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Abstract
In this work, we address an issue of deeply penetrating stray X-ray contamination of a therapeutic
electron beam. The issue is of particular importance in the Intraoperative Electron Radiation Therapy
(IOERT) - one of the most modern and promising ways to treat cancer. In a typical IOERT treatment, the
irradiation is performed in a regular, nonshielded operating room thus reduction of stray X-ray radiation is of
great importance.
The sources of stray X-ray contamination are located within the beam forming system. This system is
responsible for formation and delivery of a therapeutic beam of uniform spatial dose distribution over entire
target area. The beam is formed by scattering in metallic foils what inevitably leads to considerable
production of stray X-ray radiation.
Until recently, due to limited computing resources and lack of adequate methods, designing of an
electron beam forming system was to a large extend an art of trial and error guided by extremely simplified
physics models and a small set of empirical rules of thumb that have an unknown range of applicability. Most
often this resulted in a much higher than otherwise achievable levels of unwanted beam contamination.
Here we consider the so called Kozlov and Shishov rule [1] for selection of the primary scattering foil.
This foil is one of the major sources of stray X-ray contamination. The Kozlov and Shishov rule dates back to
an empirical observations made in 1970s in a context of a long obsolete device. Using a comprehensive new
method developed in our earlier works [2,3] we put, for the first time, the Kozlov and Shishov rule to scrutiny.
We demonstrate, on an example of a mobile accelerator for IOERT that is currently under development at
NCBJ, that the simple recipe of [1] does not in fact lead to an optimal solution. We present a new approach
capable of finding a solution of beam forming system that truly minimizes therapeutic beam contamination.
[1] A. P. Kozlov, V. A. Shishov, Acta Radiol. 15 (1976) 493–512
[2] P. Adrich. Nucl. Instr. Meth. Phys. Res. A 817 (2016) 93–99
[3] P. Adrich, Nucl. Instr. Meth. Phys. Res. A 817 (2016) 100–108
2
NCBJ and LINACs for science, industry, medicine
prototype 1.3 GHz cavities for Tesla-FEL
at DESY
cavity for LINAC4@CERN
(proton beam for LHC)
mobile industrial radiography
mobile linac for Intraoperative
Electron Radiation Therapy
6 MV external beam RT
stationary dual energy industrial radiography
science industry medicine
3
IOERT
• IOERT = IntraOperative Electron Radiation Therapy
• delivery of high dose of electron radiation directly into exposed tumor bed during oncological
surgery inside an unshielded operation room
• Sparing of healthy tissue
• Elimination or significant shortening of external beam RT following surgical intervention
• Electron beam energy range 4 – 12 MeV
Pictures form presentation of dr Roberto Orecchia
http://www.eurama.info/public/pdf/atti_2011/16_04_2011/003_oreeurama2011_eliot.pdf 4
Why electrons?
5
Electrons (megavoltage)
Relatively short but well defined and easily
controllable range
Large dose at surface
Primary beam, directly ionizing -> high dose rate
(liniacs) ~kGy/min
Compact, lightweight and relatively cheap
accelerators
X-rays (megavoltage)
Deeply penetrating
Little surface dose
Secondary beam, non directly ionizing -> low
dose rate (liniacs) ~Gy/min
Protons
Deeply penetrating
Very well defined range
Little surface dose
electrons X-rays, protons
R. Mohan, A. Mahajan, B. D. Minsky, Clin. Cancer Res. 19 (2013) 6338-6343
Dose due to stray X-ray radiation
6
pe
rcen
tage
de
pth
do
se
[%
]
Depth in water [cm]
XS
R10 R10 + 10 cm
Shielding against stray X-ray radiation in IOERT
Stray X-ray radiation:
• danger of exposure of personnel and patients
• workload limitation (3-4 patients a week)
7
Shielding against stray X-ray radiation in IOERT
Stray X-ray radiation:
• danger of exposure of personnel and patients
• workload limitation (3-4 patients a week)
Common radioprotection measures:
• beamstopper
• lead curtains
• mobile shields
Source: http://www.soiort.com/en/solutions-eng/liac-10-mev-eng/ 8
Shielding against stray X-ray radiation in IOERT
Stray X-ray radiation:
• danger of exposure of personnel and patients
• workload limitation (3-4 patients a week)
Common radioprotection measures:
• beamstopper
• lead curtains
• mobile shields
… and their drawbacks:
• restricted mobility and maneuverability of the
accelerator
• mechanical collisions with other equipment,
e.g. operation table
Source: http://www.soiort.com/en/solutions-eng/liac-10-mev-eng/ 9 Source: http://www.isiort.org/fileadmin/templates/pdf/p3/hensley.pdf
Shielding against stray X-ray radiation in IOERT
Stray X-ray radiation:
• danger of exposure of personnel and patients
• workload limitation (3-4 patients a week)
Common radioprotection measures:
• beamstopper
• lead curtains
• mobile shields
… and their drawbacks:
• restricted mobility and maneuverability of the
accelerator
• mechanical collisions with other equipment,
e.g. operation table
Source: http://www.soiort.com/en/solutions-eng/liac-10-mev-eng/ 10 Source: http://www.isiort.org/fileadmin/templates/pdf/p3/hensley.pdf
The less X-rays the better!
Dual foil system - principle of operation
12
Foil material
13
Scattering power 𝑇
𝜌~
𝑍2
𝐴
Collisional stopping power 𝑆𝑐𝑜𝑙
𝜌~
𝑍
𝐴
Radiative stopping power 𝑆𝑟𝑎𝑑
𝜌~
𝑍2
𝐴
The higher the atomic number Z the better
𝑺𝒄𝒐𝒍
𝑻~
𝟏
𝒁
𝑺𝒓𝒂𝒅
𝑻~𝑪𝒐𝒏𝒔𝒕
Literature
A. Brahme, The optimal choice of scattering foils for electron therapy, Technical Report TRITA-EPP-7217, Royal Institute of
Technology, Stockholm, Sweden, 1972
Kozlov and Shishov method for designing primary foil
Scattering foil
𝜙(𝑟 = 0)
𝜙(𝑟𝑏)
𝑘 =𝜙(𝑟𝑏)
𝜙(𝑟 = 0)
Primary scattering foil has
acceptable thickness if
0.5 < k < 0.7
k = 0.6 proposed as optimal
BUT
is it really optimal in IOERT?
A. P. Kozlov, V. A. Shishov, Forming of Electron Beams from A Betatron by Foil Scatterers, Acta Radiol. Ther.
Phys. Biol. 15 (1976) 493
14
Is Kozlov and Shishov method optimal?
15
Verification of Kozlov and Shishov method
17
To make fair comparisons we require that each system delivers
therapeutic beam of minimal possible X-ray contamination but
of otherwise similar dosimetric properties, i.e.:
(1) flatness ≤ 5%
(2) therapeutic range ≥ 𝑅90 𝑚𝑖𝑛
(3) minimal dose due to X-rays
Flatness, therapeutic range
𝑓 =𝐷𝑚𝑎𝑥
𝐷𝑚𝑖𝑛− 1 ∙ 100%
Irradiation field
𝐷𝑚𝑎𝑥
𝐷𝑚𝑖𝑛
Flatness:
Therapeutic beam: f < 10%
pe
rcen
tage
de
pth
do
se
[%
]
Depth in water [cm]
R90
Therapeutic range = R90
Verification of Kozlov and Shishov method
19
For a given primary foil, fulfilling of conditions
(1) flatness ≤ 5%
(2) therapeutic range ≥ 𝑅90 𝑚𝑖𝑛
(3) minimal dose due to X-rays
depends on the secondary foil…
How to find an optimal secondary foil for each of the
considered primary foils?
Verification of Kozlov and Shishov method
20
How to find an optimal secondary foil?
1. Calculate functions representing behavior of real
system in respect to parameters H and R of the
secondary foil:
𝑓 𝐻, 𝑅 – flatness of off-axis dose profile
𝑅90 𝐻, 𝑅 – therapeutic range
𝑋𝑆 𝐻, 𝑅 – stray X-ray contamination
2. Select an optimal secondary foil (H,R) by requiring
𝑓 𝐻, 𝑅 ≤ 5%
𝑅90(𝐻, 𝑅) ≥ 𝑅90 𝑚𝑖𝑛
𝑋𝑆 𝐻, 𝑅 is minimal
= 30 mm for 10 MeV, and 37 mm for 12 MeV beam 𝑅90 𝑚𝑖𝑛
Calculation of 𝑓 𝐻, 𝑅 , 𝑅90 𝐻, 𝑅 , 𝑋𝑆 𝐻, 𝑅
• Geant 4.10.4
• Enhanced precision electromagnetic
physics („option 3”)
• Bremsstrahlung splitting (x20)
•
0.5 mil. hours
3000 CPU cores ───────── ≈ 7 days
Cluster at Świerk Computing Center
Results – minimization of stray X-ray contamination
May seem that Kozlov and Shishov method (k = 0.6) is quite good…
… as long as one is looking for and able to find the minimum of XS(H,R)
~10% reduction of dose due to X-rays for k = 0.4 compared to k = 0.6
24
Results – minimization of dose uniformity (flatness)
Systems optimized for X-ray contamination Systems optimized for dose uniformity
• Thinner foil results in 20% reduction of dose due to X-rays
• Also better therapeutic range in systems optimized for X-ray contamination
25
Results – comparison with existing system
E [MeV] Dose due to stray X-ray radiation [%] Reduction
Minimum found here Intraline prototype
10 0.18 0.31 42%
12 0.26 0.39 33%
26
Conclusions
• Kozlov and Shishov method is not strictly correct in case of beam forming
system of the type used in modern IOERT.
• New design method is capable of finding a solution that ensures
minimization of stray X-ray contamination without compromising dose
uniformity or therapeutic range.
• 30 to 40% reduction in dose due to stray X-ray contamination is possible by
means of relatively simple redesign of scattering foils. Benefits include:
• reduction of dose to healthy tissues,
• increase in allowable patient workload,
• reduction in size and weight of beam stopper and other shielding
devices -> simplification of IOERT delivery.
27
Development of novel IOERT accelerator at NCBJ
• Energy range 4-12 MeV
• Dose rate 10 Gy/min
• Applicator diameter 3-12 cm
• Battery powered for transport
IntraLine-IOERT prototype
30
www.ncbj.gov.pl/sites/default/files/folder-akcelerator_calosc_ang_druk_02.pdf