Radiation Limits
By: Radiological71
16 January 2008

There are two main distinctions in the biological effect of radiation.
The usual terms are "stochastic" (random) effects, and "non-stochastic" (deterministic or acute effects). An acute effect is associated with killing cells. Surface skin burns from contact with radioisotopes ("beta burns") are an example. A stochastic effect is usually associated with a modification (mutation) of a cell. The mutation may or may not be passed on, may or may not manifest itself, and is less predictable.

Acute Effects

An acute effect is one with a definite cut off, below which the effect does not occur. An example of an acute effect is the production of cataracts with radiation. Below a level of irradiation of 1 Sievert (Sv) cataracts of the eye are effectively unknown. Unlike many other effects, the radiation effect on cataracts is relatively well known and studied. Many patients in the world receive irradiation for cancers in the head and neck region, and one of the "critical structures" to which dose is calculated and measured is the lens of the eye. The USA Department of Health limits the lens of the eye to a dose value of 0.15 Sv (150 mSv) for a radiation worker. The value of 0.15 Sv is also used in USA National Council for Radiation Protection (NCRP) 116 report. For a member of the general public, the radiation limits are ten times less than those of a radiation worker. Since the biology of a radiation wroker and a non-radiation worker is identical, either someone is being over exposed or the limit for the general public is generously low.

The cataract formation low dose limit for a single exposure of radiation appears to be about 2 Sv, and about 4 or 5 if the exposures occur in multiple fractions.However, neutron irradiation has a much greater relative biological effectiveness than gamma rays, so the neutron induced cataract formation is about 10 times lower compared with x-rays or gamma rays. Radiation induced cataracts usually form at he back of the lens, whereas age related cataracts tend to progress from the front of the lens backwards Other examples of deterministic effects are death LD50/60 4.5 Sv whole body dose, and sterility in men (3 Sv gonad dose).

When a single large dose is confined to the skin, early transient reddening (erythema) occurs at 2 Sv, and appears in a few hours. Main erythema, permanent epilation and dry desquamation occur at the 6 Sv to 10 Sv range and appear at around 10 days to 4 weeks. Moist desquamation, ulceration and necrosis are typical in the 15 Sv to 20 Sv range and manifest themselves around 4 to 10 weeks. More information on this can be found on the CDC website. Examples of these effectects are seen in some difficult radiology proceedures.


Picture following is graphic in nature.
















Figure 1: The result of ulceration after 5 hours of fluoroscopic x-rays


Stochastic Effects

Stochastic or random effects are always present even at low doses.
A Stochastic effect has a probability of occurring, and that probability increases with the radiation dose. The typical example of a stochastic effect is the creation of cancer. The International Committee on Radiation Protection (ICRP) and the NCRP use a value of 5% per Sievert for whole body irradiation. The value of 5% per Sv is really only valid for small doses, for example sugggesting that 20Sv = 100% chance of cancer is not true. For example, if one million people receive 0.1 Sv, there is 0.5% chance of cancer for each person, or 5,000 excess cases of cancer would be expected. Out of the 5% chance for one Sievert, the ICRP assigns 1.1% for Stomach cancer, 0.85% for colon cancer, 0.85% for lung cancer, and lesser amounts for the others. The 5% figure is based on a whole body uniform dose, i.e. gamma ray or x-ray irradiation.

The USA whole body annual dose limit for a radiation worker is 50 mSv or 5 rem [10 CFR 20.1201]. At a rate of 5% per Sievert, this corresponds to a 25% chance of cancer sometime during a persons life, if the 5% rule is accuate. (100 years x 0.05 Sv/yr x 5%). A 10 mSv (1 rem) exposure every year would correspond to a 5% chance of the radiation producing cancer.The ICRP dose limit for radiation workers is 0.02 Sv (20 mSv). For a member of the general public, the dose limit is 5 mSv or 0.5rem.[10CFR 20.1301] The NRC requires that any additional radiation from a unit site visit or facility produces no more than 1m Sv (0.1 rem) per year. Nasa limits for astronauts are much larger than a usual radiation worker, with 1 to 4 Sv for an entire career. Internally ingested radioisotopes are a different subject. An injested isotope can be selectively absorbed in a particular organ. For example 131I in the thyroid or 89Sr in the bones. The "whole body dose equivalent" in such a case would be small, but the effect would be much larger than the simple 5% per Sv rule.


Dose Rate

The two main problems that dose limits have are firstly the lack of experimental data. This factor can be partially overcome by studies of certain events, such as Hiroshima and Nagasaki, cell culture measurements, and also following patients treated for radiation therapy. The second problem is the dose rate. Dose delivered at a very low rate can be repaired by the cells themselves. During a normal cell livetime, a cell will repair several thousand double strand DNA breaks [Goodsell 2005]. There is some limit, different for each organ and almost certainly each individual, above which the rate of DNA damage exceeds the repair mechanism. Consequently 5 Sv in 5000 irradiations of 1 mSv per day over 13.7 years will be a lot less dangerous than one irradiation of 5 Sv. For an unrestricted area, for a member of the general public (not an astronaut, or a radiation worker) the dose rate is set as 20 micro Sv/hour (2 mrem per hour) [10CFR20.1302]

References

Offline
NCRP Report number 116:Limitation to exposure of ionizing radiation 1993
Online, available to general public internet
Cutaneous Radiation Injury: Fact Sheet for Physicians
Wilde, G.A clinical study of radiation cataract formation in adult life following gamma irradiation of the lens in early childhood Br J Ophthalmol 1997;81:261-266 (April)
ASTRO Radiation/Cancer biology practise examination and study guide 2007
Goodsell DS. Fundamentals of Cancer Medicine The Molecular Perspective: Double-Stranded DNA Breaks The Oncologist, Vol. 10, No. 5, 361-362, May 2005
Radiation Limits for Astronauts
Title 10 CFR 20 USA Federal Regulations
1 Sv = 1 J/Kg x RBE = 100 rem. For X-rays, Q=1, so 1Sv Xrays = 1 Gy.
Radiological71



www.alpharubicon.com
All materials at this site not otherwise credited are Copyright 1996 - 2008 Trip Williams. All rights reserved. May be reproduced for personal use only. Use of any material contained herein is subject to stated terms or written permission.