radiation measurements and monitoring · optically stimulated dosimeter ... “palm” side wear...

90Y Physics

Radiation Measurements and

Monitoring

Mack L. Richard, MS, CHP

Indiana University Medical Center

Phone: (317) 274-0330

Email: [email protected]

What type of measurements are

required?Dosage (activity) to be administered to the patient.

Pre-treatment radiation levels from dosage.

Radiation levels during dosage administration.

Contamination surveys of IR staff prior to leaving

area.

Survey of patient following administration.

Post-treatment radiation levels from waste.

Contamination survey of IR suite.

Contamination survey of patient recovery area.

Dose Calibrator for Dosage Assay

Determine calibration factors (settings)

– Beta emitter – dependent on geometry, dose vial construction

– Optional with unit doses – may use activity provided by vendor

SIR-Spheres®

– NIST calibrated dose for initial set up

– Calibration sheet w/ patient dose for periodic check

TheraSpheres®

– Request calibration sheet w/ initial patient dose and recheck periodically

Geometry, Geometry, Geometry!!!

“Out of the box” assay: 82.2 mCi

After “shaking” vial, subsequent assays over 1 minute:

– 76.5 mCi

– 77.5 mCi

– 77.8 mCi

– 78.5 mCi

– 80.0 mCi

– 81.0 mCi

When setting up dose calibrator, be sure to use the

same geometry for setup & subsequent assays

Dose Assay

“Shaken, not stirred”

Dosage Assay

SIR-Spheres®

– Since dose must be drawn from “stock” vial, the vial

should always be shaken at dose calibrator setup,

prior to initial assay, and prior to subsequent assays

following drawing of dose to assure uniform

geometry.

Theraspheres®

– Since this vendor provides a “unit” dose, the vial

should shaken and tapped on a hard surface to free

any microspheres trapped around the septum and

then allowed to settle at dose calibrator setup

subsequent assays.

Types of Instruments for Radiation

Measurements

Geiger Mueller (GM) survey

meter

– Good for finding

contamination

– Can be used for making

“comparative” measurements

– Not as good for measuring

true exposure rate due to

energy dependency

– Can “saturate” at high count

rates

GM Energy Response

GM Energy Response with Energy

Compensation

90Y Beta Efficiency

For contamination monitoring, the beta

detection efficiency is determined by the

type of GM detector, beta window

thickness, & beta particle energy

For 90Y betas, the typical 4π beta efficiencies are:

– End window GM probe ~10% (detector “sees” 1 of every 10 beta particles emitted)

– “Pancake” GM probe ~30%

Types of Instruments for Radiation

Measurements

Ion chamber

– Most common use is for

measuring exposure rate

from gamma or x-ray

sources

– Good for measuring

exposure rate from

patient following

administration

– “Pressurized” ion

chambers will provide

better sensitivity

Ion Chamber

What Radiation Units to Use?

Dosage – “Traditional”

Units (US)– 1 Curie (Ci) =

37 GBq

1000 millicuries (mCi)

– 1 millicurie (mCi) = 37 MBq

0.037 GBq

0.001 Ci

Dosage - SI Units

– 1 Gigabecquerel (GBq) =

0.027 Ci

27 mCi

1000 MBq

– 1 Megabecquerel (MBq) =

0.027 mCi

27 microcuries ( Ci)

0.001 GBq

Note: 1 Bq = 1 90Y atom “decaying” or “disintegrating” per second (dps) – with 90Y, 1 beta particle is emitted with every disintegration


Exposure Rate from Brems Radiation

– Milliroentgens/hr (mR/hr)

– This corresponds to a dose equivalent of about 1

millirem/hr to an individual

– SI unit of dose equivalent is the millisievert (mSv) – 1

mSv = 100 mrem

– These are the appropriate units to use when

measuring brems radiation only


Contamination monitoring

– Appropriate units are counts per minute (cpm); however,

if comparative measurements are being performed,

other units such as mR/hr can be utilized

– It should be understood that the units of mR/hr do not

reflect either the dose rate or exposure rate from beta

radiation

– Contamination monitoring is usually the “all or none”

principal

– By knowing the detector efficiency, one can estimate the

amount of contamination on a surface

– Detector efficiency is very geometry dependent

Quantifying Contamination

Example: If one detects 100,000 cpm under the

window of a pancake probe with a 30%

efficiency for 90Y betas, how much activity is

present under the detector window?

– 100,000 cpm/0.3 counts/disintegration = 333,333

disintegrations/minute =

5556 dps (Bq), or

5.556 kBq, or

0.15 Ci

This amount deposited over 1 cm2 of bare skin will

deliver a skin dose equivalent rate of ~1127 mrem/hr

Personnel Monitoring

“Body badges” for

monitoring whole

body exposures

“Ring badges” for

measuring hand

exposures – very

important with

sphere treatments

Primary Standards

Annual, occupational dose equivalent limits:– Total effective dose

equivalent (TEDE) from external & internal sources – 5,000 mrem

– Committed dose equivalent (CDE) to extremities, skin, or individual organs -50,000 mrem

– Lens dose equivalent (LDE) - 15,000 mrem

Personnel Monitoring Devices

Film BadgeOptically Stimulated Dosimeter

Direct Reading Dosimeter Ring TLD Dosimeter

Whole Body Badge Use

When Pb aprons are routinely used, a “collar” badge may be used to measure head (eye) & neck dose equivalent

It is important to avoid reversing the position of the WB and collar badge

Special calculations may be employed to determine EDE from multiple badges

21


2 badge method

– 1 badge worn under Pb apron

– 1 badge worn at collar, outside Pb apron

Effective dose equivalent (EDE) calculated:

EDE = 1.5Bu + 0.04Bo

Example – Bu = 20 mrem, Bo = 300 mrem

EDE = 1.5(20) + 0.04(300)

= 42 mrem**Generally rounded to nearest 10 mrem

22


Must wear badges correctly & return both for

proper EDE calculation

Previous example

– Under apron badge not returned – assigned EDE

= 300 mrem (7.5X calculated EDE!!)

– Badge positions reversed (e.g., assume both

badges receive ~160 mrem) – calc EDE ~250

mrem (6.25X calculated EDE!!)

Single badge at collar outside Pb apron – EDE =

0.18(Bo)

Previous example single badge EDE ~50 mrem

Ring Badge Use

Wear on hand expected

to receive the highest

dose equivalent (can

wear one ring on each

hand)

Wear with label on the

“palm” side

Wear under gloves

Summary of Instrumentation

Use1. Dosage (activity) – Dose Calibrator

2. Pre-treatment radiation levels from dosage – GM or Ion Chamber

3. Radiation levels during dosage administration – GM or Ion

Chamber

4. Contamination surveys of IR staff prior to leaving area - GM

5. Survey of patient following administration – Ion Chamber or Energy

Compensated GM

6. Post-treatment radiation levels from waste for quantification of

delivered dosage – Same instrument and geometry as item 2.

7. Contamination survey of IR suite - GM

8. Contamination survey of patient recovery area – GM

9. Personnel monitoring – Whole Body (1 or 2) and Ring Badges

Q1. A dose calibrator typically

reads out in what units?

a) Counts per minute (cpm)

b) Millicuries (mCi)

c) Disintegrations per minute (dpm)

d) milliroentgens per hour (mR/hr)

Q2. Which instrument generally provides the

best measurement of exposure rate from

brems radiation?

a) An ionization chamber

b) A geiger mueller (GM) survey meter

c) A dose calibrator

d) A whole body personnel monitoring

badge

Q3. The best way to determine the dose

equivalent to one’s hand during a 90Y

administration is:

a) With an ion chamber

b) With a GM survey meter

c) With a “ring” badge

d) With a dose calibrator

Q4. The best instrument to use for

measuring contamination on IR staff

following a 90Y administration is:

a) A dose calibrator

b) An ion chamber

c) A whole body personnel dosimeter

d) A GM survey meter

Q5. If you wear “under apron” and “collar”

personnel monitors (badges) & accidentally

switch them several times over the time you

wear them:

a) It won’t affect the calculated effective

dose equivalent (EDE)

b) The calculated EDE will be artificially high

c) The calculated EDE will be artificially low

d) The badges will be unreadable

radiation measurements and monitoring · optically stimulated dosimeter ... “palm” side wear...

Documents