How to minimize stray X-ray contamination

of a therapeutic electron beam

Przemysław Adrich

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

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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!

Major sources of stray X-ray radiation

Dual foil system - principle of operation

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Foil material

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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

16

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

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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

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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

10 MeV beam

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12 MeV beam

23

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

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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

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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%

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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.

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www.ncbj.gov.pl

Thank you for your attention!

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

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www.ncbj.gov.pl/sites/default/files/folder-akcelerator_calosc_ang_druk_02.pdf

IntraLine-IOERT prototype accelerator

Very large working space

access to any tumor location without

necessity of moving the operation table

Protected with 3 patents

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