Neutron dose evaluation in radiotherapy
Francesco d’Errico
University of Pisa, Italy
Yale University, USA
Photoneutron production in accelerator head
• Photoneutrons produced by interaction of photon beam with accelerator components
• Produced mainly in the target, primary collimator, flattener and jaws/collimators
• Typical materials are copper,tungsten, gold, lead and iron
• Neutron production in electron mode is lower than in photon mode• Direct production of neutrons by electrons
is at least 2 orders of magnitude lower
• Lower electron current
Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production in accelerator head
• Photon must have energy greater than binding energy of nucleus in atom
• Sn =Separation Energy
• Neutron production in primary laminated barrier • Lead has lower Sn than Iron
• Pb-207 (22.1%): Sn =6.74 MeV (NCRP 79)
• Pb-208 (52.4%) : Sn = 7.37 MeV
• Iron• Fe-57 (2.1%): Sn= 7.65 MeV
• Fe-56 (91.7%): Sn =11.19 MeV
• Lead has a higher neutron yield than iron
• Steel is a better choice for reducing neutron production
Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production
• Photoneutron spectrum from
accelerator head resembles a
fission spectrum
• Spectrum changes after
penetration through head
shielding
• Concrete room scattered
neutrons will further soften the
spectrum
• Spectrum outside the concrete
shielding resembles that of a
heavily shielded fission spectrum
Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production
• Two Processes
–Direct Emission
•Average energy of direct
neutrons is ~ few MeV
•Spectra peak at energies > 2
MeV
•Have a sin2J angular
distribution, therefore
forward peaked
•Contributes about 10 to 20%
of neutron yield for
bremsstrahlung with upper
energies of 15 to 30 MV
Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production
• Two Processes
–Evaporation Neutrons
•Dominant process in heavy
nuclei
•Emitted isotropically
•Spectral distribution is
independent of photon energy
for energies that are a few
MeV above neutron production
threshold
•Average energy is ~ 1 -2 MeV
•Spectra peak at ~ 200-700 keV
Nisy E. Ipe, 2007 AAPM Summer School
Recent treatment modalities – IMRT
• SmartBeam IMRT "paints" a dose to the tumor with pinpoint precision, while sparing healthy normal tissue.–Minimizes hot spots
– Improves target inhomogeneity
– Provides detailed dose painting to the target
– Sculpts dose around critical structures more effectively
–Allows treatments to occur in conventional 10 - 15 minute time slots
–Dose resolution, with up to 500 segments per field
– Spatial resolution of 2.5 - 5mm
Recent treatment modalities – VMAT RapidArc™
• RapidArc uses a unique
algorithm that provides
excellent treatment delivery
control.
• Its single gantry rotation
speeds treatment delivery so
clinicians can develop
treatments that take one-half
to one-eighth the time of
conventional IMRT
treatments—just two minutes
in many cases.
• A RapidArc treatment may also
result in less radiation leakage
and scatter, so peripheral tissues
receive a lower overall dose.
Photoneutrons: NCI-WG 2001 recommendations
• Total-body dose for IMRT patients is higher, generally increasing with the number of monitor units used for treatment. The potential for complications related to this increased dose should be recognized and considered.
• Increased neutron production for high-energy machines used for IMRT should be considered.
• Increased workload values for IMRT (may be 2–5 times larger than in conventional therapy) should be considered for the leakage/transmission part of the treatment room shielding.
• Because IMRT is inherently less efficient (per MU) than conventional RT, vendors should consider the use of more internal shielding in the design of future IMRT machines.
Neutron dosimetry Conversion coefficients
Neutrons
– particle fluence to ambient or personal dose equivalent
– influence of phantom (multiple scattering) and quality factor
Neutron radiation protection dosimetry
•Utilization of physical phenomena
resembling dose (equivalent)
deposition in tissue.
•Measurements of LET spectra and
convolution over Q(L).
•Design of systems mimicking
the fluence to dose equivalent
conversion coefficient
Neutrons are particles without charge, difficult to detect
TL induced by secondary particles from nuclear reactions:(n,) (n,p) (n,d) etc
TLD for neutrons have high concentration of isotopes withhigh cross section to neutrons
in LiF ~ 7,4% of Li is 6Li
n Radiation LiF 6Li(n, )3H
Neutron detection by thermoluminescence
Plastic Nuclear Track Detectors
• Particles of ionizing radiation cause molecular size damage in solid material
• The damage can be enlarged to microscopic range by chemical etching
Atomic force microscopy of latent tracks
• Latent tracks 70-100 nm and must be enlarged to 5-30 µm to be visible with optical microscopy
• Tracks are enlarged by chemical or electrochemical etching
• Track density is proportional to the exposure value [dose]
• With calibration the [equivalent] dose can be deducted
Typical microscopic view-field with developed
tracks
Overlapping
Tracks
Identified
Tracks
• Neutrons are non ionizing radiation, but CR-39 (C12H18O7) is sensitivity to fast neutron recois
• Neutrons interact with H producing recoil protons
• Recoil protons create latent tracks in the CR-39 material
Neutron response mechanism
• In practice, converter layers are utilized:
- Converters modify (improve) the energy dependence of the reponse
- Converters also protect the CR-39 chip against alpha particles from environmental
radon
Neutron energy dependent response
• Good energy dependence of response
• But, relatively low sensitivity
Superheated emulsions
• Fluorocarbon droplets kept in a steady superheated state by
emulsification in compliant gels.
• Bubble nucleation triggered by neutrons above selectable
threshold energies.
• Can be totally insensitive to photons.
Neutron energy dependence of emulsions
• Composition closely tissue equivalent
• Insensitive to photons
• Response resembling kerma equivalent coefficient
• The emulsions can measure dose equivalent in phantom without disrupting
charged particle equilibrium
Bubble counting by scattered light
Photo
diodes
LEDs
• Instant read out
• Rate insentive
• Position sensitive
Clinical simulations of prostate radiotherapy
BOMAB = BOttle MAnnikin ABsorber phantom, industry standard (ANSI 1995) for calibrating whole body counting systems
Sagittalplane section view
≈5 cm
≈5 cm
BOMAB CT scan and simulated organs
Sagittalplane section view
≈5 cm
≈5 cm
Transverseplane section view
Rectum
Bladder
ProstatePTV
Boostvolume
Treatment modalities in Pisa and Krakow
Modalities
PisaVarian Clinac 2100 C
MU/2 Gy
KrakowVarian Clinac 2300 CD
MU/2 Gy
6 MV single 10x10 cm2 field (ref)15 MV single 10x10 cm2 field (ref)
15 MV 5-field MLC6 MV IMRT
6 MV VMAT (RapidArc)
251218
266432
481
6 MV single 10x10 cm2 field (ref)18 MV single 10x10 cm2 field (ref)
6 MV 4-field MLC18 MV 4-field MLC6 MV IMRT
18 MV IMRT
240199277218466350
6 MV Tomotherapy
Used for reference
Dosimeter in-phantom placement
BTI sensitive detector length: 7.0 cm
SDD sensitive detector length: 2.5 cm
Measurement points center of sensitive length
D’’ = 6.8
D’’ distance from outer flange – legs side.
Dimensions in cm
LE
G
S
D’’ = 12 D’’ = 16.8 D’’ = 31.8 D’’ = 46.8 D’’ = 56.8
D’’ = 12 D’’ = 27 D’’ = 42
IMRT neutron dose as a function of nominal energy
1,E+00
1,E+01
1,E+02
1,E+03
0 5 10 15 20
μSv
/Gy
Penetration distance along the sagittal plane (cm)5 cm is bladder, 10 cm is prostate and 15 cm is rectum
18 MV - Krakow
6 MV - Pisa
MLC multiple field treatments
1,E+00
1,E+01
1,E+02
1,E+03
1,E+04
-10 0 10 20 30 40 50
μSv
/Gy
Off-axis distance (cm)
PTV Krakow 18 MV SDD
PTV Pisa 15 MV SDD
PTV Krakow 18 MV PADC
PTV Pisa 15 MV PADC
Reference phantom literature comparison - SDD
0,E+00
1,E+03
2,E+03
3,E+03
4,E+03
5,E+03
6,E+03
0 50 100 150 200 250
This work (SDD, 20 MV)
This work (BDPND, 20 MV)
d'Errico 2000 (SDD, 18 MV)
d'Errico 2000 (SDD, 15 MV)
Awotwi-Pratt 2007 (SDD, 15 MV)
Lin 2007 (BDPND, 15 MV)
This work (SDD, 12 MV)
μSv
/Gy
Off-axis distance (mm)
Conclusions and Prospects
• The “Pisa BOMAB” proved a viable approach
• The selected neutron dosimetry methods appear reliable and reproducible
• But, measurement times are long and additional campaigns are needed for a systematic analysis
• Tomotherapy appears to deliver the smallest neutron doses
• A small neutron contamination is present already at 6 MV and warrants further investigation