Biological Effects of Non-Ionizing Radiation: Measurement Environments and Dosimetry

Marcus Stiemer, Lars-Ole Fichte, Robert Hollan, Sebastian Böhmelt, Niels Kielian, Marco Rozgic, Michael Dudzinski

Theory of Electrical Engineering Helmut Schmidt University / University of the Federal Armed Forces Hamburg

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This presentation contains results jointly obtained with

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Bundeswehr Institute of Radiobiology (Dr. Andreas Lamkowski, Dr. Michael Abend, Dr. Matthias Port)

Central Institute of the Bundeswehr Medical Service Kiel, Berlin branch (Dr. Kerstin Grutza)

Bundeswehr Research Institute for Protective Technologies and CBRN Protection (WIS) in Munster (Dr. Matthias Kreidlow, Dr. Stefan Potthast, Dr. Martin Schaarschmidt, Dr. Sebastian Lange, Dr. Frank Sabath)

Federal Standards Laboratory (PTB) (Dr. Thomas Kleine Ostmann, Reiner Pape)

Electromagnetic exposure of biological tissue

• Electromagnetic fields are used for more than 100 years to control cancer and for coagulation of hemorrhages. These applications are based on the thermal effect of the electromagnetic fields on biological tissue

• Around 1950, for the first time hardware was available to convey sufficiently high power at frequencies within today’s wireless communication bands

• At the same time the question of long-term non thermal effects in a domain where temperature effects can be excluded arose due to the regular use of radar systems

• Many comprehensive scientific studies have been carried out in the 50th of the last century with different results, leading, e.g., to different standards in different countries or even to inconsistent thresholds in some cases

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Electromagnetic exposure of biological tissue

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• At the beginning of the 70th, a specific absorption rate of 4 W/kg has been identified as harmless for mammals, and with safety factors of 10 and 50 the values 0.4 W/kg and 0.08 W/kg for people who are occupationally exposed to electromagnetic fields and who are exposed in an uncontrolled environment have been derived

• As a consequence of the demands of wireless communication, RF radiation is

becoming ubiquitous. Hence, people living in developed countries are permanently exposed to electromagnetic fields

• This change in our living conditions together with modern techniques of

biomedical analysis justifies renewed studies of the interaction of non ionizing electromagnetic radiation and biological systems

Clarification of the existence of effects

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• To identify basic mechanisms, model systems can efficiently be used (e.g., suspensions of human cells)

• If there is any long-term effect of non-ionizing radiation, it must

become visible by changes in the inter-cellular signal paths DNA -> transcriptome -> protein synthesis

-> changes of cellular functions

• If such effects are non-thermal, there should be a difference between the cellular reaction with respect to heating and with respect to the electromagnetic field

Agenda

• On suitable notions of an electromagnetic dose – Effects of RF radiation (manly 800 – 3000 GHz) – Parameters of RF radiation

• Technical devices to expose biological systems (tissue or

model systems) to RF radiation with defined parameters – Reproducibility of electromagnetic parameters – Exclusion of non-electromagnetic influences

• Dosimetry for RF exposure systems

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Electromagnetic fields

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Waves are reflected and scattered by matter, simulation by N. Meier, 2012

Wave vector k

Electrical field E

Magnetic field H

Name Frequency Wave length 𝞬-ray X-ray > 30 PHz < 10 nm extreme UV near UV visible 400 – 789 THz 380 – 750 nm IR terahertz microwave RF (high) 0.3 – 3 GHz 10 cm - 1 m RF (middle) 0.3 – 300 MHz 1 m – 1 km RF (low) < 300 kHz > 1 km

Interaction with biological cells

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damage of of DNA? Non ionizing radiation:

Ionizing radiation:

damage of DNA (e.g. single strand or double strand breaks)

builds radicals in cell by ionization necrosis

no severe consequences

significantly altered

cellular repair mechanisms

For low radiation: impact correlated with absorbed energy

For low radiation: thermal impact correlated with absorbed power other impacting mechanism maybe not

polarizes molecules (and structures)

impact on cell metabolism for certain frequencies

increases temperature proteins may degenerate, chemical equilibria change

catalytic effects?

Ionizing radiation • Activity is measured in Becquerel [decays per s], and radiation is conveyed as

alpha- beta- or gamma-radiation

• The energy absorbed by biological tissue is measured in Gray [energy per mass].

• The type of radiation (alpha-, beta- or gamma-) and its energy can be accounted for by impact factors. This yields to the equivalent dose measured in Sievert

• The effective dose is obtained by taking the equivalent dose for all organs, multiplication with organ depending sensitivity factors, and computing the sum over all affected organs (also measured in Sievert)

• For low radiation a risk assessment based on an effective dose is possible

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Thermal dose of RF radiation

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• So far, only thermal effects of non-ionizing radiation (denaturation of proteins) are considered as a risk for human health

• Thermal risks depend directly on temperature

• The temperature level obtained during radiation results from the balance of absorbed power and cooling power

• The absorbed power per unit mass (specific

absorption rate, SAR) is employed to measure the dose of electromagnetic radiation

• Same SAR-value may lead to different risks for different type of tissue (e.g., eyes)

Temperature curve during heating by radiation 0 °C

0 s 900 s

0.5 °C

A notion of doses for RF fields beyond the SAR?

• If molecule resonances or other phenomena evoked by RF radiation can lead to severe alteration of cells, an energy based notion of a dose may be more adequate than a power based for low radiation

• The influence of the type of radiation (continuous wave, modulated, pulsed

etc.) has still to be identified • Scattering of waves at the radiated tissue need probably be considered • Known non-thermal effects (not relevant above 100 MHz): activation of

nerves, electroporation (cell membrane becomes penetrable due to displacement of lipid chains), cell polarization and change of the transmembrane potential

• Identified effects at higher frequencies (THz regime): polarization of molecules inside the cell, slight disturbances of spindle apparatus

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Deterministic devices

• Transversal electromagnetic (TEM) cells provide to a certain degree homogeneous electromagnetic fields with a certain defined direction

• Many experiments are required, relocation of the test sample takes much time

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Image taken from Popovic et al., IEEE Trans. Biomed. Eng. 1998

• Space is limited if high frequencies

are aimed at

• Kleine Ostmann et al: TEM-cell with conditioning for small samples of cell solutions, 2014

• Lamkowski et al. worked with an open TEM cell, 2016

Electromagnetic reverberation chambers

• Complete shielding of fields by metal walls -> reproducibility • Standing waves result from multiple reflections

at the walls • Geometry is varied by a stirrer • Under favorable conditions, statistically well

distributed samples are obtained

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Hollan et al.: Proc. Kleinheubacher Tagung 2016 (submitted)

Barbay et al.: Proc. EMC Europe 2017 (submitted)

Simulated example • 800 MHz,

• 80 stirrer positions (step: 4,5°)

• conductivity of walls 0,09 MS/m

• conductivity of the antenna 58,6 MS/m

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Inès Barbary Michael Hagel

Guaranteeing stable environmental conditions

• The ERC is placed in an incubator to guarantee a stable environment for the biological samples

• Constructive problems to solve: RF-proof ventilation, arrangement of antennas and samples, guarantee of good field distribution in a small volume

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Hollan et al.: Proc. Kleinheubacher Tagung 2016 (submitted)

Field distribution

• The absolute value of all field components must be Rayleigh distributed

• Rayleigh distribution corresponds to Weibull distribution with shape parameter 2

• In this way, a least usable frequency (LUF) can be identified

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Barbay et al.: Proc. EMC Europe 2017 (submitted)

Perform measurements for many stirrer positions and take the sample of the electrical fields arising in certain positions

Problems with small ERCs

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• The LUF of small ERCs is sometimes higher than the frequencies of interest

• In these cases, particular constructive arrangements may help

• Schlie et al.: Knowledge base for the influence of constructive means on the field quality

• Schiffner et al.: ERICA – a software for the statistical analysis of ERC data written in the statistical programming language R

• Barbary et al.: Simulation of ERC with the software PROTHEUS

Hollan et al.: Proc. Kleinheubacher Tagung 2016 (submitted)

SAR dosimetry

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Differential equation for the temperature T(t) of a sample heated by the electromagnetic field with specific heat absorption rate fSAR, specific heat capacity c, mass m and heat transfer coefficient 𝛼 (approximately homogeneous temperature and electrical fields are assumed for the sample)

Solution

Renaming of variables

Specific heat capacity of blood (Blake et al., British Journal of Anesthesia 2000)

Temperature curve during heating by radiation

0 °C

0 s 900 s

0.5 °C

Identification of 𝜸 𝐚𝐧𝐝 𝒘 𝐛𝐲 𝐚 𝐥𝐞𝐚𝐬𝐭 𝐬𝐪𝐚𝐫𝐞𝐬 𝐟𝐢𝐭

SAR dosimetry

• Temperature measurement can be done with an IR camera (e.g., Lamkowski et al. 2016), which also allows for a measurement of the spatial distribution of the temperature or by a fibre-optic device

• Mathematical models for heat transport (e.g., diffusion) or non homogeneous heating (e.g., due to the skin effect) may be incorporated to obtain more precise results

• Other authors (Lalléchère et al. 2010 or Kleine Ostmann et al. 2014) work with a numerical simulation to obtain SAR values

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Summary and outlook

• Identification of non-thermal effects of RF radiation is an ongoing process

• SAR can be controlled by temperature measurement and suitable

mathematical models • For low SAR, a further notion of dose might be important

• Monitoring of the way of cellular signal propagation is an adequate

tool for the identification of non thermal effects

• ERCs are a suitable tool for field exposure experiments. They are cheap, close to reality, portable, and due to their efficiency suited for large measurement campaigns

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