L 15: RT Protection

Question 1

Dose equivalent

Answer 1

Biological effects of radiation depends on absorbed dose and quality oif the type of radiation

H = D.Q
SI Unit = 1J/kg

H- Equivalent dose
D- Absorbed dose
Q- Quality factor
* 1 = x-rays, gamma rays, electrons
* 5 = Thermal neutrons
* 20 = Neutrons, alpha

Question 2

Effective dose equivalent

Answer 2

• Whole body radiation exposures are not uniform.
• For a given exposure, internally or externally dose quivalents are different for various tissues.
• This concept was adopted by ICRP & NCRP
• Formula in the picture, it is the sum of the weighter dose equivalents of the irradiated tissues or organs.
• Weighting factor Wr
  X-rays, gamma rays, electrons, positrons and muons = 1
  Protons = 2
  Neutrons = 2-20
  Alpha and heavy nuclei = 20

Question 3

ICRP

Question 4

NCRP

Answer 4

recommendations for radiation workers is based on
1. at low radiation levels the nonstochastic effects are essentially avoided;
2. the predicted risk for stochastic effects should not be greater than the average risk of accidental death among workers in “safe” industries; and
3. the ALARA principle: As low as reasonably achievable by maintaining 1) distance 2) shielding 3) time: reducing time near source

Question 5

Background radiation

Answer 5

3mSv/year
300/360 mrem/year
It is mainly from
* Terrestrial
* Cosmic
* Radio active elements in body ( K-40, beta/gamma rays).

Question 6

Radiation exposure from medical procedures

Answer 6

~20 mrem/year from medical procedures
Now a a days 60mrem/year

Question 7

Effects of radiation

Answer 7

Stochastic Effects
Non-Stochastic Effects

Question 8

Stochastic Effect
Board question

Answer 8

• The probability of occurrence increases with increasing
  absorbed dose but the severity in affected individuals does not depend on the magnitude of the absorbed dose.
• all-or-none phenomenon
• No threshold dose, dose risk relationship is linear

Question 9

Non - Stochastic Effect/
Deterministic Effect
Board question

Answer 9

• It increases in severity with increasing absorbed dose in
  affected individuals, owing to damage to increasing number of cells and tissues.
• additive phenominon, dose dependent; Eg: organ atrophy, fibrosis, lens opacification, etc.
• There is a threshold dose

Question 10

Dose limitations

Answer 10

• Radiation worker - 50mSv/5rem/ year
• General Public - 0.5rem/ year

Question 11

Annual Recommendations

Answer 11

lens was changed to 50mGy from 150mSv
Fetus and embryo is board question

Question 12

Structural shielding

Answer 12

Protection aganist 3 types of radiation
1) Primary RT
2) Scatter RT
3) leakage

Question 13

Primary barrier

Answer 13

A barrier sufficient to attenuate the useful beam to the required degree is called the primary barrier.
p = 0.1 rad/week - occuptional & 0.01 in controlled areas
Greatest decrease in dose is achieved by increasing the HVL

Question 14

Secondary barrier

Answer 14

• The required barrier against stray radiation (leakage and scatter) is called the secondary barrier.
• The transmission curve for the primary beam should be used to determine the leakage barrier thickness
• A barrier designed for primary radiation provides adequate protection against leakage and scattered radiation.
• If a barrier is designed for stray radiation only, the thickness is computed for leakage and scattered radiations separately.
• If the thicknesses of the two barriers differ by at least 3 HVLs, the thicker of the two will be adequate. If the difference is less than three HVLs, one HVL should be added to the larger one to obtain the required secondary barrier.

Question 15

Factors for calculation of barrier thickness

Answer 15

1. Workload
2. Use Factor
3. Occupancy Factor
4. Distance

Question 16

Workload (W)

Answer 16

• The actual “beam on” time
• 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 dose/wk at 1 meter distance from source.
• Weekly dose delivered at 1 meter from the source
• Unit = rad(cGy)/week @ 1meter

Question 17

Use Factor (U)

Answer 17

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

Question 18

Occupancy Factor (T)

Answer 18

• Fraction of the operating time during which the area of interest is
  occupied by the individual.
• Work areas, office/nurse station T=1
• Corridors, restrooms, elevators T=1/4
• Waiting rooms stairs T=1/8-1/6

Question 19

Distance (d)

Answer 19

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.

Question 20

Door Shielding

Answer 20

6mm pb is required for door shielding
* sandwich model for door
* lead - polyethylene - lead
* polyethylene Thermalizes neutrons/slowing down the neutrons
* Concerete should always be the last material of shielding the wall when energies are > 10MeV
* Lead and then concrete (inside the room to outside)

Question 21

Neutron contamination

Answer 21

• Increases as beam energy increases.
• These are produced by high-energy photons and electrons incident on the various materials of target, flattening filter, collimators, and other shielding components
• 10-20 Mv >20Mv= constant
• 16- to 25-MV x-ray therapy mode the neutron dose equivalent along central
  axis is approximately 0.5% of the x-ray dose and falls off to about 0.1% outside the field.
• Reflections from the walls cause a reduction in the neutron fluence and, depending on the accelerator configuration, a decrease in neutron fluence of two orders of magnitude (10−2) from machine location to the inside of the maze can be expected
• The shielding required for the door can be further reduced by the maze design.
• Sandwich method is used for the door: Pb/steel few inches of polyethylene material to thermalize neutrons and then Pb/steel to capture gamma rays produced by neutron capture.
• These radiations have a spectrum of energies ranging up to 8 MeV, but most have energies in the region of 1 MeV

Question 22

Leak Test

Answer 22

A Source is considered leaking if presence of 0.005 micro Ci or more of removable contamination is measured.

Question 23

Radiation protection survery

Answer 23

After the installation of radiation equipment, a qualified expert must carry out a radiation protection survey of the installation.

Question 24

Ion Chamber

Answer 24

Used for low level x-ray measurements

Question 25

Geiger-Muller Counters

Answer 25

* More sensitive than ion chamber * It can detect individual photons or individual particles that could never be observed in an ionization chamber. * slow recovery time (~50 to 300 μs) * G-M counter could significantly underestimate radiation levels when used to count radiation around pulsed machines such as accelerators.

Question 26

Neutron Detectors

Answer 26

* The detector, after exposure to the neutron field, is counted for β- or g-ray activity. * Detected by ionization chamber * boron trifluride filling gas

Question 27

NRC Regulations Nuclear regulatory commission

Answer 27

**10 CFR Part 35** * Regulates naturally occuring isotopes * It does not regulate x-ray generating machines like LINAC. These are regulated individually by states

Question 28

Radiation safety committee

Answer 28

* RSO, Nurse * Meet every quater

Question 29

Medical Event

Answer 29

1. A dose that differs from the prescribed dose > 20% 1. Fractionated dose differed by 50 % 1. An administration of a wrong radioactive drug containing byproduct material. 1. An administration of a radioactive drug containing byproduct material by the wrong route of administration. 1. An administration of a dose or dosage to the wrong individual or human research subject. 1. An administration of a dose or dosage delivered by the wrong mode of treatment. 1. A leaking sealed source. 2. Physicians ans patients involved must be notified within 24 hours and written report in 15 days

Question 30

Technichal requirements

Answer 30

* A licensee may not release a patient administered a radiopharmaceutical from confinement for medical care until either the **measured dose rate at a distance of 1 m from the patient is less than 5 mrem/h or the activity remaining in the patent is less than 30 μCi.** * In the case of a permanent brachytherapy implant, the patient must not be released until the measured dose at a distance of **1 m from the patient is less than 5 mrem/h or 0.01mSv/h**. * A record of patient surveys has to be retained by the licensee for 3 years.

Question 31

FDA

Answer 31

Regulates the manufacture and sale of radiation producing machines and treatment planning systems

Question 32

DOT Department of transportation

Answer 32

Establishes rules for safe transport od radioactive materials including packing and labeling

Question 33

Boron

Answer 33

BF3 filling gas Used inside moderators to detect thermalized neutrons It reduces gamma emission shielding requirements

Question 34

Problem: how many cm of concrete is used as a primary barrier for a linear accelerator

Answer 34

Question 35

rem to mSv

Answer 35

0.001rem = 0.02 mSv

Question 36

Dose limit to public member in unrestricted area in one week

Answer 36

0.02 mSv/ 0.002 rem

Question 37

Leakage

Answer 37

Is from the linac head

Question 38

Scatter

Answer 38

is from the primary beam

Question 39

Daily effective equivalent to US public

Answer 39

0.017 mSv/ day 6.2 mSv/year

Question 40

Important must learn

Answer 40

Question 41

Records should be saved for

Answer 41

3 years

Question 42

Xrays

Answer 42

10 mrem

Question 43

USA background radiation

Answer 43

300 mrem

Question 44

Ct whole body

Answer 44

1000 mrem

Question 45

World wide background radiation

Answer 45

1 msv

Question 46

When should any person be monitored in the department

Answer 46

When they have a reasonable chance of exceeding 10% of regulatory dose limit. NRCP recommendation

Question 47

radiation protection model

Answer 47

Linear no threshold model