dwf clin
,Qclin
f msr
w ,Qmsr
D
=
MH2QOf clin
clin
M
f msr
Qmsr
clin , f msr
kQfclin
,Qmsr
Motivations
• Several papers addressed nonstandard beam dosimetry
– Sanchez-Dobaldo et al. (2003, 2005), Capote et al. (2004), Bouchard et
al. (2004, 2009, 2012), Das et al. (2008), Francescon et al. (2008, 2012,
2014), Scott et al. (2012), Underwood et al. (2013), Fenwick et al.
(2013), Charles et al. (2013), Czarnecki and Zink (2013)
• A comprehensive overview of the problem might be lacking
– This can be helpful to prepare our community for the upcoming IAEAAAPM CoP
• The goal of the present work is to:
– Address the fundamental differences between absorbed dose
measurements in standard and nonstandard beams
– Highlight potential misconceptions in the applicability of cavity theory
in nonstandard beams
3
Why do small fields require correction
factors?
• One isolates 4 main effects related to the characteristics of
the detector
1.
2.
3.
4.
The presence of extracameral components
The atomic properties of the sensitive volume
The density of the sensitive volume of the detector
Volume averaging
• To characterize these effects quasi-individually, we define a
series of geometries in which absorbed dose is evaluated
4
Monte Carlo factorization of perturbation factors
• Standard cavity theory approach (e.g., Bouchard et al. 2009)
• “Fano” cavity theory (this work)
Pencil Beam Decomposition Method
• Pencil beam decomposition method to model dose
in the cavity
(xi,yi)
Dose response function
Fluence weight
distribution in
[0,1]
dcav(xi,yi)
6
Pencil Beam Decomposition Method
• Cavity response to pencil beams: DRF
7
Pencil Beam Decomposition Method
• Perturbation function
– We define
Pencil Beam Decomposition Method
• Perturbation function
– From the definition
– Having the property
– We calculate kQ factors
Denominator is field factor (also
known as output factor)
9
Monte Carlo simulations
EGSnrc user code cavity
C++ library (egs++)
ECUT = AE = 512 keV
PCUT = AP = 1 keV
Range rejection up to 10 keV (ESAVE = 521 keV)
All physics “on” (spin effects, atomic relaxation,
etc..)
• 1.25 MeV parallel photon pencil beams
• Geometry/simulation passes Fano test to 0.1%
or better
•
•
•
•
•
•
Air detector
π x 5 x 2 mm3 volume
1 mm graphite wall
10 cm
30 x 30 x 30 cm3
H2O phantom
Silicon detector
π x 2 x 2 mm3 volume
1 mm aluminium wall
1. Extracameral components
• Components such as wall, electrodes, stems, etc., can affect
the detector dose response
Air detector
Air cavity
Silicon detector
Silicon cavity
2. Atomic properties
• Assuming the detector to be a cavity with the
electron density of water, relevant atomic properties
are
– The atomic number (photo-electric effect, pair production)
– The I-value (stopping power)
– The density effect parameter δ (stopping power)
• All have an effect on the interaction cross sections
(i.e., electronic cross sections)
2. Atomic properties
• Monte Carlo simulations of cavity dose response to 1.25 MeV
photon pencil beams in cavity of identical electron density
Air cavity
Air-density water cavity
Silicon
Silicon-density water
3. Density perturbation effects
• Monte Carlo simulation of cavity dose response to 1.25 MeV
photon pencil beams in water cavities made of different
density
Air-density water
Water cavity
Silicon-density water
Water
Perturbation functions
Remind that
• They govern quality correction factors
cavity
wall
π x 52 x 2 mm3 air cavity
cavity wall
π x 12 x 2 mm3 silicon cavity
4. Volume averaging
• In nonstandard beams, volume averaging effect can
be as important as density perturbation effects
Density perturbation effect
Scott et al. Phys. Med. Biol. 2012
Volume averaging effects
16
Density perturbation effects
• Results show that by far, detector density is the most
important effect, taking volume averaging effects
aside
• This can be explain by the interaction between lack
of CPE and the detector density using Fano’s
theorem
17
Fano’s theorem
• In a medium of uniform properties irradiated
by uniform photon fluence, the fluence of
charged particles is uniform and independent
of the mass density distribution in space.
Water cavity
Vapor cavity
Dense-water cavity
18
• We decompose the beam
Beam directed at cavity
Broad beam
=
+
Beam not directed at cavity
• By Fano’s theorem
• And we find
19
Fano calculations of DRF
20
How small is small?
• Y. Kamio and H. Bouchard, Phys. Med. Biol. 59 (2014)
4973-5002
– Pencil beam decomposition method (as in this talk)
– Tolerance on kQ