Y Physics Radiation Measurements and Monitoring

Transcription

1 90 Y Physics Radiation Measurements and Monitoring Mack L. Richard, MS, CHP Indiana University Medical Center Phone: (317) mrichar@iupui.edu

2 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.

3 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

4 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

5 Dose Assay Shaken, not stirred

6 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.

7 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

8 GM Energy Response

9 GM Energy Response with Energy Compensation

10 90 Y Beta Efficiency For contamination monitoring, the beta detection efficiency is determined by the type of GM detector, beta window thickness, & beta particle energy For 90 Y 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%

11 Types of Instruments for Radiation 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 Measurements

12 Ion Chamber

13 What Radiation Units to Use? Dosage Traditional Units (US) 1 Curie (Ci) = 37 GBq 1000 millicuries (mci) 1 millicurie (mci) = 37 MBq GBq Ci Dosage - SI Units 1 Gigabecquerel (GBq) = Ci 27 mci 1000 MBq 1 Megabecquerel (MBq) = mci 27 microcuries ( Ci) GBq Note: 1 Bq = 1 90 Y atom decaying or disintegrating per second (dps) with 90 Y, 1 beta particle is emitted with every disintegration

14 What Radiation Units to Use? 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

15 What Radiation Units to Use? 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

16 Quantifying Contamination Example: If one detects 100,000 cpm under the window of a pancake probe with a 30% efficiency for 90 Y 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 kbq, or 0.15 Ci This amount deposited over 1 cm 2 of bare skin will deliver a skin dose equivalent rate of ~1127 mrem/hr

17 Personnel Monitoring Body badges for monitoring whole body exposures Ring badges for measuring hand exposures very important with sphere treatments

18 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

19 Personnel Monitoring Devices Film Badge Optically Stimulated Dosimeter Direct Reading Dosimeter Ring TLD Dosimeter

20 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 Personnel Monitoring 2 badge method 1 badge worn under Pb apron 1 badge worn at collar, outside Pb apron Effective dose equivalent (EDE) calculated: EDE = 1.5B u B o Example B u = 20 mrem, B o = 300 mrem EDE = 1.5(20) (300) = 42 mrem* *Generally rounded to nearest 10 mrem 21

22 Personnel Monitoring 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(B o ) Previous example single badge EDE ~50 mrem 22

23 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

24 Summary of Instrumentation Use 1. 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 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

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26 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)

27 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)

28 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

29 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

30 Q3. The best way to determine the dose equivalent to one s hand during a 90 Y administration is: a) With an ion chamber b) With a GM survey meter c) With a ring badge d) With a dose calibrator

31 Q3. The best way to determine the dose equivalent to one s hand during a 90 Y administration is: a) With an ion chamber b) With a GM survey meter c) With a ring badge d) With a dose calibrator

32 Q4. The best instrument to use for measuring contamination on IR staff following a 90 Y administration is: a) A dose calibrator b) An ion chamber c) A whole body personnel dosimeter d) A GM survey meter

33 Q4. The best instrument to use for measuring contamination on IR staff following a 90 Y administration is: a) A dose calibrator b) An ion chamber c) A whole body personnel dosimeter d) A GM survey meter

34 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

35 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