Radiobiology – Wikipedia

Radiobiology (also known as radiation biology, and uncommonly as actinobiology) is a field of clinical and basic medical sciences that involves the study of the action of ionizing radiation on living things, especially health effects of radiation. Ionizing radiation is generally harmful and potentially lethal to living things but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis. Its most common impact is the induction of cancer with a latent period of years or decades after exposure. High doses can cause visually dramatic radiation burns, and/or rapid fatality through acute radiation syndrome. Controlled doses are used for medical imaging and radiotherapy.

The following table shows radiation quantities in SI and non-SI units.

Ionizing radiation related quantities
Quantity	Unit	Symbol	Derivation	Year	SI equivalence
Activity (A)	becquerel	Bq	s−1	1974	SI unit
curie	Ci	3.7 × 1010 s−1	1953	3.7×1010 Bq
rutherford	Rd	106 s−1	1946	1,000,000 Bq
Exposure (X)	coulomb per kilogram	C/kg	C⋅kg−1 of air	1974	SI unit
röntgen	R	esu / 0.001293 g of air	1928	2.58 × 10−4 C/kg
Absorbed dose (D)	gray	Gy	J⋅kg−1	1974	SI unit
erg per gram	erg/g	erg⋅g−1	1950	1.0 × 10−4 Gy
rad	rad	100 erg⋅g−1	1953	0.010 Gy
Equivalent dose (H)	sievert	Sv	J⋅kg−1 × WR	1977	SI unit
röntgen equivalent man	rem	100 erg⋅g−1 x WR	1971	0.010 Sv
Effective dose (E)	sievert	Sv	J⋅kg−1 × WR × WT	1977	SI unit
röntgen equivalent man	rem	100 erg⋅g−1 × WR × WT	1971	0.010 Sv

Gray (unit) – Wikipedia

The US National Academy of Sciences Biological Effects of Ionizing Radiation Committee “has concluded that there is no compelling evidence to indicate a dose threshold below which the risk of tumor induction is zero”

Phase	Symptom	Whole-body absorbed dose (Gy)
1–2 Gy	2–6 Gy	6–8 Gy	8–30 Gy	> 30 Gy
Immediate	Nausea and vomiting	5–50%	50–100%	75–100%	90–100%	100%
Time of onset	2–6 h	1–2 h	10–60 min	< 10 min	Minutes
Duration	< 24 h	24–48 h	< 48 h	< 48 h	N/A (patients die in < 48 h)
Diarrhea	None	None to mild (< 10%)	Heavy (> 10%)	Heavy (> 95%)	Heavy (100%)
Time of onset	—	3–8 h	1–3 h	< 1 h	< 1 h
Headache	Slight	Mild to moderate (50%)	Moderate (80%)	Severe (80–90%)	Severe (100%)
Time of onset	—	4–24 h	3–4 h	1–2 h	< 1 h
Fever	None	Moderate increase (10–100%)	Moderate to severe (100%)	Severe (100%)	Severe (100%)
Time of onset	—	1–3 h	< 1 h	< 1 h	< 1 h
CNS function	No impairment	Cognitive impairment 6–20 h	Cognitive impairment > 24 h	Rapid incapacitation	Seizures, tremor, ataxia, lethargy
Latent period		28–31 days	7–28 days	< 7 days	None	None
Illness		Mild to moderate Leukopenia Fatigue Weakness	Moderate to severe Leukopenia Purpura Hemorrhage Infections Alopecia after 3 Gy	Severe leukopenia High fever Diarrhea Vomiting Dizziness and disorientation Hypotension Electrolyte disturbance	Nausea Vomiting Severe diarrhea High fever Electrolyte disturbance Shock	N/A (patients die in < 48h)
Mortality	Without care	0–5%	5–95%	95–100%	100%	100%
With care	0–5%	5–50%	50–100%	99–100%	100%
Death	6–8 weeks	4–6 weeks	2–4 weeks	2 days – 2 weeks	1–2 days

Nuclear Regulatory Commission (nrc.gov)

0751 – H122 – Basic Health Physics – 12 – Dosimetric Quantities and Units. (nrc.gov)

The above Nuclear Regulatory Commission link would explain in detail the differences between Absorbed Dose [ gray, Gy ], and Dose Equivalent [ sievert, Sv ].

Generally speaking, we can detect ionizing radiation using Dose Rate which uses the S.I. unit of μSv/hr as a display for the ionizing radiation. Most of the time, it would be gamma rays, X-rays, through the upper energy range of the Ultraviolet electromagnetic waves that can be detected as the ionizing radiation.

Whereas, for gray (Gy), it is used as a calculated Absorbed Dose, with the known variables of the Body Tissues, and also the source of ionizing radiation. It is then reconverted back to Sv to assess the gravity of the Absorbed Dose, in Equivalent Dose (Sv).

[Formal Citations from Genicot, Jean Louis. (2014). Re: What is the difference between Sievert and Gray? A practical question concerning the SI units for ionizing radiation.. Retrieved from: https://www.researchgate.net/post/What_is_the_difference_between_Sievert_and_Gray_A_practical_question_concerning_the_SI_units_for_ionizing_radiation/5416e1a0d2fd64e71e8b45b0/citation/download. ]

Citing from Dr. Jean Louis M Genicot’s discussion reply on the differences of Sievert and Gray.

Jean Louis M Genicot

Belgian Nuclear Research Centre

The gray (Gy) is the only actual physical dose ! 1 Gy = 1J/Kg

But the effect of the radiation dose on the body changes with the radiation type. Beta and Gamma are quite equivalent but Alpha is much more dangerous.

So we use the equivalent dose to consider the danger of different radiations.

For instance, 1 Gy (1 J/kg) of Alpha is 20 times more dangerous than 1 Gy of gamma.

So 1 J/kg (physical unit) of Gamma absorbed dose corresponds to 1 Sv of equivalent dose

and

  1 J/kg (physical unit) of Alpha absorbed dose corresponds to 20 Sv of equivalent dose!

Now the different tissues of the body are differently sensitive to radiations

So we use the effective dose to consider the effect of a radiation on organs.

The ovaries are 20 times more sensitive than the skin.

For calculation:

Absorbed dose:  D (Gy)

Equivalent dose : H : Wr . D (Sv)

Effective dose:   E = Wr . Wt . D (Sv)

For simplicity, the sum of all Wt = 1

So  1 Gy of Gamma on the skin (Wr = 1, Wt = 0.01) gives 1 Sv of equivalent dose and 0.01 Sv of Effective dose.

But 1 Gy of alpha on the ovaries (Wr = 20, Wt = 0.20) gives 4 Sv of effective dose,  400 times more risky than gamma on the skin (the previous case).

There are still a lot of confusion in the use of these units because, at high doses, all radiation are deadly so the Sv is only valuable for low doses but we find publications in radiotherapy mentioning doses of 100 Sv !!!!

The sievert is used to consider the RISK of appearance of a disease. It is only statistically valuable: With a dose of 100 mSv,the risk to see a cancer in the next 20 years in two times higher than with a dose of 50 mSv BUT ONLY THE RISK maybe not the reality for a person considered alone.

A dose of 100 Sv effective dose leads certainly to the death as well as a dose of 50 Sv !!!!

Another confusion comes from the same unit (Sv) to speak about equivalent dose and effective dose.

Some people propose to use the Taylor (Ty) for the Effective dose. This is not yet approved because most of the time we know what we are speaking about.

I think that it should be useful to use different unit for different entities.

Jean Louis Genicot