topic1 health physics
TRANSCRIPT
INTRODUCTION
• Months after the discovery of x-rays,
they were immediately applied to the
„healing arts‟.
• Clarence Daly (Thomas Edison‟s
assistant) was the 1st American victim
of radiation fatality.
- In 1931, 1st dose-limiting
recommendations were made by
NCRP.
- “providing radiation protection for
workers and the general public is the
main practice of health physics.”
- HEALTH PHYSICISTS design
equipment, calculate and construct
barriers, and develop protocols to
maintain radiation-exposure ALARA.
HEALTH PHYSICS
Health Physics is concerned with
providing occupational radiation
protection and minimizing radiation
dose to the public.
Health Physicist is a radiation scientist
concerned with the research, teaching
or operational aspects of radiation
safety.
CARDINAL PRINCIPLES OF
RADIATION PROTECTION
• all health physics activity in radiology
is designed to minimize radiation
exposure to pts and personnel.
- 3 cardinal principles designed for
nuclear activities
• 1. time
• 2. distance
• 3. shielding
CARDINAL PRINCIPLES OF
RADIATION PROTECTION
1. Keep the time of exposure to radiation
as short as possible.
2. Maintain a large distance as possible
between the source of radiation and
the exposed person.
3. Insert shielding material between the
radiation source and the exposed
person.
Minimize TIME
• The dose to an individual is directly
related to the duration of exposure.
• If the time is doubled, the exposure is also
doubled.
• TIME = exposure rate X exposure time
TIME: Sample Solving
A radiation worker is exposed to 230
mR/hr from a radiation source. If the
worker remains at that position for 36
minutes, what will be the total
occupational exposure?
Occupational exposure (time)
36 minutes
= 230 mR/hr . = 138 mR
60 mins/hr
TIME: Sample Solving
The parent of a pt is asked to remain next
to the pt during fluoro, where the radiation
exposure level is 600 mR/hr. If the
allowable daily exposure is 50 mR, how
long may the parent may remain?
Occupational exposure (time)
= exposure / exposure rate
= 50 mR / 600 mR/hr
= 5 minutes
Sample Solving
A fluoroscope emits 4.2 R/min at the
tabletop for every mA of operation (4.2
R/mAmin). What is the pt exposure in a
BE exam that is conducted at 1.8 mA and
requires 2.5 mins of fluoroscopic time?
4.2 R
Pt exposure = mAmin = (1.8 mA)(2.5 min)
= 18.9 R
• In radio, exposure time is kept low to
reduce motion blur. In fluoro, exposure
time must also kept low to reduce pt
and personnel exposure.
• 5-minute reset timer in all fluoro
machines reminds the radiologists that
a considerable fluoro time has
elapsed.
-records the amount of x-ray beam
on-time
Maximize DISTANCE
-as the distance between the radiation
source and the person increases, the
radiation exposure decreases.
DISTANCE: Sample Solving
An x-ray tube has an output intensity of
2.6 mR/mAs at 100-cm SID when
operated at 70 kVp. What would be the
radiation exposure at 350 cm from the
target?
I1 = (d2)2 = (2.6 mR/mAs) (100 cm)2
I2 (d1)2 (350 cm)2
= (2.6 mR/mAs) (0.08)
= 0.21 mR/mAs
-during fluoro, the RT should remain a
large distance from the radiation
source. In radio, the distance is usually
fixed depending on the type of exam.
-Isoexposure lines= are lines on the plot
plan that represents the positions of
equal radiation exposure in fluoro
room.
-at normal position, the exposure rate is
300 mR/hr
“TWO STEPS BACK is equivalent to 5
mR/hr exposure rate to the RT.”
-however, this exposure reduction
method doesn‟t commend the rule of
thumb/ inverse square law.
Sample Solving:
What is the approximate occupational
exposure of an RT at a position where the
exposure rate is 300 mR/hr, and farther back
where the exposure rate is 20 mR/hr during a
fluoro exam lasting 4 mins, 15 secs?
Occupational exposure =
1st position: (300 mR/hr)(4.25mins)= 21.25 mR
2nd position: (20 mR/hr)(4.25mins)= 1.4 mR
Maximize SHIELDING
Shielding in diagnostic radiology
consists of lead.
“the amount that a protective barrier
reduces radiation intensity can be
estimated by:
- HVL
- TVL
*Protective apparel
- apron – 0.5 mm Pb = 2 HVLs
Dose Limits
*MPD- radiation dose that would be
expected to produce no significant
radiation effects.
- rad dose below MPD, no responses
- rad dose at MPD level, risk might occur
- replaced by DL
WHOLE-BODY DOSE LIMITS
*Radiation Workers
= DL – 50,000 mrem/wk (1902)
– 50 mSv/yr
– 150 mSv/yr = eye lens
– 500 mSv/yr = body organs
– 10 mSv/yr = whole body x age
– 5 mSv = during pregnancy
- 0.5 mSv/monthly
Dose Limits Recommended by NCRP
A. Occupational exposures
1. Effective dose a. Annual: 50 mSv
b. Cumulative: 10 mSv x age
2. Equivalent annual dose for tissues and organs
a. Eye lens 150 mSv
b. Thyroid, skin, hands, feet 500 mSv
B. Public exposures (annual)
1. Effective dose, freq expo 1 mSv
2. Equi dose for tissue and organs
a. Eye lens 15 mSv
Dose Limits Recommended by NCRP
b. Skin, hands, feet 50 mSv
C. Education & training expo (annual)
1. Effective dose 1 mSv
2. Equivalent dose for tissues and organs
a. Eye lens 15 mSv
b. Skin, hands, feet 50 mSv
D. Embryo-fetus exposures
1. Total equivalent dose 5 mSv
2. Equivalent dose in 1mo 0.5 mSv
E. Negligible individual dose (annual) 0.1 mSv
“essentially, all of our radiation exposure
occurs during fluoroscope and the
trunk is shielded by a lead apron,
radiation monitoring/ measuring device
is positioned in the COLLAR, ABOVE
THE PROTECTIVE APRON.”
DOSE LIMITS FOR TISSUES AND ORGANS
• 50 mSv/yr whole-body DL is an
effective dose (NCRP)
SKIN
-DL: 500 mSv/yr
-nonpenetrating rays (a, B, grenz)
EXTEMITIES
-DL: 500 mSv/yr
-personnel monitoring devices are
worn on wrist or finger
PUBLIC EXPOSURE
-DL: 5 mSv/yr if exposure is infrequent
-DL: 1 mSv/yr if exposure is frequent
=used by physicists to compute
protective barrier thickness
“radiation exposure of the general public
is rarely measured because it is not
necessary. Most radiology personnel do
not receive even this level of exposure.”
EDUCATIONAL CONSIDERATIONS
-DL: 1 mSv/yr = students under 18
-ICRP
= issued a recommendation
including an annual whole-body DL of
20 mSv.
X-RAYS AND PREGNANCY
Radiobiologic Considerations
“radiation-response exposure in utero is
both time-related and dose related.”
Time Dependence
- Irradiation during the 1st 2 weeks of
pregnancy is least hazardous.
- The most likely biologic response to
irradiation during the 1st 2 weeks of
pregnancy is resorption of embryo. No
other response is likely to occur.
- No concern on the possibility of
radiation-induced congenital
abnormalities
Time Dependence
- 2nd to 10th week= Major Organogenesis
- If irradiation occurs with higher dose,
congenital abnormalities may result
*early – skeletal deformities
*late – neurologic deficiencies
- During 2nd and 3rd trimester the principal
response would be the appearance of
malignant disease during childhood.
Time Dependence
- No radiation- responses during
pregnancy would likely to occur at less
than 25 rad.
- Such dose level is highly unlikely yet
possible with patients who receive
multiple x-ray examinations of the
abdomen or pelvis.
- There are no other significant
responses after irradiation.
Dose Dependence
- after the utero is irradiated with dose
of 200 rad, it is nearly certain that each
of the noted effects will occur.
- spontaneous abortion during the 1st 2
weeks of pregnancy is unlikely at
radiation doses less than 25 rad.
- a 1% increase in congenital
abnormalities is estimated to follow
after a 10-rad fetal irradiation.
- Relative Risk is used to assess
childhood malignancy radiation risks.
Pregnant RT
- DL: 0.5 mSv/mo (pregnant women)
- DL: 5 mSv (fetus for the entire
pregnancy)
- *most of RT receive < 1 mSv/yr
- “the length of the apron (0.5 mm Pb =
attenuate 90% of rays @ 75 kVp) need
not extend below the knees, but
wraparound aprons are preferred
during pregnancy.”
Pregnant RT
- “the pregnant RT should be provided
with a second personnel monitoring
device.” – must be positioned under
the Pb apron at waist level
- Attenuation by the maternal tissues
overlying the fetus reduces the dose to
the fetus (30% or 300 uSv)
Management Principles
1) NEW EMPLOYEE TRAINING
2) PERIODIC IN-SERVICE TRAINING
3) COUNSELING DURING PREGNANCY
NEW EMPLOYEE TRAINING
*orientation and training
*each RT should be provided with a copy
of the facility radiation protection
manual and other appropriate materials
*a female employee must voluntarily
notify her supervisor when she is
pregnant or suspects that she is
pregnant
Pregnancy in Diagnostic Radiology
Human Responses to Low-Level X-ray
Life-span shortening 10 days/rad
Cataracts None below 200 rad
Leukemia 10 cases/ 106/ rad/ yr
Cancer 2 cases/ 104/ rad
Genetic effects Doubling dose= 50 rad
Death from all causes 2 deaths/ 104/ rad
Effects of Irradiation In Utero
0-14 days Spontaneous abortion
2-10 weeks Congenital abnormalities
2nd – 3rd trimester Cell depletion: no effect @ < 50 rad
Latent malignancy
0-9 months Genetic effects
Protective Measures for Pregnant RT
*Two occupational radiation monitors
*Dose limit: 5 mSv/9mos, 0.5 mSv/mo
IN-SERVICE TRAINING
*conducted at monthly intervals
*at least twice each year such training
should be devoted to RADIATION
PROTECTION, and a portion of these
sessions should be directed at the
potentially pregnant employee
*EMPHASIZE
*the effective RDL is 50 mSv/ yr
*environmental background radiation is approximately 1 mSv/yr
*occupational exposures are closer to the latter than the former
COUNSELING DURING PREGNANCY
*review of pregnant employee‟s radiation
exposure hx
*appropriate schedule modification
*In ONCOLOGY, pregnant RT is not
advised to participate in brachytherapy
*In NUCLEAR MEDICINE, pregnant RT
should handle only small quantities of
radioactive material and should not
elute or inject radioactive material.
The Pregnant Patient
- “if the state of pregnancy is known, the
radiologic exam should not be
conducted.”
- if radiographic exam is too essential to
be done, proper techniques of
minimizing pt dose must also be
observed, consent form must be
signed
-use of high kVp technique is done
The Pregnant Patient
• Elective Booking
-a safeguard against the irradiation of
an unsuspected pregnancy
-determine the time of the pt‟s previous
menstrual cycle
-xray exam in which the fetus is not in
or near the primary beam may be
allowed
The Pregnant Patient
• Patient Questionnaire
- “Are you or could you be pregnant?”
-‟‟What was the date of your last
menstrual period?”
The Pregnant Patient
• Posting
• Heath Physics is concerned with the
research, teaching and operational
aspects of radiation exposure.
• Occupational radiation exposure in
measured in mSv (mrem)
• The description of such exposure is
effective dose (E).
-effective dose accounts for radiation
type and the relative radiosensitivity of
tissues and organs.