Topic1 Health Physics

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.