Small photon field dosimetry: present status
Maria Mania Aspradakis
Cantonal Hospital of Lucerne
Switzerland
IPEM report 103: main scope
• Educate on the physics and challenges in
the dosimetry of small MV photon fields.
• Review commercially available detectors
and measurement methodologies suitable
for implementation in the clinic.
• Give recommendations of good practice in
order to reduce uncertainty in the
determination of dosimetric parameters
• Explain the need to commission TPSs
specifically for small fields.
• To point out directions along which future
work and research efforts are required in
this challenging field of dosimetry.
Aspradakis M, IDOS, IAEA, Vienna, Nov 2011
Other reviews on small field dosimetry
 R. Alfonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjäll, T. R.
Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich, and S.
Vatnitsky, “A new formalism for reference dosimetry of small and
nonstandard fields,” Med. Phys. 35, 5179–5187 (2008).
 I. J. Das, G. X. Ding, and A. Ahnesjö, “Small fields: Nonequilibrium
radiation dosimetry,” Med. Phys. 35, 206–215 (2008).
 H Palmans (2011) CN-182-INV006, Small and composite field
dosimetry: the problems and recent progress. IDOS Conference,
Vienna.
Outline of presentation
•
Small MV photon field conditions
•
Present status on:
reference dosimetry
output factor determination
Discussion in this presentation restricted to dose determination in
static MV photon fields
Small MV photon field conditions
•
For the selected energy and medium, the field size is
not large enough to ensure lateral CPE (lack of LEE).
•
The entire source is not in the detector’s-eye-view
(source occlusion).
•
The detector is not small enough and perturbs
fluence significantly (detector issues)
Detector size relative to field size
Small field conditions exist when one of the edges of
the sensitive volume of a detector is less then a lateral
charged particle equilibrium range (rLEE) away from
the edge of the field
rLEE
g
cm
2
  5 .973  TPR
20
10
Li et. al., Med. Phys. 22, (1995), 1167-1170
 2 . 688
Slide courtesy: H. Palmans
In narrow fields: source occlusion
source occlusion by
the collimators
point source used in
calculation
finite source size used
in calculation
Very narrow dose profile
Partial view of
extended direct
beam source
from the point of
measurement
Treuer et al PMB 38 (12), 1992
Radiation detector
measures in nonuniform dose region
Overlapping penumbras  apparent field widening
Das et al., Med. Phys. 35: 2008, 206-15
definition of field size?
Uncertainty in output factor correction introduced
due to field size definition
Cranmer-Sargison et al, Med. Phys. 38, 2011. 6592-6602
Benhmakhlouf et al, Med. Phys. 41,2014, 041711
Slide courtesy: H. Palmans
Detector issues in small field dosimetry
• Energy dependence of detector response
• Perturbation effects
 Volume averaging
 Ionization chambers: wall, central electrode, air cavity
different from water
 Solid state detectors (e.g. diodes): housing, shielding,
coating of silicon chip
Detector issues in small field dosimetry
Perturbation effects: volume averaging
2.5 mm
5 mm
5.8 mm
6.6 mm
16.25 mm
23 mm
0.3%
1.4%
Pantelis el al, Med Phys 37 (5), (2010)
Correction for volume averaging
Azangwe et al, Med. Phys. 41, 072103 (2014)
A correction for volume
averaging can be derived
from the ratio of the detector
response in its central part to
the detector response over its
whole volume
Georg et al, 2nd ESTRO
Forum, Pre-meeting
workshop 2013
Correction for volume averaging
Morin et al MP, 40(1), 2013
Ralston et al PMB, 57, 2012
Detector issues in small field dosimetry
Perturbation effects: detector construction
Crop et al., Phys Med Biol 54:2951 (2009)
C. McKerracher & D.I. Thwaites
Radiotherapy and Oncology 79 (2006) 348–351
Detector perturbation
the influence of detector density at small field sizes
D water  voxel
D detector
in water
 voxel in water
D water - voxel in water
D water - voxel  with  detector
density in water
Scott et al PMB, 57 (2012) 4461-4476
15MV
Small field reference dosimetry
IAEA/AAPM formalism for reference dosimetry
Small static MV fields: reference dose determination
Alfonso el al (2008), Med Phys 35 (11)
f msr
machine specific
reference field
Q msr
beam quality of machine
specific reference field
msr
ref
D w,msrQ msr  M Q msr

N

k

k
D, w, Q o
Q, Q o
Q msr , Q
msr
f
f
f
,f
IAEA/AAPM formalism for reference dosimetry
field instrument correction factors
k
f msr , f ref
Q msr , Q

N D, w, Q msr
N D, w, Q

D
f msr
w, Q msr  pcsr 
M
f msr
Q msr
M
D
f ref
w, Q
f ref
Q
ref detector
field instrument
Determined through:
•Experiment: by a primary standard
•Experiment: using dosimeters that can measure reference dose traceable
to a primary standard and which have sufficiently low uncertainty (alanine,
radiochromic film, diamond, liquid ion-chambers ...)
•Calculation: Monte Carlo simulations (MC)
IAEA/AAPM formalism for reference dosimetry
small static MV photon fields
Reference dosimetry on Cyberknife: chamber factors calculated with MC
Franscescon et al (2012), PMB 57 3741-3758
Gago-Arias el al (2013), Med Phys 40 (1), 011721-1, &
Erratum Med Phys 40, 011721-1-10, 2013
f msr , f ref
k Q msr , Q
Exradin A12
PTW 30006
NE 2571
PTW 31014
PTW 31014
IBA CC13
Specification of a reference-class ionisation chamber
Not all micro-chamber designs are considered suitable for reference
dosimetry
McEwen , Med Phys 37, 2010, 2179-93
Small field relative dosimetry - output factor , Scp
6 M V m e a s.
re la tiv e D o s e
1 .0
0 .8
0 .8
IC
P iP
0 .6
D iG re
0 .6
D iY e
MOS1
0 .4
0 .4
MOS2
D IA
0 .2
0 .2
0 .0
0 .0
0 .5
1 .0
1 .5
2 .0
2 .5
0 .0
0
2
4
6
8
10
12
14
16
S E S / cm
Sauer & Wilbert Med. Phys. 34, 2007, 1983-1988
18
IAEA/AAPM formalism for relative dosimetry
output factor determination in small static fields
D
f clin
w, Q clin
D
f msr
w, Q msr
f clin , f msr
 Q clin ,Qmsr
f
f clin
 D w,f clinQ
M Q clin
M
Q clin
clin
clin

f msr 
f
f
M Q msr  D w,msrQ msr M Q clin
msr
 M Qf clin f , f
clin
clin
msr

k

Q clin , Q msr
f msr
 M Q msr
Small field
detector
specific
correction
factors
Alfonso el al (2008), Med Phys 35 (11), new CoP: IAEA TECDOC###
Small fields: relative dosimetry – output factor Scp
Detector specific correction factors
6MV
Siemens
Elekta
Francescon et al Med. Phys. 38(12), 2011
IAEA/AAPM formalism for relative dosimetry
Small field detector correction factors
Accounts for three main detector perturbation effects:
k
f clin , f msr
Q clin , Q msr
  p vol Q
Ratio of
volume
averaging
correction
factors
Q clin
msr
p

Q clin
fluence
Ratio of
charged
particle
fluence
perturbation
correction
factors
Q msr
p

Q clin
spectrum
Q msr
Ratio of correction
factors to account
for the spectral
dependence of
photon energy
absorption in the
detector medium
p
Only different degree in CPE
& spectral effects considered

Q clin
fluence
Q msr
p

Q clin
spectrum
Q msr
These result confirm previous
conclusions that unshielded diodes
a better choice of detector than
shielded diodes.
The corrections for mini-ionization
chambers used in this study (active
volume between 0.015 cm3 and 0.05
cm3) were generally lower than 10%
and
for micro-chambers (active
volume<0.015 cm3) lower than 3%.
active volume > 0.1 cm3
corrections of 20%-30% !
Azangwe et al Med. Phys. 41 (7), 2014
IAEA/AAPM formalism for relative dosimetry
Small field detector correction factors
k
f clin , f msr
Q clin , Q msr
Benmakhlouf et al Med. Phys. 41 (4), 2014
IAEA/AAPM formalism for relative dosimetry
Small field detector correction factors
p
k
f clin , f msr
Q clin , Q msr
p
Q clin
spectrum

Q clin
fluence

Q msr
Benmakhlouf et al Med. Phys. 41 (4), 2014
Q msr
Summary & conclusions
•
Research in small field dosimetry, after the publication of IPEM
report 103, has lead to improved understanding of detector
response in such fields.
•
Current research efforts in small field dosimetry focus on the
determination of detector-specific output correction factors.
•
Detectors requiring output corrections greater that 5% are not
recommended for dose determination in small fields.
•
The new IAEA TECDOC will include a consistent set of such data and
will be an international code of practice for small static field
dosimetry.
Thank you for your attention!
‘Sunset over the Libyan sea’, Ierapetra, Crete
Maria.Aspradakis@luks.ch