neutron dose evaluation in radiotherapy -...

Neutron dose evaluation in radiotherapy

Francesco d’Errico

University of Pisa, Italy

Yale University, USA

Radiation therapy with a linear accelerator (LINAC)

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


• 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


Schematic of Varian accelerator head

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



• 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



• 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


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.

Recent treatment modalities - Tomotherapy

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

3He(n,p)3H10B(n,)7Li 6Li(n,)3H

Some neutron capture cross sections

Albedo-based neutron dosimeters

TL albedo neutron dosimeter response

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

How CR-39 PNTDs Work

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

Atomic force microscopy of latent tracks

How CR-39 PNTDs Work

Result of 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

Sensitization by CO2 pretreatment

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.

Some current technologies

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

Isocenter neutron doses

Neutron doses from various x-ray beams

18 MV

x-rays

15 MV

x-rays

10 MV

x-rays

In vivo photoneutron measurements

SDD fluence response vs quality-factor weightedkerma coefficient and photoneutron spectrum

Bubble counting by scattered light

Photo

diodes

LEDs

• Instant read out

• Rate insentive

• Position sensitive

Study of a prostate treatment

Characteristics of various phantoms for out-of-field measurements

Clinical simulations of prostate radiotherapy

BOMAB = BOttle MAnnikin ABsorber phantom, industry standard (ANSI 1995) for calibrating whole body counting systems

Irradiation channels (“pipes”)

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

Organs of interest: bladder, prostate, rectum

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

Treatment modalities – 5 fields with MCL

Treatment modalities – IMRT

Treatment modalities - VMAT

Dose profiles from different treatment plans

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)

TrueBeam 10 cm x 10 cm field results

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

neutron dose evaluation in radiotherapy -...

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