Khan Outline: Chapter 16 Radiation Protection

Front 1

What is a quality factor?

Back 1

The Quality factor in radiation protection is a conservative value based on a range of relative biological effectiveness related to linear energy transfer (LET) of the radiation.

Front 2

List the values of different quality factors and their sources

Back 2

X-Rays and electrons: 1

Thermal neutrons: 5

Fast neutrons, heavy particles: 20

Front 3

List sources of background radiation with their respective percents

Back 3

- Terrestrial

- Cosmic

- Inhaled

- In the Body

Front 4

What are some of the low-level radiation effects

Back 4

Exposures to low-level radiation may produce

a) genetic effects, such as radiation-induced gene mutations, chromosome breaks, and anomalies
b) neoplastic diseases, such as increased incidence of leukemia, thyroid tumors, and skin lesions.
c) effect on growth and development, such as adverse effects on the fetus and young childern
d) effect on life span, such as diminishing of life span or premature aging
e) cataracts or opacifications of the eye lens.

Front 5

Define ALARA

Back 5

As Low As Reasonably Acheivable

Front 6

When does one start monitor for pregnancy and what are the exposure limits?

Back 6

The pregnant woman who is a radiation worker can be considered as an occupationally exposed individual, but the fetus cannot. The total dose-equivalent limit the an embryo-fetus is 5 mSv (0.5 rem), with the added recommendation that exposure to the fetus should not exceed 0.5 mSv (0.05 rem) in any 1 month.

Front 7

Define Negligible risk

Back 7

A negligible individual risk level (NIRL) is defined by the NCRP (2) as "a level of average annual excess risk of fatal health effects attributable to irradiation, below which further effort to reduce radiation exposure to the individual is unwarranted"

The NIRL is regarded as trivial compared to the risk of fatality associated with ordinary, normal societal activities and can therefore, be dismissed from consideration.

The concept of NIRL is applied to radiation protection because of the need for having reasonably negligible risk level that can be considered as a threshold below which efforts to reduce the risk further would not be warranted.

Front 8

List all factors that go into a shielding calculation. Give typical values for each.

Back 8

Workload (W)

-The workload is usually stated in terms of weekly dose delivered at 1 m from the source. This can be estimated by multiplying the number of patients treated per week with the dose delivered per patient at 1 m.

-W is expressed in Gy/week at 1 m

Use Factor (U)

-Fraction of the operating time during which the radiation under consideration is directed toward a particular barrier.

Occupancy factor (t)

-Fraction of the operating time during which the area of interest is occupied by the individual

Distance (d)

-Distance in meters from the radiation source to the area to be protected. Inverse-square law is assumed for both the primary and stray radiation.

Maximum permissible dose equivalent (P)

-The maximum dose per week allowed in a given area outside the treatment room.

-For structural shielding, P is taken as 0.1 mSv/week for controlled areas and 0.02 mSv/week for noncontrolled areas.

Front 9

What is the different between primary and secondary barriers?

Back 9

A barrier sufficient to attenuate the useful beam to the required degree is called the primary barrier.

The required barrier against stray radiation (leakage and scatter) is called the secondary barrier.

Front 10

What is the leakage radiation for 5 to 50 kVp?

Back 10

Leakage exposure shall not exceed 0.1 R in any 1 hour at any point 5 cm from the source assembly.

Front 11

What is the leakage radiation for greater than 50 kVp and less than 500 kVp?

Back 11

The leakage exposure rate at a distance of 1 m from the source shall not exceed 1 R in any 1 hour. In addition, these assemblies shall limit exposure rate to 30 R/h at 5 cm from the surface of the assembly.

Front 12

What is the leakage radiation for greater than 500 kVp?

Back 12

The absorbed dose due to leakage at any point outside the maximum field size, but within a circular plane of radius 2 m that is perpendicular to and centered on the central axis at the normal treatment distance shall not exceed 0.2% of the useful beam dose rate at the treatment distance.

The leakage dose rate from the source assembly at any point at a distance of 1 m from the electron path between the source and target shall not exceed 0.5% of the useful beam dose rate at the treatment distance.

The neutron contribution to the dose within the useful beam shall be kept well below 1% of the x-ray dose. Outside the useful beam, the neutron dose should be reduced to as low as practicable.

Front 13

What is the leakage radiation for Cobalt teletherapy?

Back 13

The leakage dose rate from the source housing with the beam in the off position shall not exceed 2 mrad/hr on the average and 10 mrad/hr maximum in any direction, at a distance of 1 m from the source.

With the beam in the "on" position, the leakage dose rate from the source housing shall not exceed 0.1% of the useful beam dose rate, both measured at a distance of 1 m from the source.

In addition, for sources that give rise to a useful beam dose rate of less than 1,000 rad/hr at 1 m, the leakage from the source housing shall not exceed 1 rad/h at 1 m from the source.

Front 14

What makes up a door to a high energy treatment room?

Back 14

In most cases, the required shielding is less than 6mm of lead.

A few inches of a hydrogenous material such as polyethylene may be added to thermalize the neutrons and reduce the neutron dose further.

Front 15

What makes a good neutron shield?

Back 15

Concrete barriers designed for x-ray shielding are sufficient for protection against neutrons.

Front 16

Is neutron production more of a problem for high or low energy radiation beams?

Back 16

Hight energy radiation beams.

The neutron contamination increases rapidly as the energy of the beam is increased from 10 to 20 MV, and then remains approximately constant above this.

Front 17

What are the criteria for brachytherapy storage?

Back 17

The storage room typically has the following:

-Lead lined safes with lead-filled drawers
-A shielded area for source preparation
-A sink for cleaning source applicators. The sink should be provided with a filter or trap to prevent loss of any source.
-The storage area should be ventilated by a direct filtered exhaust to the outdoors. This precaution is taked so that if a source ruptures, the radionuclide is not drawn into the general ventilation system of the building.
-The door to the radioisotope facility must be kept secure under lock and key

Front 18

When a source considered to be leaking?

Back 18

A source is considered to be leaking if a presence of 0.005 uCi or more of removable contamination is measured.

Front 19

List the characteristics for the Ionization Chamber.

Back 19

Used for low level x-ray measurements and has a large volume to obtain high sensitivity

A direct current voltage is applied between the outer shell and the central electrode to collet ionization charge produced by radiation in the internal air volume when the chamber is exposed to radiation. The output current is directly proportional to the exposure rate.

Front 20

List the characteristics for the Geiger-Muller Counter.

Back 20

The G-M tube is much more sensitive than the ionization chamber. It can detect individual photons or individual particles that could never be observed in a ionization chamber.

Not a dose-measuring device.

Useful for preliminary surveys to detect the presence of radiation.

Slow recovery time. Could significantly underestimate radiation levels when used to count radiation levels when used to cout radiation around pulsed machines such as accelerators.

Front 21

List the characteristics for Neutron Detectors.

Back 21

Neutron measurement in or near the primary x-ray beam can be made with passive detectors such as activation detectors, without being adversely affected by pulsed radiation.

An activation detector can be used either as a bare threshold detector (e.g., phosphorus detector) or inside a moderator (e.g., gold foil surrounded by polyethylene).

Outside the treatment room, it is common practice to use two detectors that respond to predominantly one or the other radiation- x-rays or neutrons.

The neutron survey meter is calibrated using a Pu-Be neutron source that has been calibrated by the National Institute of Standards and Technology (NIST)

Front 22

What is an area survey?

Back 22

Area surveys are required to check environmental safety. They should be evaluated by taking into account actual operating conditions, workload, use factor, occupancy factor, and attenuation and scattering of the useful beam by the patient.

Areas outside the treatment room that are accessible to any individual should be designed as controlled or noncontrolled, depending on whether the exposure of persons in the area is monitored or not. Exposure levels in these areas should be measured with the beam orientated in various possible directions. The transmitted radiation through the primary barrier should be measured with the beam of maximum size directly incident at the barrier. Measurements outside the secondary barrieres should be made with a phantom at the treatment position. Other operational conditions such as total body irradiation may present special treatment conditions that should be considered in the area survey.

Front 23

List the different types of personnel monitoring and their accuracy.

Back 23

Film badges-

TLD's-

Pocket Dosimeters-

Front 24

What is the NRC? Can you disagree with the NRC?

Back 24

The U.S. Nuclear Regulatory Commission (USNRC). They control the use of all reactor-produced material (byproduct materials) in this country. You can not disagree with the NRC.

Front 25

What is an RSO?

Back 25

A Radiation Safety Officer.

- Appointed and has the responsibility of implementing the radiation protection program.

-The RSO must ensure that the program is conducted in accordance with the approved procedures and regulatory requirements.

Front 26

What are the administrative requirements for a radiation program?

Back 26

A written radiation program is required that includes the ALARA principles. The program must be communicated to the workers and be reviewed periodically to ensure that a responsible effort is being made to keep individual and collective occupational doses ALARA.