Health Physics - Fisica medica

Where n is simply viewed as a measure of producing biological damage than low LET. However,

shoulder width, that is related to the RBE increases with increasing LET up to a certain

radiosensitivity of the cell. ~100 /

value: the max of RBE occurs when

. LET values greater than this is said to produce

“overkill”: the ionization density is so high that much

more energy is deposited that needed to produce the

effect, so it’s wasted.

Deterministic somatic dmg

The key mechanism for biological damage is max at •

LET = 100 Kev/µm: for this value the spacing of : deterministic somatic damage are those

ionizing events equals the diameter of a DNA double for which there is a threshold dose and

helix. Greater LET energy may have closer spacing of whose severity (above the threshold

ionizations, but such additional path doesn’t yield increases with the dose).

additional double strand breaks. Properties:

→Appear only when a threshold dose,

characteristic for each effect, is exceeded.

→the value of the threshold dose is also a

function of the temporal distribution of the

dose.

→the latency period id usually short.

Biological effect on humans →The severity of the clinical manifestation

increases with the increasing of the dose.

→an identical dose will lead to predictable

: the probability of a certain damage is zero for effect.

small doses, but above a certain threshold rapidly →immediate and reproducible effects are

increases to 1. Above the threshold, the severity of the only observed above a certain threshold, this

damage increases with the dose. This type of effect is doesn’t mean that below the threshold we

called deterministic. don’t have any damage, but they don’t have

: the result is very different if the cell is not killed clinical manifestation.

but simply modified. The reproduction of a modified →on the whole-body level (the effective

cell can give rise, after a period of latency, to a dose), the threshold is 500 mSv.

neoplasia. The probability of induction of neoplasia

following the irradiation generally increases with Damages:

increasing the dose, probably without a threshold, o Radiologist skin effect: the lesion is

roughly proportional to the dose. This type of effect is characterized by dry and thin skin.

called stochastic (because it’s randomly distributed in o Blistering lesion surrounded with

the population of the cell). large inflammatory halo.

o Epilation following the high-dose CT

We can have 2 types of effects on human, after the perfusion scan, used to show which

irradiation of ionizing radiation. areas of the brain and provides

detailed information about delivery

of the blood.

o The acute radiation syndrome: when

the whole body or a large part of it is

Somatic damage subjected to an high radiation dose,

there are a series of characteristic

: the somatic damage occurs in the exposed clinical responses known as the

individual and can be early or delayed. The delayed Acute Radiation Syndrome. It can be

damage can be: divided into 3 phases: initial or

• prodromal phase, latent phase, and

stochastic, then probabilistic and random, it the manifest illness phase. The type

can happen after years with a probability and the time and the severity of the

which depends on the dose. No threshold symptoms are dose dependant.

dose. →

• The doses in all body: 3-5 Gy

deterministic, therefore not random. it occurs damage to bone marrow (30-60

relatively quickly, above a certain threshold →

days), 5-15 Gy dagame to lungs

dose. →

(10-20 days); > 15 Gy damage to

nervous system (1-5 days).

Stochastic somatic dmg

• NB: the absence of epidemiological evidence

: stochastic somatic damage includes at low doses may be related to the possible

leukaemia and solid tumours. In this inexistence of radio-induced effects, or to

pathology only the probability of appearance “masking” of the effects, which, although

is a function of the dose and NOT the severity present, are not evident because they are

and there is no threshold dose. included in the statistical fluctuations of the

natural incident of the cancer.

Properties:

→ they don’t require the exceeding of a The conventional approach for

threshold dose. radioprotection is based on the the ICRP’s

→are probabilistic and randomly distributed linear no threshold model, which implies the

in the exposed population. ionizing radiation is always harmful no matter

→ they are demonstrated by radiobiological how small the dose and it’s used by

experimentation and epidemiological extrapolating the high dose data at low

evidence. doses.

→the frequency of appearance is greater if

the doses are high.

→occurs after years = latency period (2 yrs Stochastic genetic dmg

•

for leukaemia and 5 yrs for solid tumours).

→show They can be observed in the progeny of

no gradual manifestation with the irradiated people and the most important

dose received.

→are study was on the descendants of survivors of

indistinguishable from tumours →

Hiroshima No statistically significant

induced by carcinogens. difference appeared between the two groups

as regards both the psychophysical

The elaboration od the dose-effect curve has development and malformations of genetic

occur over the years based on two main origin. However, experimental studies

sources of info: indicate that hereditary damage can occur.

1. Data obtained from experimentation The genetic risk Is calculated by extrapolation

on animals; from the experiments on lab animals. It’s

2. Data obtained from the observation clear that for doses associated with

of irradiated humans in two groups:

▪ occupational and medical exposure, the risks

Groups of people exposed to high

and acute doses. are small compared with the spontaneous

▪ Groups of people exposed to small incident of genetic anomalies. The risk

and chronic doses. coefficient is estimated about 5 % per Sv.

We have obtained a curve for doses higher An acute exposure of 100 people to 100 mSv

than 0.5 S, for lower doses many curves are would add approximately 1 additional cancer

proposed. It’s quite difficult because we are case to 41 normally expected, but there is no

taking into consideration cancer that aren’t way of identifying which person has the

present only in irradiated population: in fact, additional cancer cause by the irradiation.

for small doses the eventual increase of

cancer incident is not detected with

confidence because, even if it’s present, it’s

masked by statistical fluctuation.

If we look at the “excess of solid cancer” wrt

to dose we can see that in the low dose

range, the error bars extend below the 0

→

error this could possibly indicate that low

doses have zero risk or even less than zero

risk, so they can have a beneficial impact on

health (it can reduce the hormesis).

In utero irradiation Organ dose

• • : it’s defined as:

The embryo and the fetus are really sensitive

= []

to the ionizing radiation, and this vary from

the stage of development. Where ε is the total energy given to a tissue

and m is the mass.

The gestation period can be divided into 3

phases and each stage is different for

radiation response: Equivalent dose

•

→Preimplantation stage: before the Def: the equivalent dose is proportional to

implantation of the embryo the effect of the the biological effect that can be expected

radiation is “all-or-nothing” – so they can from radiation absorbed dose.

cause the embryo death or nothing.

→Organogenesis: embryonic malformation

→

occur more frequently during this period = ∑ []

,

high radiosensitivity.

→Fetal growth phase: the frequency and the Where D is the absorbed dose averaged over

severity of the malformation decrease, while tissue and radiation type.

the risk of a defective development of central

nervous system is significant. Effective dose

•

In the period between the third week from : it takes into account the fact that

the conception and the end of gestation it’s different tissue have different response to

likely that exposure can cause stochastic radiation.

effect like the increasing of the probability of = ∑ []

leukaemia and tumours.

E is the sum over each irradiated body part

of the product of dose equivalent and the

tissue weighting factor, which represent the

contribution of that organ to the total

“detriment”.

main dosimetric quantities The health detriment means an estimate of

the risk of reduction in length and quality of

Exposure life occurring in a population following the

• exposure.

: it describes the ability of radiation to

ionize the air.

= [ ] The probability of biological damage depends not only

on the absorbed dose but also on the type and energy

Where dQ=absolute value of the total charge →

of radiation the variation of the effect for various

of the ions of single sign when all electrons radiation sources is now expressed as radiation

released by photons in the volume of mass weighting factor (w ), it was called in the past quality

dm are completely stopped in air R

factor. High LET radiation corresponds to high w and

Absorbed dose leads to higher biological effects.

• : it’s useful to define an average absorbed

dose for tissue or organ. It’s defined

̅

=

Where dε = the average energy given by

ionizing radiation to an element of volume

with mass dm (energy absorbed per unit

mass)

the background radiation →Small quantities of radioactive substances

the environmental radiation comes from the sky, the enter in our body via food. The main

earth and the air and can be defined as natural or 40 14

radioisotopes present are K and C. Food

artificial. rich in potassium contain a small fraction of

K40, which has an half life of 1.25 billions of

years. About 5000 nuclei of K-40 decay per

second in the human body.

Natural background Banana are rich in Potassium and it’s created

the Banana equivalent dose which is about

0.1 µSv. However, the dose isn’t cumulative,

Primordial radionuclide

• because the amount of potassium in human

body is nearly constant due to homeostasis

: the radioactive material which are and any excess is quicky compensated.

present on the earth since its formation are

called primordial radionuclides and they have Cosmic rays

half-life comparable with the age of the •

earth. Cos