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Evaluation of the near threshold 7 Li( p , n ) 7 Be accelerator-based irradiation system for BNCT. A treatable protocol depth (TPD)-based characterization of neutron fields. Gerard Bengua. Presentation Outline. Introduction New protocol-based evaluation indices

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evaluation of the near threshold 7 li p n 7 be accelerator based irradiation system for bnct

Evaluation of the near threshold 7Li(p,n)7Be accelerator-based irradiation system for BNCT

A treatable protocol depth (TPD)-based characterization of neutron fields

Gerard Bengua

presentation outline
Presentation Outline
  • Introduction
  • New protocol-based evaluation indices
  • Characterization of BDE Materials
  • TPD-based evaluation of near-threshold proton energies
  • Effects of variations in 7Li-target thickness
  • Gaussian proton beams for the near threshold 7Li(p,n)7Be reaction
slide4

Introduction

Boron Neutron Capture Therapy

highly selective killing of tumor cells with simultaneous sparing of normal cells

slide5

Introduction

Boron Compound

that preferably delivers higher 10B concentration in tumor than in healthy cells

Method for determining 10B concentration

for accurate timing of neutron irradiation

Success of

BNCT

Neutron irradiation field

that can provide a high flux of thermal neutrons at sites where 10B accumulates

slide6

Introduction

(Bayanov et al, 1998)

Accelerator-based Neutron Sources (ABNS)

can produce epithermal beam source with small fast neutron, thermal neutron and -ray contamination and feasible for a hospital-based BNCT

Nuclear Reactors

capable of producing high neutron flux with stable therapeutic properties for BNCT

Radioisotopic source

252Cf

Neutron sources for BNCT

introduction1
Introduction
  • Accelerator-based neutron source for BNCT
  • Advantages:
    • Hospital-based implementation of BNCT
    • Familiarity of oncologist, medical physicist and technicians with accelerators in hospitals
    • Public acceptance
introduction2
Introduction
  • Accelerator-based neutron source for BNCT
  • Candidate reactions for ABNS
introduction3
Introduction
  • Accelerator-based neutron source for BNCT
  • Candidate reactions for ABNS
  • The 7Li(p,n)7Be reaction
  • Moderated neutron usage – Ep ~ 2.5MeV
  • Near-threshold neutron production
    • lower ave. neutron energy = no need for dedicated moderators
    • neutrons are kinematically collimated
    • more compact irradiation system

Threshold energy: 1.881MeV

introduction4
Introduction
  • Accelerator-based neutron source for BNCT
  • Candidate reactions for ABNS
  • The 7Li(p,n)7Be reaction
  • Moderated neutron usage – Ep ~ 2.5MeV
  • Near-threshold neutron production
    • feasibility for BNCT irradiation demonstrated by Tanaka et al (2002)
introduction5
Introduction
  • Accelerator-based neutron source for BNCT
  • Candidate reactions for ABNS
  • The 7Li(p,n)7Be reaction
  • Recent research output of our group
  • Proposed new and more comprehensive dose evaluation indicesfor BNCT based on actual dose protocol;
  • Established a method for the characterization of neutron fields for BNCT;
  • Evaluated the various components of the near-threshold 7Li(p,n)7Be-ABNS for BNCT using the  and ;
slide13

Background and Definitions

Treatable Region:

(1) the tumor dose from HCP is greater than or equal to the treatment protocol dose for tumors;

(2) the dose to healthy tissue do not exceed the tissue tolerance dose from either HCP or gamma rays set by protocol;

Dose components of the intra-operative BNCT for brain tumor(Nakagawa et al., 2003)

TheTreatable Protocol Depth (TPD)is the maximum depth of the treatable region relative to the surface facing the incident neutron beam. For symmetric geometries, this is located at the central axis.

slide15

Normalized dose components based on dose protocol

The

Treatable Protocol Depth (TPD)

is equal to whichever is shallower between PD(hcp) and PD().

BNCT dose components

slide16

38

( Unit : mm )

BDE*

H2O Phantom

(180diam×200)

38

( Unit : mm )

Proton

Central axis

BDE*

H2O Phantom

(180diam×200)

38

( Unit : mm )

Proton

Central axis

H2O Phantom

(180diam×200)

Proton

Central axis

TPD = 2.11 cm

TPD = 2.91 cm

*BDE – Boron dose enhancer

TPD = 0.45 cm

slide18

The relationship between BDE thickness and PD(hcp), PD() and TPD.

TPDmax

peak of the TPD versus BDE thickness curve

BDE(TPDmax)

the BDE thickness corresponding to TPDmax

objective
Objective
  • To determine possible optimization criteria for selecting appropriate BDE materials for BNCT.
  • In particular,
    • Evaluate the effects of various candidate BDE materials on the BNCT dose components;
    • 2. Examine the characteristics of candidate BDE materials in relation to the Treatable Protocol Depth (TPD) and other pertinent figures of merit;
calculation parameters
Calculation Parameters

Incident proton energy: 1.900MeV (mono-energetic)

7Li-target thickness: 2.33m

BDE diameter: 18cm

BDE thickness: Variable (chosen in order to attain the TPDmax for each BDE material)

BDE material: Variable

calculation method
Calculation Method

Flowchart of Calculation

Procedures

Neutron production

-ray production

Particle transport

Dose calculation

Evaluation of dose distribution

calculation method1
Calculation Method

Flowchart of Calculation

Procedures

Neutron production at the 7Li-target

  • Lee et al.’s code: calculation of neutron yields from thin 7Li-targetfor the near threshold 7Li(p,n)7Be reaction
  • Bethe’s stopping power formula: derivation of 7Li-target thickness

Neutron production

-ray production

Particle transport

Dose calculation

Evaluation of dose distribution

calculation method2
Calculation Method

Flowchart of Calculation

Procedures

Production of gamma-rays from proton-induced reactions

Neutron production

-ray production

Particle transport

Dose calculation

Evaluation of dose distribution

*Neutron induced production of gamma-rays are included in the MCNP simulation

calculation method3
Calculation Method

Flowchart of Calculation

Procedures

Neutron and gamma-ray transport in the irradiation system

Neutron production

-ray production

  • Monte-carlo n-particle (MCNP) transport code (MCNP4C2, MCNPX)
  • Particle tally: neutron and gamma-ray flux
  • Estimated relative error of calculated data < 5%
  • S(,) thermal neutron scattering tables

Particle transport

Dose calculation

Evaluation of dose distribution

calculation method4
Calculation Method

Flowchart of Calculation

Procedures

Calculation of absorbed dose in tumor and healthy tissue

Neutron production

-ray production

  • Absorbed dose = flux * KERMA factor
  • Tissue composition: H(11.1), C(12.7), N(2.0), O(74.2)

Particle transport

Dose calculation

Evaluation of dose distribution

calculation method5
Calculation Method

Flowchart of Calculation

Procedures

Evaluation of dose distribution

Evaluation indices: Treatable protocol depth (TPD), Heavy-charged particle protocol depth (PD(hcp)), Gamma-ray protocol depth (PD())

Applied dose protocol: Intra-operative BNCT dose protocol for brain tumors

Neutron production

-ray production

Particle transport

Dose calculation

Evaluation of dose distribution

slide31

TPDmax, BDE(TPDmax) and Tumor dose rate at TPDmax forthe candidate BDE materials evaluated in this study

summary
Summary
  • The following parameters together with other practical considerations may be used for choosing suitable BDE materials for BNCT:
    • TPDmaxdeeper is better for deep-seated tumors
    • BDE(TPDmax)thinner is better from the view point of dose rate reduction and material handling
    • TPD versus BDE thickness curvesmaller dependence of TPD on BDE thickness is better to avoid large variations in TPD for small changes in BDE thickness
slide33

TPD-based evaluation of near threshold proton energies for the 7Li(p,n)7Be production of neutrons for BNCT

slide34

Objective

To evaluate the characteristics of neutron fields from the 7Li(p,n)7Be reaction at near-threshold incident proton energies with the treatable protocol depth (TPD) as the primary index of evaluation

slide35

Background

Incident Proton Energy: 1.885 MeV

Incident Proton Energy: 1.900 MeV

slide36

Background

Incident Proton Energy: 1.885 MeV

Incident Proton Energy: 1.900 MeV

slide37

Background

Higher dose rate for all BNCT dose components

Higher proton energy

More effective for treatment

Incident Proton Energy: 1.885 MeV

Incident Proton Energy: 1.900 MeV

calculation method6
Calculation Method

Simulation parameters

slide39

PD(hcp) and PD() curves generated by

near-threshold proton energies

slide41

Higher proton energy

Higher ave.neutron energy

Lower relative difference between dose to tumor and to healthy tissue

TPD curves generated by near-threshold proton energies

The Colored arrows indicate the TPDmax

slide42

TPDmax, BDE(TPDmax), HCP dose rate at TPDmax and the required proton current to deliver 15 Gy per hour at TPDmax

slide43

Central axis distribution of HCP dose rate to tumor

Colored region indicates the depths within the treatable region

Gray-shaded region indicates the depths beyond the treatable region

slide44

Central axis distribution of HCP dose rate to tumor

As an example, consider a tumor located at 3cm.

The choice of the suitable proton energy will depend on the desired HCP dose rate to tumor.

variation in 7 li target thickness for near threshold 7 li p n 7 be neutron production for bnct

Variation in 7Li-target thickness for near threshold 7Li(p,n)7Be neutron production for BNCT

objective1
To investigate the range of allowable 7Li-target thickness in the production of neutrons for BNCT via the near-threshold 7Li(p,n)7Be reactionObjective
background
Background
  • 7Li-target thickness for near-threshold neutron production at 1.900MeV  tmin = 2.33 m; minimizes gamma production in 7Li-target
  • Thicker 7Li-targets may be needed to extend target life-time if solid targets are used.
  • Thicker targets = larger gamma ray component in neutron field
slide49

Gamma ray yield for the proton-induced reactions in the 7Li-target and aluminum backing material.

tmin=2.33m

calculation method7
Calculation Method

Simulation parameters

slide52

Dependence of TPD on the 7Li-target thickness and BDE thickness

tupper is the limit of the 7Li-thickness that will result in the deepest attainable TPD for each BDE thickness

tmin=2.33m

slide53

Dependence of TPD on the 7Li-target thickness and BDE thickness

Range of usable 7Li-target thickness

for the BDE thickness used

tmin=2.33m

summary2
Summary
  • While thinner 7Li-targets are desirable because they produce less gamma rays, thicker targets may be used for as long as they do not reduce the attainable TPD.
slide60

Gaussian proton beams for neutron production with the near threshold 7Li(p,n)7Be reaction for BNCT

objective2
To evaluate the influence of incident proton energy fluctuations on the TPD in the near threshold 7Li(p,n)7Be accelerator–based BNCTObjective
background1
Background
  • Ideal condition: mono-energetic incident proton energy
  • Real condition: fluctuating incident proton energy
  • Influence on stability of neutron production at near threshold energies

Cross-section for the 7Li(p,n)7Be reaction adapted from Liskien (1975)

calculation method8
Calculation Method

Simulation parameters

slide64

Effect of Gaussian proton beams on

the BNCT dosecomponents

  • Dose rates of all dose components increase with incident proton energy spread.
  • Relative change in dose rate is greater for hcp than for gamma rays.
slide66

TPD curves for mono-energetic and Gaussian proton beams

Fluctuations in the incident proton energy will result in a significant reduction in TPD for irradiations without BDE

slide67

TPD curves for mono-energetic and Gaussian proton beams

Using suitable BDE material and thickness will improve the attainable TPD even for highly fluctuating incident proton beams

Fluctuations in the incident proton energy will result in a significant reduction in TPD for irradiations without BDE

summary3
Summary
  • An acceptable limit of the energy fluctuation for Gaussian incident proton beam would be about ±10keV.
  • Introducing a suitable BDE material and thickness in the irradiation field can narrow down the difference in attainable TPDmax for an ideal mono-energetic beam and a Gaussian proton beam.
summary4
Summary

As we come closer to the realization of BNCT irradiation using ABNS, it is apparent that both near-threshold and moderated neutron usage of the 7Li(p,n)7Be reaction will be implemented depending on specific treatment requirements.

Regardless of the approach used in the neutron production and the design of the irradiation system, the new protocol-based evaluation indices and the method for evaluating neutron fields we defined in our study will be effective tools in providing a simple and more comprehensive way of evaluating the worthiness of neutron fields from ABNS for BNCT.

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