Radiation Risk Assessment

8/13/2019 Radiation Risk Assessment

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Radiation Protection Course for Radiation Oncology Residents.

Radiation Risk Assessment

Mara Pinto Monedero

Medical Physicist

Kantonsspital Aarau (KSA)


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Contents

o Introductiono What is Risk Assessment?

o Hazards identificationo Interaction of ionizing radiation with tissues

o Ionizing Radiation Effects

o Dose-response relationo Linear Quadratic Model

o Linear Threshold Model

o Linear No Threshold Model

o Exposure assessmento Radiation exposure in medicine

o

Risk characterization

Radiation Risk Assessment. Radiation Protection Course for Radiation Oncology Residents.


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Introduction

Risk analysis process


Health risk assessment from the nuclear accidentafter the 2011 Great East Japan Earthquake andTsunami. WorldHealth Organozation(WHO).


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What is Risk Assessment?

Risk assessmentpredicts the likelihood of occurrence of adverseevents from a defined exposure based on scientific evidence.


o Hazard Identification

o Dose-Response relationship

o Exposure assessment

o


Type and nature of adverse effects that radiation can cause.

Relationship between radiation exposure andadverse health effects that can cause.

Measure or estimate the intensity, frequency, andduration of human exposure to radiation.

Estimate qualitatively or quantitatively the risk of adversehealth effects under defined exposure conditions.


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Hazard Identification

Ionizing Radiation


Electromagnetic radiation (UV, X-Rays, Gamma Rays)

Particulate radiation (atomic or subatomic particles)

Radiation with sufficient energy to cause ionization in the medium.


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Sources Ionizing Radiation


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1980 USA Population 2006 USA Population

Total Dose (mSv) 2.98 6.24

Natural 80.5% 49.7%

Medical 17.8% 48.1%

Others 1.5% 2.2%

Medical Exposures % of Total Exposures % Medical Exposures

CT Exams 24.0% 50.0%

Nuclear Medicine 12.8% 26.7%

Diagnostic X-Ray 4.8% 10.0%

Interventional Radiology 6.4% 13.3%


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https://www.naz.ch/en/aktuell/tagesmittelwerte.shtml


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Ionizing Radiation Effects


Interactions with tissue


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Interactions with tissue


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o Deterministic (Non-Stochastic) Effects

o Stochastic Effects

Somatic (Cancer).

Hereditary Defects.

Severity increases with dose.

Mechanism involves effects (often cell kill) on many cells.

No threshold.

Dose threshold.

Probability increases with dose but severity is independent of dose.


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Dose-Response Relationship

Dose


o The amount of radiation deposited within the body.

o The amount of energy deposited per unit of mass (Unit: Gy)

o The biological response to an agent (radiation), i.e. a change

in structure, function, morbidity or mortality.

Response

Dose-Response Models

o A mathematical description of the relationship between

dose and response.


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What is consideredlow/high dose?


The National Academy of Sciences in their

Biological Effects of Ionizing Radiation reports

(BEIR), as well as other scientificorganizations that study effects of radiation,

use a definition of low dose to be between

0.1-0.15 Sievert (Sv).

Doses above 1 Sv are considered high

doses.


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What is the Evidence?


o Radiogenic cancer elevated risk is currently only

consistently able to be demonstrated in those groups of

study populations exposed to high-dose radiation (>1 Sv).

o Cancer and other health effects have not been observedconsistently at low doses (


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What is the Evidence?


o Mayor epidemiologicalinvestigations that form thebasis of current cancer dose-response estimates inhuman populations areshown in the table.

o In this studies most individuals

received a large dose.

o Extrapolation (using models)

to low dose level.

Difficult to quantify risks from low

doses directly.


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Low Dose Extrapolation


o Cell culture and animal studies

have provided information

demonstrating tumorigenesis

from high levels of radiation

exposure.

o At lower dose levels bothdeterministic and stochastic

effects are not observed or the

studies have not been

comprehensive enough.

Risk for cancer induction at low dosemust be extrapolated from the high

dose data


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Linear Quadratic Model


o Assumes that the incidence ofinduced cancer at low doselevels increase linearlywithradiation dose up to a certain

level.

o At high dose levels, theincidence increases muchmore rapidly (quadraticrelationship)


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Linear Threshold Model


o Assumes that damage is directlyproportional (linear)to the doseof radiation at all dose levels.

o

There is athresholdbelowwhich too few cells are affectedor cellular repair mechanismcompensate for any damagethat occurs.

o Threshold near 0.01-0.5 Sv


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Linear No-Threshold Model (LNT)


o Assumes that damage is

directly proportional (linear)to

the dose of radiation at all

dose levels.o No Threshold.

This is the most conservative

approach for radiation protection and

this hypothesis is used by advisoryand regulatory agencies .


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Exposure Assessment


Radiation exposure in Radiology


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Exposure Assessment



Ionizing Radiation Diagnotstic Procedures

Diagnostic

Procedure

Effective dose

(mSv)

Equivalence in

thorax X-Ray

Equivalence in natural

radiation background

(Madrid)

X-Ray Extremity < 0,01


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Exposure Assessment



Ionizing Radiation Diagnotstic Procedures

Diagnostic ProcedureEffective dose

(mSv)

Equivalence in

thorax X-Ray

Equivalence in natural

radiation background (Madrid)

Lumbar Column (AP) 1,3 65 8 months

Intravenous Pyelogram

(IVP)2,5 125 15 months

X-Ray Upper GI tract 3 150 43 months

X-Ray Lower GI tract 7 350 3,5 years

Head CT 2,3 115 1 year

Thorax CT 8 400 4 years

Abdomen/Pelvis CT 10 500 5,1 years


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Risk characterisation


Radiation exposure in RadiologyProbability of mortal cancer induction in different diagnostic

procedures (per 1000000 exposures)

Procedure Cases

RX Abdomen AP 25

RX Lumbar Spine AP 36

RX Lateral Lumbar Spine 16

RX Mamography (one proyection) 5

RX Thorax PA 1

RX Thorax Lateral 2

RX Lower GI Tract 236

RX Coronary Angioplasty 1100

RX Ablation 850

RX Thorax-Abdomen-Pelvis CT 690

RX Abdomen CT 430

NM Bone Gammagraphy 300

NM Pulmonary Gammagraphy 330


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Radiation exposure in RadiologyProbability of mortal cancer induction in different diagnostic

procedures (per 1000000 exposures)

The excess of risk is low, considering the natural risk of developing a cancer

(almost one case per four population)

The benefits of a diagnostic procedure always overcome the risk, if the

procedure isjustified.

The risks of NOT performing an exam include missing a diagnosis and/or

initiating treatment too late to improve the medical outcome.

The potential life reduction due to the disease of concern in conjunction

with the latency period for radiation induced cancer needs to be considered

relative to the normal life expectancy of the patient.



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Radiation exposure in RadiologyVirtual Colonoscopy

o Alternative colorectal cancer screening tool

o Concern about potential radiation risks

o Effective dose 8 mSv per screen

o Screening 50-80 every 5 years

8 to 28 radiation-related cancers per 10,000 screened

360 to 520 cancers prevented per 10,000 screened



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Radiation exposure in RadiologyRisk comparison



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Radiation exposure in RadiologyRisk comparison



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Exposure Assessment


Radiation exposure in Radiation Oncology


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Exposure Assessment



o Simulation

Imaging radiation

o CT imaging, 4D-CT

o PET imaging, PET-CT imagingo

IGRT: kV- Imaging, Cone Beam-CT, MV Imaging.o Imaging during radiation therapy (gating, tracking)

Therapeutic radiation

o Radiation therapy

o Intensity Modulated Radiation Therapy (IMRT)


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Exposure Assessment



Dose for 1 exposure

0.1-1 mSv

kV imaging MV imaging

Treatmentwith 30fractions and twosetup fields:

Dose for 1 exposure

0.5-5 mSv

Dose: 6-60 mSv

Treatmentwith 30fractions and two

setup fields:

Dose: 30-300 mSv

IGRT kV imaging vs MV imaging


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Exposure Assessment



Dose for 1 CT:

10-20 mSv

kV Cone Beam CT MV Cone Beam CT

Treatmentwith 30

fractions and one CB-CT:

Dose for 1 CT:

20-50 mSv

Dose: 300-600 mSv

Treatmentwith 30

fractions and one CB-CT:

Dose: 600-1500 mSv

IGRT Cone Beam CT

Attention!: Local doses can be large!

Example: H&N irradiation 3-5 Gy extra parotid dose


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Exposure Assessment


Radiation exposure in Radiation OncologyImaging dose vs Therapeutic Dose

Hlgt RA.Assesment of Therapeutic, Scatter and Imaging Doses in radiation Oncoloy and Implications for

Cancer Risk. Dissertation ETH Zrich.

Scheme CBCT Portal MV Planar kV-kV Planar kV-MV

Scheme CBCT 26/23 8/0 0/0 0/0

Scheme kV-kV 0/0 8/0 26/23 0/0

Scheme kV-MV 0/0 8/0 0/0 26/23

Scheme MV 0/0 20/20 0/0 0/0

Fractionation

3D-CRT 26 x 2.0 Gy = 52Gy

IMRT,VMAT 23 x 2.2 Gy = 60.6 Gy


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Exposure Assessment


Radiation exposure in Radiation OncologyImaging dose vs Therapeutic Dose

Hlgt RA.Assesment of Therapeutic, Scatter and Imaging Doses in radiation Oncology and Implications forCancer Risk. Dissertation ETH Zrich.Local relative increase of dose because of the applied

imaging scheme varies between below 0.5% and 30%.


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o Second cancer: histologically distinct cancer that develops afterthe first cancer.

o Second cancer account for 6%-10% of all cancer diagnoses in USA

o With the new treatment modalities (IMRT, IMAT, proton and heavy

ion therapy), larger number of secondary cancers are expected.

Risk of secondary malignacies after radiotherapy

Increase in long term survivors who are at risk of developing second cancer.

Increase in beam-ON time (IMRT) to deliver the same target dose but

different dose distribution: Low dose to a large volume .

Secondary neutron dose in proton and heavy therapy and in photon therapy

E> 10 MV.


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Risk factors for second cancers

Age

o Increased susceptibility of tissue to mutagenic effect of therapy.

o Higher rate of cell proliferation during early stages of development.

o Longer period of follow up, which allows second cancer withtypically long latencies to emerge.


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Age

Children are 2-3 times more sensitive to the

effects of radiation exposure as compared with a

middle age person, and 5-10 times more sensitive

compared to the elderly.


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Gender

o Female sex is associated with increased risk of second primarycancers.

E.g: increased of secondary breast cancers, increased occurrence of

thyroid cancer in female survivors.

Others

o Certain chemoterapeutic agents.

o Familiar cancer syndromes.

o Gene-environment interactions.

o Lifestyle choices.


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Latent Period of Cancers


o Latent period: time interval between irradiation and occurrence of

malignancy.

The risk and the length of the latent

period depend on the persons age at

the time of exposure.



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Epidemiology of Second Primary Cancers after Radiation Therapy


o Most radiation-associated cancers develop within or at the edge of

the radiation field.


o

o

o sarcomas, lung-, bladder- and rectal cancer in patients receiving

RT for prostate cancer.

o

brain tumors or prophylaxis irradiation.

o sarcomas, skin-, breast- and thyroid cancer after pediatricprimary cancer.


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Observations with Hodgkin-patients


o Cumulative cancer risk for a patient irradiated at 30 years, at the

age of 60.


Males: 18% (Comparison to general population: 7%)

Females: 27% (Comparison to general population: 9%)


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Uncertainties in Estimate ofCancer Risk


o Extrapolation to low dose or low dose rate exposure.

o Population transfer models.

o Age at the time of exposure and projection to lifetime risk.

o Dosimetry uncertainties.

o Missclasification of cancer.


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o BEIR VII -Health Risks from Exposure to Low Levels of IonizingRadiation- US population (2006)

o UNSCEAR- Effects of Ionizing Radiation-multiple populations

(2008)

o

Health Protection Agency- Risk of Solid Cancers followingRadiation Exposure: Estimates for the UK Population -UK (20011)

Risk Assessment Reports


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Take Home Points


o LNT model accepted by advisory and regulatory agencies for

risk assessment in radiation protection.

o LNT might overestimated the incidence of induced cancer at

low doses.

o Evidence is:

o Radiation risk studies are affected by high uncertainties.

o Importance of justification and optimization.

D>1Sv, higher risk of developing radiogenic cancer.

D< 0.1Sv, developing of radiogenic cancer has not been consistently

demonstrated.


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Bibliography


Health risk assessment from the nuclear accident after the 2011 Great East

Japan Earthquake and Tsunami, based on a preliminary dose estimation.

World Health Organization (2013).

Low-dose Extrapolation of Radiation-related Cancer Risk. International

Comission of Radiation Protection (ICRP) publication 99.

Effects of Ionizing Radiation. United Nations Scientific Committee on the

Effects of Atomic Radiation (UNSCEAR) Report to the General Assembly,

2006.

Risk of Solid Cancers following Radiation Exposure: Estimates for the UK

Population. Report of the independent Advisory Group on Ionising

Radiation. Health Protection Agency.

Schneider U. Modeling the Risk of Secondary Malignancies after

Radiotherapy. Genes 2011 (2) 1003:1049.

Hlgt RA.Assesment of Therapeutic, Scatter and Imaging Doses in radiation

Oncoloy and Implications for Cancer Risk. Dissertation ETH Zrich.


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Bibliography


RSNA/AAPM Physics Modules: Estimating Cancer Risk from Imaging

Procedures. Hendde W, OConnor MK.

RSNA/AAPM Physics Modules: Radiaton Dose and Risk. Peck D, Applegate K,

Slovis T.

RSNA/AAPM Physics Modules: Radiaton Effects. Brown K, Parlade A, Dunne

A.

Nickoloff E et al. Radiation Dose Descriptors: BERT, COD, DAP, and other

Strange Creatures. Radiographics 2008 (28): 1439-1450.


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Radiation Risk Assessment