Biological Aspects of Radiation

•	Roentgen (R)
–	Measures exposure
•	RAD (Radiation Absorbed Dose) 
–	Absorbed dose
–	SI unit = gray
•	REM (Radiation Equivalent Man)
–	Effective dose
–	SI unit = seivert

Radiosensitivity
Dose-Response Relationships
Linear Responses
Linear Responses
Linear Threshold
Non-Linear Responses
Non-Linear Non Threshold
Non-Linear Responses
Non-Linear Threshold

RELATIVE TISSUE RADIO SENSITIVITIES
•	LET= linear energy transfer (another way of expressing the radiation quantity) 
•	RBE= Relative Biologic effectiveness
  
•	As LET ▲ the ability to produce biologic damage ▲ therefore LET and RBE are directly related. 
CELL SURVIVAL AND RECOVERY (LD50)
•	The dose of radiation to the whole body that will cause 50% of the irradiated subject to die. 
•	Usually follows with the number of days ex: LD 50/60 (Lethal dose that will cause 50% to die in 60 days)
Somatic Effects
SHORT-TERM VERSUS LONG-TERM EFFECTS
•	Short Term Somatic Effects: Begin a short time after a large dose of radiation. 
•	Long Term Somatic Effects: Begin after several doses of radiation over a period of time (months)
•	Cataractogenesis - the formation of cataracts due to large doses of radiation
•	Life - Span Shortening - when the average life span of a person is shortened due to large doses of radiation
•	Carcinogenesis - the formation of tumors (cancer) due to large doses of radiation
•	Embryological Effects - the effects on a fetus/embryo from exposure to radiation. The first trimester is the most sensitive to radiation. Sensitivity decreases in the 2nd and 3rd trimesters 
ACUTE VERSUS CHRONIC EFFECTS
•	Acute Radiation Syndrome.
•	1) Initial Stage - Begins within 48 hours after exposure. Flu like symptoms set in (i.e. nausea, fatigue, loss of appetite, etc.)
•	2) Latent Stage - Lasts about a week. During this time, the initial symptoms disappear, and the person feels better. At the same time, changes are taking place internally.
•	3) Manifest Illness Stage - the person show signs of fever, infection, hemorrhage, nausea, vomiting, shock, and Death may follow.
•	4) Recovery Stage - if the dose was not lethal enough, the body starts to heal. This healing may take weeks.
•	There are 3 types of Acute Radiation Syndrome: Central nervous System (CNS), Gastrointestinal (GI), and Hematopoietic (Blood forming organs).
Eye/Thyroid
•	Eyes- non linear threshold response curve

–	Doses of 200 rads can develop cataracts
–	700 rads everyone will develop cataracts
–	15 year latent period

•	Thyroid- linear nonthreshold curve 
•	Latent period for benign nodules is 5-35 years
•	Latent period for malignant nodules is 10-35 years
Reproductive (sterility)
•	Temporary occurs at doses of 200-250 rads for males and last up to one year. Doses at 200 rads can cause temporary sterility in females. 

•	Permanent occurs at 500-600 rads for males and females.
 
•	Doses of  >10 rads can lead to depletion of mature sperm in males or can suppress and delay menstruation in females.
Systemic Responses
•	CNS
•	Occurs with doses >5000 rad with a mean survival rate of 0-3 days 
2. Hemopoietic
•	Consists of bone marrow, circulating blood and lymphoid tissue.   Damage occurs in doses 200-1000 rads with a mean survival rate of 10-60 days 
3. skin
•		
4. GI
•	Occurs with doses 1000-5000 rad with a mean survival rate of 4-10 days 
Embryonic and Fetal Risks
•	Death, congenital abnormalities, malignancy, genetic effects caused by doses >20 rad over varies stages.  
•	Embryonic cells grow at rapid rates and are sensitive. 
•	Estimated fetal doses
–	Dental x-rays= .0001 rad
–	UGI= .001 rad
–	Chest (2 view) = .00007 rad
–	Abdomen (multiple views) .245 rad
–	CT  Abdomen (10 slices) 2.6 rad
10 day rule
•	Identifies the first 10 days following the onset of the menstrual cycle as the safest time to schedule elective procedures of the abdomen/pelvis.
Genetic Impact
GOALS OF GONADAL SHIELDING

•	 3 guidelines 
–	a) gonads lie in or within 5 cm of well collimated field
–	b) Patient is within reproductive age <55 y/o 
–	c) as long as the shield does not obscure the  area of interest. 

•	Types of shields flat, shadow, shaped breast 

Minimizing Patient Exposure
Exposure Factors
kVp  and mAs
•	 Use less time
•	 high kV  
•	 mAs is directly proportional to dose 
•	Higher kV uses less mAs
•	Lower kV will be absorbed by patient 
•	mAs directly proportional to dose

Shielding
Beam Restriction
1.Purpose of primary beam restriction is to limit the exposure to area of interest 

2. Types of beam restrictors 
•	Collimator 
–	most complex and expensive, can make any size desired
•	Cones/ cylinders 
–	have fixed openings used in dental and with collimator for C7, L5-S1
•	Aperture diaphragm 
–	thin piece of metal with a hole ;fixed openings; least expensive 
Filtration
(Getting rid of the low energy photons) 
Repeat Exposure Reduction
•	Patient positioning 
•	Accurately position patients to avoid repeats
 
•	Automatic exposure control (AEC) 
•	Optimal technique if centering is correct, therefore decreases patient dose 

•	Patient communication 
•	Communicate with the patient that they need to breath, hold still etc. to avoid repeats 

Image receptors 
•	As screen speeds increase patient dose decreases. 
•	Digital has less dose due to the ability to manipulate images and less repeats. 
•	Some techs use more technique on digital images to ensure compensation
Grids
•	as grid ratio increase you will use more mAs: therefore more patient dose 
Fluoroscopy
•	Pulsed-turns fluoro on and off to prevent continuous exposure 
•	Exposure factors-Low mA (1-5) used 
•	Grids-higher ratios will increase dose 
•	Positioning-accurate positioning will decrease dose 
•	fluoroscopy time-less time will decrease dose