MAGNETIC may come in the path of the

MAGNETIC
MODULATED PROTON BEAM THERAPY-  CURVED
BEAM APPROACH IN RADIOTHERAPY-A NEW CONCEPT.

Dr.Gurku H.A

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Narayana Hrudyalaya Hospital,
Bangalore, India

Abstract

One of the
important properties of the proton beam i.e. its behavior in presence of
magnetic field has not been extensively investigated in field of radiotherapy.
If a proton beam passes through the magnetic field acting perpendicular to the
velocity, it bends in a direction which is perpendicular to both. By bending
the proton beam, we can achieve the dose distribution within the target volume
by completely sparing the OAR adjacent to it. Also, combining with technologies
of magnetic scanning of pencil beam within the target volume and MRI guided
proton beam therapy, we can provide the advantage of a more conformal dose
distribution with complete avoidance of an OAR which may otherwise come in the
path of proton beam therapy.

 

 

 

 

 

 

 

 

 

 

INTRODUCTION:

Proton beam therapy is one the latest technological advances
in the treatment of cancer. It involves the use of positively charged particle
proton for irradiating the tumor. With the properties like bragg peak effect,
it produces dose deposition over the narrow range with minimal exit dose. Being
the charged particle one of the important properties of proton i.e. its
behavior in the presence of a magnetic field has not been extensively evaluated
and utilized in the field of radiotherapy.

This paper discusses the potential benefit of the bending the
proton beam by using the magnetic field during the treatment of cancer patient
by proton therapy with the possibility of avoiding the important structures
(organ at risk) which otherwise may come in the path of the straight beam.

PHYSICS :

According to the Lorentz force law, when a charged particle
like proton moves in a direction at an angle ? with the direction of the magnetic
field, it experiences a magnetic force with magnitude F given by:

                            
F=?q?v Bsin?

Thus, a charge moving parallel to the magnetic field experiences
zero magnetic force (sin 0=0).

                        
F=?q?vBsin0 =0

And if a charge is moving in a direction perpendicular to the
direction of a uniform magnetic field, it moves in a circle at a constant
speed. Force (F) is perpendicular to the velocity (v) and is of constant magnitude.
(Sin90=1)

                            

                                           F= qvB

                                            Where q= charge

                                                         
V =velocity

                                                         
B = magnetic field

    Also,                                F = ma

                                                         
m=mass

                                                          
a=acceleration

Constant acceleration perpendicular to the velocity gives
circular motion

                                            a=v2/R

                                          qvB=m (v2/R)

Radius of the circle is

                                         R=mv/qB

 

RIGHT HAND RULE:

The direction of the magnetic force on positively charged
particle can be given by right hand rule. The direction of the velocity vector ‘v’
is given by the direction of the index finger. The middle finger gives the
direction of the magnetic field vector ‘B’ and the thumb points in the
direction of the force ‘F’.

 The direction of the
force is opposite to the direction given by the thumb for negatively charged
particle like electron.

USE OF MAGNETIC FIELD
IN PROTON THERAPY-PRESENT STATUS:

Apart from the acceleration of proton beam in cyclotron or synchrotron,
magnetic field at present is being investigated and utilized in two ways:

1.    
Magnetically
scanned proton pencil beam

2.    
MRI
guided proton therapy.

MAGNETIC SCANNING:i

As the dimensions of the proton beam emerging from the
accelerator are small and the bragg peak is also narrow so in order to cover 3D
target volume, beam is modified to get the required dose. There are two methods
followed, to achieve that. One is passive scattering where a passive mechanical
is used to spread beam laterally and the other method is magnetic scanning of
pencil proton beam where the positively charged proton beam is deflected
magnetically. In spot scanning individual pencil beams are sequentially
deposited and a highly conformal plan is achieved by changing the dosage and
position of each pencil beam individually.

MRI GUIDED PROTON
THERAPYii:

In proton beam therapy, directing the beam and also predicting where the
beam will stop is more important than in photon therapy. This uncertainty in
the range requires additional margins, which can lead to decrease in the
benefit provided by the proton therapy bragg peak effect especially in moving targets.
Thus, to counter the impact of motion, image guidance in real time, like MRI
guided proton therapy is being investigated.

DOSIMETRIC EFFECT OF
MAGNETIC FIELD IN PROTON BEAM -EVIDENCE SO FAR

Most of the data for dosimetric effects of magnetic field on
dose distribution have been studied especially in case of MRI guided proton and
photon therapy.iiiiv
Effect of transverse magnetic field on the dose distribution in case MRI guided
photon beam therapy revealed an increase of up to 30-40% of the dose especially
at the tissue air interface due to the electron return effect(ERE). For
proton therapy, the impact of a transverse magnetic field from ERE is
negligible. This is due to the fact that the electrons released by the proton
irradiation have a very low energy, making the electron path lengths very
short. So, the total number of electrons scattering away from a tissue air
interface is far less than for photon therapy. In fact, so much less that the
ERE can be neglected in dose calculations.

Most of the studies done to
analyze the magnetic deflection of proton beam considered it as an adverse
effect and not advantageous due to the deflection caused in an otherwise
well-designed plan. Dosimetric analysis of intensity modulated proton therapy
in a transverse magnetic field of 1.5 T by Hartman showed that the generation of an IMPT plan in a magnetic field was
feasible and the impact of the magnetic field was small, and the resulting dose
distributions were equivalent for 0 T and 1.5 T.v

However,
Raaymakersvi
paper addressed the magnetic field dose effects for proton irradiation in the
presence of a 0, 0.5 T and 3T magnetic field. It demonstrated that the average
energy as a function of depth does not depend on the magnetic field strength
and the dose distribution of a 5 ×
5 cm2 proton beam showed the shift of the field by 1 mm at the Bragg
peak, caused by the proton beam curvature, in case of 0.5 T. For 3.0 T the
profile was the same, with the lateral shift of 5 mm. Fig 1

 

Figure 1 raaymakers 2008

                                                                                         

The
dose distribution for the layered, water–air–water phantom showed, the lateral
dose shift due to the proton beam curvature was same  for 0.5T 
but  for 3.0 T the lateral shift
of the Bragg peak was 8 mm with no impact of the magnetic field  at 
tissue–air interfaces. Fig 2

Figure 2 raaymakers 2008

 

For
0.5 T the curvature of a 90 MeV proton beam in a vacuum was 2.8 m. Over a depth
of 6 cm, lateral displacement was 0.2 mm. As the average energy of the protons
decreases with depth, the curvature also decreased, resulting in a 1 mm lateral
displacement. For 3.0 T the curvature of the proton beam was more pronounced.
The lateral shift of the Bragg peak increased with increasing penetration depth.
The 2-cm wide air gap also caused a longer proton beam track and thus a larger
lateral shift.

Wolf et al gave the analytical solution of the proton
deflection in a magnetic fieldviito examine
the trajectory of a slowing-down proton beam in a magnetic field. Their results showed
that end of range displacement varies with the 3rd power of the
energy.  A 90 MeV beam in a 3 T field showed a 5-mm
lateral displacement of the Bragg peak while a 200 MeV beam showed greater than
5 cm deflection in water. As the proton beam passes through a medium the energy of the beam
decreases and at the point of bragg peak the energy becomes zero. But even
though the energy of the particle becomes small, they do not curl up because
the slowing down occurs so rapidly that the residual range is not sufficient
for spiral motion.

One recent study by Wenchengeviii
shao, investigated the bragg peak positions of proton beam therapy under the
influence of magnetic field inside the cancer patients with modulation to
achieve proper coverage and uniform dose within the tumor. This
study was the first to introduce the concept of using magnetically modulated
proton beam in the treatment of cancer. Using GEANT4 to simulate the proton
transportation under magnetic field they first passed the proton beam through a
gap between vital organs which may be surrounding the tumor and then bending
the beam towards tumor by magnetic field. Next they constructed and
investigated the magnetic modulation and dose deposition in an ideal water
phantom with central tumor and surrounding cuboid vital organs and also in
abdominal phantom for pancreatic tail tumor and liver tumor.

 The vital organ
doses were found to be approximately 50%, 30%, 30%, and 15% for the single,
opposing, orthogonal, and box fields, respectively in the water phantom. The
organ volume receiving proton irradiations for the opposing, orthogonal, and
box fields increased by two, two, and four times compared with that for the
single field. The tumor was adequately covered by a 95% dose line, and the
maximum tumor doses were less than 110%. They also
repeated the procedures for abdominal anatomies like tumors at the pancreatic
tail and liver. In
pancreatic tumor case, the proton beams were curved and bypassed the kidney to
generate uniform doses inside the tumor through MMPT. In the liver case, the
liver volume receiving proton irradiations was reduced by approximately 40%.
They concluded that the Braggs Peak positions can be intentionally modulated to
produce uniform tumor doses under the magnetic fields inside cancer patients
especially for the tumors inside parallel organs and also the volume receiving
proton irradiations was decreased through MMPT.

DISCUSSION:

Theoretically, the concept of bending the radiation beam
inside the patient is very appealing and the advantages based on different
sites and regions can be enormous.

For generating high magnetic field during radiotherapy,
conceptional configurations have been developed but in case of proton therapy
(being positively charged) deflection due to magnetic field outside the
patient’s body can be a problem. For this, use of shielding mechanism for
magnetic field outside the nozzle are being investigated. Two types of
mechanisms have been proposed. Passive shielding where the use of certain
materials with high magnetic permeability are utilized to cut the magnetic
field and active shieldingix
where shielding material is superconducting coils which nulls the deflection
produced by the magnetic field outside the shielding material.

Adjustable magnetic field technology for MMPT by
superconducting coils has already been realized and also high magnetic field
(up to 7 T) producing  devices in medical
setups  also have been designed and
tested.

 Potential
disadvantage for this technique is that the path travelled by the beam will be
increased (curve vs straight path) thus to achieve the proper dose distribution
the energy used to cover the distal part of the tumor may have to been
increased. Also, the region receiving the low dose volume may be increased as
compared to conventional proton beam therapy while completely avoiding the OAR
which may otherwise come in the path of the beam. But there is also the
potential of using beam entry points away from the actual region of the target
volume and then bending it to reach the target. This can provide unlimited
possibilities and probabilities for creating the best possible plan.

Various sites where this technique may be beneficial may
include pancreas, liver, nasopharynx, prostate etc. These are the sites where
the target is surrounded by the organ at risk and their avoidance with
conventional straight proton beams is impossible. Also in case of curved target
region like breast the beam can be bent along the curvature of the chest wall
to reduce the dose to the lung and heart. Similarly, in prostate by choosing an
appropriate entry point can help to spare bladder and rectum. In vertebral
irradiation sparing of spinal cord by bending the beam can be realized and without
it (spinal cord) acting as dose limiting factor.

As the pencil beam scanning is currently the latest
technology utilized to achieve intensity modulated proton therapy, it’s role
combined with magnetic modulated proton therapy can also be investigated. Also,
the feasibility of the magnetic scanning of the pencil beam inside the patient
body and combining with proton bending may further help to reduce the volume
receiving low dose.

CONCLUSION:

This new concept of bending the proton beam can
provide a lot of advantages and thus should be investigated with regard to its
infrastructure and feasibility of operation. Further combining with the
concepts of magnetic scanning of proton beam and MRI guided proton therapy can
provide highly conformal plan with complete sparing of adjacent OA

i D.T.L. Jones; A.N.
Schreuder
,Magnetically scanned proton therapy
beams: rationales and principles, Radiation
Physics and Chemistry, Volume 61, Issue 3-6, p. 615-618.

 

ii Oborn, B. M., Dowdell, S., Metcalfe, P. E., Crozier, S.,
Mohan, R. and Keall, P. J. (2017), Future of medical physics: Real-time
MRI-guided proton therapy. Med. Phys., 44: e77–e90. doi:10.1002/mp.12371

 

iii A
J E Raaijmakers, B W Raaymakers, S van der
Meer and J J W Lagendijk Integrating a MRI scanner with a 6 MV radiotherapy
accelerator: impact of the surface orientation on the entrance and exit dose
due to the transverse magnetic field. Physics
in Medicine & Biology, Volume 52, Number
4

 

iv A
J E Raaijmakers, B W Raaymakers and J J W
Lagendijk Integrating a MRI scanner
with a 6 MV radiotherapy accelerator: dose increase at tissue–air interfaces in
a lateral magnetic field due to returning electrons. Physics
in Medicine & Biology, Volume 50, Number
7

 

vHartman1, C Kontaxis1,
G H Bol1, S J Frank2, J J W Lagendijk1, M van
Vulpen1 and B W Raaymakers1 Dosimetric feasibility
of intensity modulated proton therapy in a transverse magnetic field of 1.5 TJ Physics
in Medicine & Biology, Volume
60, Number
15

 

vi B W Raaymakers, A J E Raaijmakers and J J W Lagendijk Feasibility of MRI
guided proton therapy: magnetic field dose effects .Physics
in Medicine & Biology, Volume
53, Number
20

 

vii . Wolf R, Bortfeld
T. An analytical solution to proton Bragg peak deflection in a magnetic field.
Phys Med Biol. 2012;57:N329–N337.

 

viii Shao, W., Tang, X., Bai, Y., Geng, C., Shu, D., Gong, C.
and Chen, D. (2017), Modulation of lateral positions of Bragg peaks via magnetic
fields inside cancer patients: Toward magnetic field modulated proton therapy.
Med. Phys., 44: 5325–5338. doi:10.1002/mp.12468

 

ix 44. Chen J,
Jiang X. Stress analysis of a 7 T actively shielded superconducting magnet for
animal MRI. IEEE T Appl Supercon. 2012;22:4903104.