MOD 6 - Late Effects of Radiation

late effects

• stochastic, meaning the effects are increased with dose but the severity isn’t

• occurs with low doses exposures over a prolonged time (whole/partial body)

opposite for late effects (type)

early effects which are deterministic (dose increase will increase chance of occurrence as well as severity)

break down of late effects

• genetic

• somatic

  • stochastic

    • carcinogenesis

    • teterogenesis

  • late tissue rxn / deterministic

    • fibrosis

    • sterility/fertility

    • cataractogenesis

carcinogenesis and late effects

• almost negligible to non-existent as there is no evidence of increased risk

• indistinguishable from normally occurring cancers

two main non-threshold risk models

• non-threshold linear model

• non-threshold linear quadratic model

Linear Non-Threshold Dose-Response Model

suggests that no dose should be completely safe and appropriate consideration of the risk vs benefit of the exposure needs to justify even small doses

Linear Non-Threshold Dose Model appropriation

• suits high-dose whole body population (not individuals) exposure response information satisfactorily

• exaggerates the actual risk at low doses typical to medical imaging and partial body exposures

Linear Threshold Dose Response Model

proposes the existence of a threshold exposure below which there is no risk of a response

consideration for Linear Threshold Dose Response Model

• an exact threshold value has not been established due to lack of sufficient life time data

• not accepted practice to disregard the possibility of a potential small dose stochastic effect

Linear Quadratic Dose-Risk Model

• what

• best application

• rate of change in response is different at different doses unlike the LNT where damage/risk was proportional to dose

• best applies for risk or leukemia and breast cancer

Radiation Hormesis Dose-Risk Models

suggests a beneficial effect of low-dose exposure creating up regulation in cellular activity, but still being investigated

• populations with known low dose exposures to radiation (5-50 mSv) have reduced cancer death rates

purpose of non-threshold dose risk models

to better estimate the potential risk, due to the stochastic nature of low dose late effects

why are late effects hard to differentiate

• since low dose exposures manifest as late effects months - years

• not uniquely identifiable as ARS

where were late effect data extrapolated from

lower dose exposures in populations resting in peripheral regions of nuclear events

medical radiation exposures are

• low dose

• partial body

• low LET (sparsely ionizing)

ALARA

As Low As Reasonably Achievable

• promotes minimizing exposure to meet the diagnostic task rather than striving for high IQ

effect of cumulative low dose exposure

while evidence suggests that infrequent low dose has negligible cumulative effect compared to a single equivalent higher dose; because of ALARA and risk vs. benefit thinking, we limit the cumulative exposure of individuals and populations

eg. stochastic late effects

• leukemia

• cancers

eg. late tissue reactions

• cataractogenesis

• organ atrophy

• fibrosis

• impaired fertility

• sterility

late tissue reactions are

deterministic, meaning the chance of occurrence and severity increases with dose

radio sensitivity of the eyes is dependent on what

with age

• higher age = less radiosensitivity

leukemia’s latent / at risk period

• between 2-7 years

• up to 20 years

influence of radiation source and damage

radiation source effects the body differently even when the dose is the same

dose response models

risk models that correlate the early and late effects

Late Effects of radiation on Fetus

• inhibited growth

• intellectual disability

• microcephaly

• genital deformities

• sensory organ damage

birth abnormalities and medical imaging tests

rate is not increased with medical imaging tests, but abnormality may be more pronounced

Vancouver annual background equivalent

1.3 mSv

Genetic Effects

effects that are manifested in future generations, this effect has been proven on mice and fruit flies, however, it is still inconclusive to humans

Why is genetic effects still in consideration

because radiation exposure damage causes recessive genetic mutations (the mutation will only be passed down if the parents have the exact same recessive mutation)

genetic effects are interrelated with

the exposure to gonads rather than fetal exposure, which will alter the DNA or chromosomes of sperm/ova germ cells

why is it hard to differentiate genetic effects caused by radiation

due to other natural mutation causing factors such as viruses, chemicals and medications

most concerned late effect

cancer

late effects of low dose radiation have

limited to no incidence compared with non-exposed populations

chances of late effects manifesting are further influenced by

genetic, environmental and lifestyle factors

who has the highest risk of late effects

pediatrics and fetus

how much radiation will cause temporary male sterility

>2Gy

how much radiation will cause infertility of the ovaries

6Gy