Neutron InteractionsPart II

Rebecca M. Howell, Ph.D.Radiation Physics

rhowell@mdanderson.orgB1.4580

Why do we as Medical Physicists care about neutrons?

• Neutrons in Radiation Therapy• Neutron Therapy

• Fast Neutron Therapy• Boron Neutron Capture Therapy

• Contamination neutrons in high energy x-ray therapy and proton therapy.

• Patient and personnel dose• Important component in shielding design

Neutron Radiotherapy

• Fast Neutron Therapy Beams

• Boron Neutron Capture Therapy

• History and Current Facilities

• Treatment sites

Fast Neutrons Methods of Production

• Neutrons can be produced in a cyclotron by accelerating deuterons or protons and impinging them on a beryllium target.

• Protons or deuterons must be accelerated to ≥50 MeV to produce neutron beams with penetration comparable to megavoltage x-rays.

Fast Neutrons Methods of Production

• Accelerating deuterons to ≥50MeV • Requires very large

cyclotron, too large for hospital.

• Accelerating protons to ≥50MeV • Much smaller cyclotron

b/c proton has ½ the mass of deuteron.

P+

n

Fast Neutrons from Deuteron Bombardment of Be

• Stripping Process – • Proton is stripped from the deuteron.• Recoil neutron retains some of the incident kinetic energy

of the accelerated deuteron. • For each neutron produced, one atom of Be is converted

to B.

Be94 B105 n

+

Fast Neutron Spectra from Deuteron Bombardment of Be

• Neutron spectra consists of a single peak, with a modal value of about 40% of the energy of the incident deuterons.

Fig 24.2a

Hall fig 24.2a

Fast Neutrons from Proton Bombardment of Be

• Knock-out Process • Protons impinge target of beryllium, where they knock-

out neutrons.• For each neutron “knocked-out”, one atom of Be is

converted to B.

Be94 nP+ B95

+

Fast Neutron Spectra from Proton Bombardment of Be

• The neutron spectra spans a wide range of energies.• Necessary to filter out the low energy neutrons to

achieve acceptable depth dose distribution.

Fig 24.2b

Hall fig 24.2b

According to Hall text book a polyethylene filter was used to harden the beam.

• Based on our knowledge of neutron interactions why would polyethylene be a good choice for removing the low energy component from the neutron beam?

• Would polyethylene alone solve the problem or make it better?

Isodose Distribution

• Neutron beam (produced from 50-MeV protons or deuterons) has comparable depth dose distribution/isodose to 6MV photon beam.

Bewley, Fig 4.3

Note: differences incearse for low isodose lines

Long Treatment Distances

• Neutron beam treatment distances are 100 to 140 cm due to large collimator• Collimator materials:

• Hydrogenous material to slow the neutrons• Absorber material to remove thermal neutrons• Pb or other high Z material to absorb -ray component

(remember that activation follows absorption, -photon is often the result)

Clinical Experience with Fast Neutrons

• First experience at Lawrence Berkeley Laboratory• Hammersmith Hospital in London

3 Neutron Therapy Facilities in the US

• Northern Illinois University Institute for Neutron Therapy at Fermilab

• University of Washington Medical Center

• Gershenson Radiation Oncology Center at Harper University Hospital, Detroit

Modern Neutron Therapy Facilities

University of Washington Medical Center

•Cyclotron accelerates protons (50.5MeV)

•Rotating gantry

•MLC equipped

Gershenson Radiation Oncology Center Karmanos Cancer Center/Wayne State University (KCC/WSU)

•Gantry mounted superconducting cyclotron accelerates deuterons (48.5 MeV)•Rotating Gantry•MLC equipped

University of Washington NeutronClinical Neutron Therapy System (CNTS)

• University of Washington CNTS Lower Floor Schematic

Rotating gantry

Fixed beam line?

University of Washington NeutronClinical Neutron Therapy System (CNTS)

• University of Washington CNTS MLC

Neutron therapy facility at the Gershenson Radiation Oncology Center KCC/WSU

• Schematic gantry mounted superconducting cyclotron

GMSCC

Neutron therapy facility at the Gershenson Radiation Oncology Center KCC/WSU

• MLC Schematic

•MLC Photo

Fast Neutron Therapy

Considerations:• Who should be treated with neutrons?

• Subgroups of patients that may benefit from neutrons.• Slower growing tumors.

• Cancers w/ good response to neutron Therapy:• adenoidcystic carcinoma (cancer of parotid glands)• locally advanced prostate cancer • locally advanced head and neck tumors • inoperable sarcomas • cancer of the salivary glands

Neutrons for Radiation TherapyA few references……………

• Fast neutron radiotherapy for locally advanced prostate cancer. Final report of Radiation Therapy Oncology Group randomized clinical trial. (American Journal of Clinical Oncology. 1993 Apr; 16(2):164-7)

• Fast neutron irradiation of metastatic cervical adenopathy: The results of a randomized RTOG study. (International Journal of Radiation Oncology Biology Physics, Vol. 9, pp. 1267-1270)

• Neutron versus photon irradiation for unresectable salivary gland tumors: Final report of an RTOG-MRC randomized clinical trial. (International Journal of Radiation Oncology Biology Physics, Vol. 27, pp. 235-240)

• Fast neutron radiotherapy for soft tissue and cartilaginous sarcomas at high risk for local recurrence. (International Journal of Radiation Oncology Biology Physics, Vol. 50, No. 2, pp. 449–456)

• Photon versus fast neutron external beam radiotherapy in the treatment of locally advanced prostate cancer: results of a randomized prospective trial. (International Journal of Radiation Oncology Biology Physics, Vol. 28, pp. 47-54)

Boron-Neutron Capture Therapy

• The idea:• Preferentially deliver Boron containing drug to the

tumor.• Then deliver thermal (0.025eV) neutrons, which

interact with the boron to produce alpha particles.• Recall 10B has large thermal cross section: = 3837

barns

• The 10B absorbs the thermal energy neutron and ejects an energetic short-range alpha particle (1.47MeV) and lithium ion (0.84MeV) which deposit most of their energy within the cell containing the original 10B atom.

Why Boron???

Several nuclides have high thermal neutron , but 10B is the best choice for several reasons:

1. it is non-radioactive and readily available, comprising approximately 20% of naturally occurring boron;

2. Emitted particles ( and 7Li) have high LETCombined path lengths are approximately one cell diameter; i.e., about 12 microns, theoretically limiting the radiation effect to those tumor cells that have taken up a sufficient amount of 10B, and simultaneously sparing normal cells

3. Chemistry of boron is well understood and allows it to be readily incorporated into a multitude of different chemical structures.

Boron-Neutron Capture Therapy

• Beam Energy Selection• Limited penetration of thermal neutrons.

• Thermal neutrons rapidly attenuated by tissue.• HVL only about 1.5cm.• Not possible to treat depths greater than a few cm.

• Can use epithermal neutrons (1eV-10keV), which are theramlized by tissue (via collisions w/ H).

• Peak in dose occurs at 2 to 3cm; • Avoid high surface doses, but still poorly penetrating!

Boron-10 Neutron Interaction

• An epithermal beam rapidly loses energy by elastic scattering in tissue.• The thermal neutrons are captured by the 10B atoms which become 11B

atoms in the excited state for a very short time (~ 10-12 s). • The 11B atoms then splits into alpha particles, 7Li recoil nuclei and in

94% of the reactions, gamma rays.

http://web.mit.edu/nrl/www/bnct/info/description/description.html

BNCT Neutron Source at MIT/Harvard

• The MIT/Harvard group makes use of a fission converter based epithermal neutron beam at the MITR-II Research Reactor.• filtered by aluminum, Teflon, cadmium,  and Lead. • provides a broad spectrum epithermal beam with low

incident gamma and fast neutron contamination while maintaining an incident neutron flux of ~5 x 109  neutroncm-2sec-1. 

• permits irradiations for clinical trials to be conducted in 1 - 4 fractions in 10 minutes or less

Treatment Sites for BNCT

• Clinical Trials for:• Glioblastoma Multiform (GBM)

• Sweet and colleagues first demonstrated that certain boron compounds would concentrate in human brain tumor relative to normal brain tissue.1

• Melanoma

1(Sweet, W.H., Javid, M., "The possible Use of Neutron-capturing Isotopes such as boron-10 in the treatment of neoplasms," I. Intracranial Tumors, J. Neurosurg., 9:200-209, (1952) )

Production of Secondary Neutrons

Secondary Neutrons Radiation Therapy

• X-Ray Therapy• Neutrons can be produced via (-n) reactions

primarily with high atomic number materials within the treatment head.

• Proton Therapy• Neutrons can be produced via (pn) reactions,

not limited to high Z materials.• At the high energies other reactions are also

possible……………..

Production of Neutrons

(,n) Reactions

Bremsstrahlung Spectrum

Particle Production Cross SectionsENDF/B-VII Incident-Gamma Data

Note: Magnitude of the cross sections 0.1- 0.6 barns

,n) cross section

High energy 18MV x-ray treatment beam has max energy of 18MeV and an average energy of 6MeV. Secondary neutron Production is possible in high Z components of linac head. No neutrons for 6MV x-ray beam all photons below threshold

Threshold energy for (,n)

• 18 MV beam has more photons above the (,n) threshold and most are in the region where cross section dramatically increases.

(-n) Reaction Cross-Sections Elements in Tissue (C,O,N)

0.00E+00

5.00E-03

1.00E-02

1.50E-02

2.00E-02

2.50E-02

6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Energy (MeV)

-n

Rea

ctio

n C

ross

Sec

tion

(b

arn

s)

C-12

O-16

N-14

Note:

•Threshold energies are much higher compared to high Z

•The magnitude of the reaction x-sections are an order of magnitude lower than for high Z (0.005 – 0.02).

•These data are from the T-2 Nuclear Information Service.

Secondary Neutron Spectra from Clinical Photon beams

• The initial distribution of secondary neutrons generated in the linac head from (,n) reactions is approximately isotropic and resembles a fission spectrum.

• Then, • The neutron energy decreases as a consequence of

their transport through the components of the treatment head (primary collimators, flattening filter, secondary jaws, MLC, etc).

• The primary mechanisms of energy loss in high Z materials in the linac head are inelastic scattering and (n,2n) reactions.

Photoneutron SpectraEffect of Collimators and room shielding

• Photoneutron spectrum for 15MeV electrons striking W target (designated 15MeV W PN bare)• Spectrum with 10 cm of W

shielding surrounding W target.

• Spectrum with 10 cm of W shielding surrounding W target inside a concrete room

• A 252Cf fission spectrum shown for comparison

NCRP-79 Fig 25

Secondary Neutron SpectraMeasured for Varian 18MV Linac

• Howell et al. Medical Physics, Vol. 36, No. 9, 4027-4038 (2009)

Production of Neutrons

(p,n) Reactions

Proton Spectra

• Clinical proton beams have a much smaller energy spread compared to photon beams (Gaussian distribution).

• Also, the maximum energies are considerably higher and clinical beam energies may include, 100 MeV, 160 MeV, 200 MeV, and 250 MeV beams.

Particle Production Cross Sections ENDF/B-VII Incident-Proton Data

Note: High Z material (e.g. Pb-207):

Magnitude of the (p,n) cross sections are higher than (,n) and continue to increase with increasing energy.

The proton beam energies can be as high as 250MeV, well above the threshold.

November 2007 November 2007 Rebecca M. Howell, Ph.D.Rebecca M. Howell, Ph.D.

Particle Production Cross Sections ENDF/B-VII Incident-Proton Data

Note low Z material (e.g. C-12): Magnitude of the (p,n) cross sections

are much lower than in high Z materials

energy thresholds for (p,n) in low Z are higher compared to high Z

Energy threshold similar to (,n)

November 2007 November 2007 Rebecca M. Howell, Ph.D.Rebecca M. Howell, Ph.D.

Particle Production Cross Sections ENDF/B-VII Incident-Proton Data

Note low Z material (e.g. C-12): Magnitude of the (p,n) cross sections

are much lower than in high Z materials

energy thresholds for (p,n) in low Z are higher compared to high Z

Energy threshold similar to (,n)

Secondary Neutron Spectra for Clinical Proton Beams

• Zhang et al. Phys. Med. Biol. 53 (2008) 187–201

References• Eric J. Hall. Radiobiology for the Radiologist 5th Ed. (2000)• Frank H. Attix. Introduction to Radiological Physics and Radiation Dosimetry.

(1986)• Patton H. McGinley. Shielding Techniques for Radiation Oncology Facilities 2nd ed.• D.K. Bewley. The Physics and Radiobiology of Fast Neutron Beams (1989)• AAPM Report 7 Protocol for Neutron Beam Dosimetry• ICRU 45 Clinical Neutron Dosimetry• NCRP 79 Neutron Contamination from Medical Electron Accelerators• NCRP 151 Structural Shielding Design and Evaluation for Megavoltage X- and

Gamma-Ray Radiotherapy Facilities (2005)• ICRP74/ICRU57 (Jointly published by both ICRU and ICRP) Conversion

Coefficients for use in Radiological Protection against External Radiation• ICRP 60 Recommendations of the International Commission on Radiological

Protection• ICRU 66 Determination of Operational Dose Equivalent Quantities for Neutrons• http://web.mit.edu/nrl/www/bnct/info/description/description.html• T2.lanl.gov• http://www.nndc.bnl.gov/nudat2

End

This concludes the material from the neutron interactions lectures that will

be covered on the exam in this course.

Extra Information

• The material in the remaining slides is covered in Radiation Protection and is beyond the scope of today’s lecture. In previous years, I have covered this material in this course. This year, I have decided to cut this material to minimize overlap with other courses.

• However, I decided to make the slides available to you should you be interested, but will not be testing you on this material.

Shielding Considerations for Secondary Neutrons

for

High Energy X-Ray Beams

Shielding for Photoneutrons

• High beams (>10MV) are contaminated with neutrons.• Produced by high energy x-rays and e-s incident

on various materials (target, flattening filter, collimators, etc.).

• Many more neutrons in x-ray beam than in e- beam.

• X-section for (e,n) reactions smaller than x-section for (,n) by factor of 10.

• In electron mode beam current is about 1000X less than in x-ray mode due to inefficiency of Brems. Production.

Shielding for Photoneutrons

• Neutron contamination increases rapidly with energy from 10 to 20 MV, then remains approx. constant above 20MV (recall that there are very few linacs with max energies greater than 25MeV).

• Neutron contamination at cax for 16-25MV x-ray beam is approx. 0.5% of x-ray dose, and falls off to about 0.1% outside the field.• Higher for IMRT more beam on time to

achieve same photon dose.

November 2007 November 2007 Rebecca M. Howell, Ph.D.Rebecca M. Howell, Ph.D.

Particle Production Cross SectionsENDF/B-VII Incident-Gamma Data

Note: Magnitude of the cross sections 0.1- 0.6 barns

,n) cross section

Neutron contamination increases rapidly with energy from 10 to 20 MV, then remains approx. constant above 20MV (recall that there are very few linacs with max energies greater than 25MeV).

Threshold energy for (,n)

Shielding for Photoneutrons

• Concrete barriers designed for x-ray shielding are sufficient for photoneutrons.

• Door must be protected against neutrons that diffuse into the maze and reach the door.• Required door shielding can be minimized with a good

maze.• Maze > 5m desirable.

• This length is chosen because the TVL for the photoneutrons entering the maze is approximately 5m (MKcGinley, pg 71)

Example of Door in High Energy Vault shielded for Neutrons

Note: Figure (5.5) is taken from Shielding Techniques 2nd ed. by Patton McGinley

Note: the average energy of the neutrons at the Maze entrance is approximately 100keV (NCRP, 1984)

Note: there are many ways to design a door, in some cases, the lead is placed before the polyethylene, and in some cases it is sandwiched between two layers of lead, this is just one example of a door design.

Door Design Photoneutron Shielding

• After multiple scattering interactions in the polyethylene (high H content) the neutrons are thermalized.• Thermal neutrons can undergo neutron capture releasing high

energy -rays (n,) -rays energies can exceed 8MeV, and have an average energy of 3.6MeV.

• How do we eliminate these high energy -rays? Add 5% Boron to the polyethylene.• Boron absorbs (high thermal absorption cross section) the low

energy neutrons before they have a chance to undergo (n,) reactions.

• But the reaction also results in a 0.48MeV .

• Lead is placed after the boronated polyethylene, where it attenuates the photons produced in the boron (0.48MeV) and any capture gamma rays generated in the maze by neutron capture in the concrete walls, ceiling, and

McGinley, 2002

Boron Interaction with Thermal Neutrons

• Materials with B are effective absorbers of thermal neutrons because of the high thermal neutron cross section• The thermal neutrons are captured by the 10B atoms which become

11B atoms in the excited state for a very short time (~ 10-12 s). • The 11B atoms then fissions producing -particles, 7Li recoil nuclei,

and in 94% of the reactions, gamma rays (0.48MeV).

Door Design for Neutron Shielding Details

1. Boronated polyethylene: The polyethylene (high H content) slows

(moderates) the fast and intermediate energy neutrons to thermal energies.

The 5% Boron absorbs the low energy neutrons (high cross section for thermal neutron absorption).

2. Lead absorbs the 0.48 MeV photon that results from the (n,) and capture gammas ( from maze ceiling, and floor).

Polyethylene 5% Boron

Steel Casing

Lead

Maz

e

Activation of Materials in Linac components

ReactionMode of Decay

T1/2Photon E

(MeV)27Al(n,)28Al 2.3 min 1.780

63Cu(n,)62Cu 9.7 min 0.51155Mn(n,)56Mn 2.6 hr 0.84763Cu(n,)64Cu 12.7 hr 1.34665Cu(n,)64Cu 12.7 hr 1.346186W(n,)187W 23.9 hr 0.479/0.686

58Ni(n,)57Ni 36.0 hr 1.378/1.920

Reproduced from Table 4.4, McGinley

Activation Material in Air

• Air is made radioactive by medical accelerators operated above 10 MeV primarily by:

• Each reaction produces a positron emitter with a relatively short half-life.• Patients/personnel can be exposed to 0.511 MeV

annihilation photons.

ReactionHalf-life

(sec)Threshold Energy

(MeV)14N(,n)13N 600 10.516O(,n)15N 122 15.7

Activation Material in Air

• Maximum permissible concentration in air (MPCa) based on a 40-hr work week and typical treatment vault (air volume).

Isotope OrganMPCa

(Bqm-3)DI

(nSvs-1)

13NSkin 1.5x106 41.7

Whole Body 8.5x106 6.95

15OSkin 7.4x105 41.7

Whole Body 8.5x106 6.95Reproduced from Table 7-5, McGinley

Activation Material in Air

• McGinley at al (1984) calculated annual total dose equivalent to radiation therapists skin• Conservative Calculation Assumptions:

• 40 patients per day• 5 days per week• 4Gy/fx• Daily treatment time = 120-s• Therapist stay time of 600s per patient

• Far below the Maximum Permissible Skin dose (0.5Sv).

• Air activation presents minimal hazard.

Neutron Dose

Two main Categories of “Neutron Dosimetry”

• Clinical Neutron Beam Dosimetry• Dose from neutron beams used for patient

treatment.

• Protection Dosimetry for unwanted neutron dose (low neutron doses)

Quantities used in Radiation Protection

Quantities used in Radiological Protection• There are two types of quantities used in

radiological protection:

1. Protection Quantities - defined by the International Commission on Radiation Protection (ICRP).

2. Operational Quantities - defined International Commission on Radiation Units and Measurements (ICRU).

ICRP Protection Quantities

• The (ICRP) defines limiting or protection quantities as dosimetric quantities specified in the human body.

• Recommended dose limits are expressed in terms of protection quantities.

• These quantities are not directly measurable but may be related by calculation to the radiation field in which the exposure occurs.

ICRU Operational Quantities

• Operational quantities were designed by the ICRU in response to ICRP recommendations on radiological protection.

• Used to demonstrate compliance with dose limits.

• Operational quantities provide a reasonable estimate of the protection quantities and serve as calibration quantities for dosimeters used in monitoring.

Physical Quantities

Fluence, Kerma, Absorbed Dose

Operational Quantities•Ambient Dose Equivalent•Personal Dose Equivalent

Protection Quantities•Equivalent Dose•Effective Dose

Measurements and Conversion Coefficients Calculations and Conversion

Coefficients

Calibration

Instrument Response

Conservative Approximation

Protection Quantities

ICRP Protection Quantities

• Absorbed Dose, DT, is the mean absorbed dose in a specified tissue or organ of the human body, T,

• Equivalent Dose, HT, is the absorbed dose averaged over the tissue or organ, T, irradiated in a radiation field consisting of several different radiations with different values of WR,

• Effective Dose, E, is the sum of the weighted equivalent doses in all the tissues and organs of the body,

• where, mT is the tissue or organ mass, DT is the absorbed dose in the mass element dm.

• where, DT,R is the average absorbed dose from radiation, R, in tissue T.

• where, HT is the equivalent dose in the tissue or organ T and WT is the weighting factor for the tissue.

TmT

T Ddmm

D1

ICRP Protection QuantitiesICRP-60 Tissue Weighting Factors

Tissue or OrganTissue Weighting Factor,

WT

Gonads 0.2

Bone marrow (red) 0.12

Colon 0.12

Lung 0.12

Stomach 0.12

Bladder 0.05

Breast 0.05

Liver 0.05

Esophagus 0.05

Thyroid 0.05

Skin 0.01

Bone Surface 0.01

Remainder 0.05

Note: In the case in which a single one of the remainder organs receives an equivalent dose in excess of the highest dose in any of the 12 organs for which a weighting factor is specified:

• a weighting factor of 0.025 should be applied to that tissue and

• a weighting factor of 0.025 applied to the average dose in the rest of the remainder organs.

Radiation Weighting Factors

Radiation Weighting Factors

Operational Quantities

ICRU Operational QuantitiesDose Equivalent

• The Dose Equivalent, H as defined by the ICRU is the product of Q and D at a point in tissue:

• where D is the absorbed dose• Q is the quality factor at this point

• The SI unit for H is the Sievert (Sv).

QDH

ICRU Operational QuantitiesDose Equivalent

• The quality factor depends on the unrestricted linear energy transfer, L, for charged particles in water, specified in ICRP publication 60.

• For photons and electrons the quality factor is unity. For neutrons, the quality factor is strongly energy dependent (ICRU Report 66).

dLDLQDQ L1

Determining Dose Equivalent

• Dose equivalent is essentially unmeasurable. • But, as shown in the previous diagram, it can be

determined by calculation or by a combination of calculations and measurements.

• Quantities needed to determine dose equivalent are: • absorbed dose and the quality factor or • fluence and fluence-to-dose equivalent conversion

coefficients.

Additional Information on Neutron Interaction

Cross-sections

This information will not be on your exam.

Transport of Secondary Neutrons

• Strongly dependent on absorbing material and incident neutron energy.

• Example: 207Pb • The next 15 slides of the presentation are

provided as an example of the data that are available for a given isotope, given time constraints, these can not be covered in detail.

Great Reference for Nuclear Data

T2.lanl.gov

T2.lanl.gov

• What Data are provided in this file?

• Let’s consider an example:

• For 207Pb, the PDF file has 179 pages of data.

View PDF files for various elements/

isotopes

Principal Cross SectionsLow Energy (Log-Log Plot)

Principal Cross SectionsHigh Energy (Linear Plot)

Inelastic Cross Sections are provided on Separate Plots

Inelastic Cross Sections

• What is (n,n*1), (n,n*2), etc.• There is a discrete energy band gap between

energy levels in the nucleus.• *1 refers to promoting a neutron into the first excited

state.• *2 refers to promoting a neutron into the second excited

state.• (n,x) – Anything above 20th excited level (not n,)

• How much energy are we talking?• http://www.nndc.bnl.gov/nudat2/

NuDat 2.4Select Levels and Gamma Search

http://www.nndc.bnl.gov/nudat2/

Enter Isotope of Interest

Search

Nuclear Level and Gamma Search

1st excited State2nd excited State3rd excited State

Threshold Reactions

Angular Distributions(provided for all elastic and inelastic Scattering)

• Angular Distribution of Elastic Scatter (0-30MeV)

• Angular Distribution of inelastic (n,n*1)Scatter (0-30MeV)

Definitions from ICRP Report 63• Elastic denotes a reaction in which incident

projectile scatters off target nucleus with total KE being conserved (final nucleus is the same as bombarded nucleus).

• Non-elastic is a general term referring to nuclear reaction that are not elastic (i.e. KE is not conserved).

• Inelastic refers to specific type of non-elastic reaction in which KE not conserved, but final nucleus is the same as bombarded nucleus.