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Electromagnetic characterization of big aperture magnet used in particle beam cancer therapy

Jhonnatan Osorio Moreno M.Pullia, C.Priano

Presented at Comsol conference 2012 Milan

Milan– 10th October 2012

Excerpt from the Proceedings of the 2012 COMSOL Conference in Milan

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General overview

1. Introduction

3. Big aperture magnet characterization

2. Validation of COMSOL simulations

4. conclusions

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Introduction

The external proton, ion or neutron beam irradiation to tumor cancer

cells

Particle beam cancer therapy is:

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Introduction

The tumor cell damage depends on the number of single and double strand breaks in DNA structure.

If the tumor tissue is irradiated with ions like

12C

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Microscopic visualization of the extremely localized DNA damage induced in nuclei of mammalian cells following irradiation with accelerated ions

Introduction

The lethal damage in the tumor cells is highly localized and effective

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12C

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Photons ProtonsRapid fall-off

X rays

12C-Ions

The particles irradiation fields have a favorable depth dose profile

Introduction

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Synchrotron

v B

F

Lorentz Force

Introduction

Highly homogeneous magnetic fields are used to control and direct the ion beam towards the patient

Bending magnets

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B ρ

Bending radius (ρ) and magnetic rigidity

Magnetic rigidity

Lorentz Force

Introduction

Carbon ion treatments require 430 MeV/u (~27 cm in depth)

to be clinically useful

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12C

Unfortunately The iron-dominated magnets only reach about B=1.8T of maximum

magnetic field (B) without loosing the optimal conditions for ion beam transport.

Introduction

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Introduction

The reality

Introduction

The design

The idea is to guarantee a magnet able to transport towards the patient a 12C ion beam with kinetic energies up to 430 MeV/u, with the following characteristics: • 1.8 T Field intensity • Field variations less than 2x10-4

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Validation of COMSOL simulations

Bending dipole (π/8)

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Validation of the simulations

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Magnetic flux density Magnetic field quality

Magnetic field profile in the GFR

Static Simulations

Magnetic length 1.696 m

B=1.53T

-2x10-4≤ ΔB/B≤2x10-4

Good Field Region (GFR)

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To validate the COMSOL calculations a comparison between dynamic measurements

and simulations has been done

Time dependent calculations The energy beam variations during the treatment require variations in the magnetic

field strength of every magnetic element in the gantry line.

Magnetic flux density in the center of the GFR dynamic and static

calculation

dI(t)/dt = 5630 A/s dB(t)/dt= 3 T/s

Tension bars

Validation of the simulations

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Time dependent calculations The energy beam variation during the treatment, requires time variations in the magnetic

Field strength of every magnetic line element.

The results show a good agreement between the calculated and the measured data

0

0.05

0.1

0.15

0.2

0.25

0.3

0.48 0.68 0.88 1.08 1.28 1.48 1.68 1.88 2.08

(Bst

atic-B

dyna

mic

s)

t(sec)

simulation measurement

Ramp end 0.5 sec

Validation of the simulations

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Big aperture magnet characterization jhonnatan.moreno@cnao.it 15

90° bending magnet

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90° conventional magnet at CNAO Facility

Its function is to transport the ions from the scanning magnets to the patient

Big aperture magnet

Patient

Scanning magnets

90° bending magnet

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90° bending magnet

Schematic view of the treatment room

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Static Simulations

This magnet has some particular features:

• Tension bars • Ferromagnetic stiffening frame • Magnet weight: 82 tons • GFR: 20 x 20 cm2

• Magnetic field quality requested in the GFR is ΔB/B ≤ 2x10-4

90° bending magnet

Magnetic field quality definition:

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Magnetic flux density simulated. No tension bars and no stiffening frame

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Contribution of the tension bars to the magnetic field in the GFR

2D Geometry simplification used in COMSOL

20 cm 20

cm

Observation line

90° bending magnet

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Magnetic line for a mobile isocenter gantry

B in the center of the magnet [T]

Delta (%)

Only iron yoke 1.8753 0 Yoke+tension bars 1.888 0.67 Yoke+ tension bars + stiffening frame

1.890 0.78

Values of the magnetic field strength in the middle of the GFR

Magnetic field: no additional ferromagnetic structures

1.874

1.876

1.878

1.88

1.882

1.884

1.886

1.888

1.89

1.892

-150 -100 -50 0 50 100 150

|B|(

T)

Xgfr(mm)

Yoke

Yoke+tension bars

yoke+tension bars+stiffening frame

Magnetic field profiles in the center of the magnet

90° bending magnet Static Simulations

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Magnetic field homogeneity

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90° bending magnet Static Simulations

No additional ferromagnetic structures

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-2.00E-05

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

1.20E-04

1.40E-04

1.60E-04

-110 -60 -10 40 90

LBM

LBM+Tension bars

LBM+tb+Stf

∆𝐵/𝐵

Xgfr(mm)

Magnetic field homogeneity contribution of the tension bars and of the tension bars + the stiffening frame, respectively

90° bending magnet Static Simulations

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3D preliminary simulations 90° bending magnet Static Simulations

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dB/dt = 0.4 T/s dI / dt = 506.6 A/s

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90° bending magnet Dynamic Simulations

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Magnetic flux lines and eddy currents distribution in the tension bars

90° bending magnet Dynamic Simulations

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Difference between static field (Bs) and dynamic field (Bd) at the end of the feeding current ramp, t = 4.5 s

Slow ramp

The eddy currents generated in the tension bars give a time constat (τ) of 1.13 s

90° bending magnet Dynamic Simulations

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Conclusions

• Static simulations of the CNAO 90° large gap dipole have shown that the 2D field homogeneity is acceptable and that the influence of the stiffening frame structure and of the tension bars cannot be neglected

• The variation in the absolute value of the field in the gap is not significant for the requirement on the excitation current and the effect on the field homogeneity is unimportant

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Thank you for your attention

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Additional slides

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Hadron From Wikipedia, the free encyclopedia In particle physics, a hadron is a composite particle made of quarks held together by the strong force. Hadrons are categorized into two families: baryons (made of three quarks) and mesons (made of one quark and one antiquark).

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90° bending magnet

GFR ( 20 x 20 cm2)

Magnetic field [T] 1.87 ΔB/B0 at GFR [-0.8x10-4, 1.03x10-4] Stored Energy [J] 1213924.48 Inductance [H] 0.47 Dissipated DC power [kW] 613.65 DC voltage [V] 269.16 Inducted Voltage [V] 236.8 Excitation current[A] 2800 Ampere-turns 182400 Magnet Weight [tons] 82

Summary of big aperture magnet simulated characteristics

Comparison table between the reduced 90° bending magnet (GRF 15 x 15 cm2) and the reference 90° CNAO bending magnet (GFR 20 x 20 cm2)

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To simulate the eddy currents in the tie dependent calculations , the tension bars Have to be simulated as a single turn coil domain.