Small Field Dosimetry
Megavoltage range
2014. 01. 28.
Jaegi Lee
2014-01-28
Introduction
• Advanced & specialized radiation treatments
– Beamlet-based intensity modulated radiation therapy
(IMRT)
– Tomotherapy
– Stereotactic radiosurgery (SRS)
– Gamma Knife
– CyberKnife
• UK IPEM (Institute of Physics and Engineering in
Medicine) Report 103
• AAPM TG-155 (unpublished)
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What is small field?
• No clear consensus definition of small field
• Commonly,
– 1) a field size of less than 3 x 3 cm2
– 2) a field with a size smaller than the lateral range of
charged particles
• More scientific approach is needed to set the criteria
which define a small field condition based on the
beam energy and the density of the medium
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3 equilibrium factors
• 1) Size of viewable parts of the beam source as
projected from the detector location through the
beam aperture
– Underdose when only a part of the source can be
viewed
• 2) Size of detector
– Volume averaging effects
• 3) Electron range in the irradiated medium
– The lateral range of the electrons is the critical
parameter to the CPE
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Partial blocking of the beam source
Underdose!
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http://medicalphysicsweb.org/cws/article/opinion/45334
Why is small field dosimetry important?
• 30% variation at 0.5x0.5 cm2 field
Ion chambers are too large for
accurate measurements
without the use of correction
The 2 mm plane-parallel chamber and other small volume detectors.
Measurements are normalized to the 10x10 cm2 reference field.
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McNiven et al., Medical Physics, Vol. 33, No. 11, November 2006
Why is small field dosimetry important?
• Many parameters for small field dosimetry
– Focal spot size and shape, collimator design and
alignment
• It is not possible to use published data for each
radiotherapy unit
– Measurement condition is different
• Radiotherapy treatment planning must be based on
data measured locally
– Accurate measurement and modeling of the individual
unit is essential
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Issues and problems
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The problem of field size definition
Das, Ding, and Ahnesjö: Small fields: Nonequilibrium radiation dosimetry, Med. Phys. 35 (1), January 2008
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The effect of source size
• MLC field profile with different x-jaw size
Das, Ding, and Ahnesjö: Small fields: Nonequilibrium radiation dosimetry, Med. Phys. 35 (1), January 2008
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Effective SSD change in electron beam
P M Ostwald and T Kron, Variation of ESSD with depth, Phys. Med. Biol. 41 (1996) 2067–2078
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Low-density inhomogeneity
• Lung
• Small fields are subject to significant
perturbations.
– Energy & density dependency
• Loss of electronic equilibrium within
and adjacent to low density materials can
result in a dose reduction along the
central axis and near the beam edge for
megavoltage photon beams
• Simple one-dimensional density scaling
fail to provide accurate dose distributions.
– Advanced treatment planning algorithms
provide more accurate dose calculations in
treatment planning.
P. J. WHITE et al., Comparison of dose homogeneity effects, IJROBP, Vol 34, Number 5, 1996
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Output factor measurement/calculation
Calc./measured ratio in small MLC with a constant
jaw setting of 12 × 12 cm2.
(a) Ratios for three different MLC transmission
factors of 1%, 1.4%, and 1.8% (Spot size 1 mm;
DLG 1.4 mm)
(b) Ratios for three different dosimetric leaf gap
settings of 1, 1.4, and 2 mm (Spot size 1 mm;
Transmission 1.4%)
(c) Ratios for three different focal spot sizes of 0, 1,
and 2 mm (Transmission 1.4%; DLG 1.4 mm).
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Kron et al.: MLC defined small fields, Medical Physics, Vol. 39, No. 12, December 2012
Absolute dosimetry
• No reference condition for some treatment
modalities used in SRS treatments (Gamma Knife,
CyberKnife, and tomotherapy)
• No simple methods to provide absolute or reference
dosimetry
– Indirectly performed by transferring, extrapolating, or
intercomparing methods (film, TLD, or small volume
ion chamber)
• RPC has undertaken intercomparing dose
measurements
– Significant deviations in dosimetry
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Overview of the dosimetry of small static fields
Alfonso et al.: Reference dosimetry of small and nonstandard fields, Medical Physics, Vol. 35, No. 11, November 2008
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Radiation detectors
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Ion chambers
• Usually, volume of ion chamber is larger than small fields
• Asymmetry in active volume & large internal electrode
Standard Imaging A1, 0.057 cc
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A L McNiven et al., Phys. Med. Biol. 53 (2008) 5029–5043
Semiconductors
• Not tissue equivalent for low-energy photons
• Part of signal is due to secondary electrons
generated in the encapsulation of the detector
• Dose rate dependency
• Angular dependency
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Diamond detector
• Used naturally grown diamond
– Expensive
• Significant variation because of size and shape
• Dose rate dependency
– Several percent
– Negligible for output factor measurement
– In depth dose measurement, overestimated by 1%
• Small angular dependency
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Radiochromic film
• QA for Stereotactic treatment using EBT2
• High spatial resolution, but scanning process limits the resolution by
0.1 mm
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T. Kron et al., Radiation Measurements 46 (2011) 1920-1923
Detector arrays
• SunNuclear IC PROFILER
• IBA MatriXX
• PTW OCTAVIUS Detector 1000 srs
– Very small detector size (2.3 mm x 2.3 mm x 0.5 mm) with high spatial
resolution (2.5 mm)
• Dose magnifying glass
– 0.2 mm spatial resolution
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Wong et al.: Silicon strip detector for IMRT dosimetry, Medical Physics 37, 427 (2010)
3-D detectors
• Sun Nuclear Arc Check
– Pseudo-3-D geometry
• PRESAGE
– MRI & optical CT
• DEFGELs
– Deformable gels
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Yeo et al.: DEFGEL: 3D deformable dosimetry, Medical Physics 39, 2203 (2012);
Advantages & disadvantages of different
detector types for small field dosimetry
Detector
Key Feature
Ionization
Chamber
Accuracy
Diodes
Small
MOSFETs
Very small
Scintillators
Small
TLD/OSL
Stand alone
Diamonds
Small
Advantages
Can be converted to absolute
dose
Robust, real-time read-out
Can be combined into detector
arrays
Tissue equivalent
Many forms available,
very high sensitivity
Tissue equivalent
Radiochromic Determines 2-D Tissue equivalent;
film
dose distribution self-developing
3-D dosimeters Determines 3-D
Tissue equivalent
/ gels
dose distribution
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Disadvantages
Relatively insensitive and large
Not tissue equivalent; dose rate de
pendence
Not very sensitive and reproducibil
ity less than other detectors
Cherenkov radiation
Light sensitivity
Expensive, dose rate dependence,
may require pre-irradiation
Expensive, not very sensitive
Complex evaluation, no 'standard'
commercial gel available
Outlook and conclusion
• Small fields are increasingly employed in modern
radiotherapy.
– High doses to small volume
– Subdivision of larger radiation fields into smaller field
segments (IMRT & VMAT)
– Non-uniform dose distribution (SIB technique)
• Accurate dosimetry becomes more important to
ensure the absolute dose
• Small field dosimetry is critical for the accurate
characterization of field edges and regions of steep
dose gradients
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Discussion & Question
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Thank you for your attention
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