Calculation of radiation doses and restriction periods for persons coming into contact with I-131 I In-111 therapy patients

Transcription

1 Calculation of radiation doses and restriction periods for persons coming into contact with I-131 I and In-111 therapy patients John Cormack and Jane Shearer Division of Medical Imaging Flinders Medical Centre South Australia

2 F L I N D E R S M E D I C A L C E N T R E, MEDICAL IMAGING DIVISION Human Radioactive Sources In-vivo radioactive sources are harder to control than in-vitro radioactive sources, and the radiation dosimetry is more complex. Little Willie, full of glee Put radium in his Grandma s tea Now he thinks it s quite a lark To see her glowing in the dark!

3 Regulatory Requirements In the USA the Nuclear Regulatory Commission (NRC) has already published amendments to its regulations pertaining to the release of radioactive patients which will necessitate an estimate of exposure to other persons to be made, in principle, for every patient. In Australia, release criteria in most states will probably be eventually based on the ARPANSA Recommendations for the Discharge of Patients Undergoing Treatment with Radioactive Substances

4 Radiation Exposure from Multiple Contact Periods Exposure Rate Time

5 Calculation of Cumulative Exposure Total = Exposure Initial Effective Exposure Rate Exposure Time E = E 0 T E = Total exposure E 0 = Exposure rate at time zero (initial exposure rate) T = Effective exposure time

6 Calculation of Effective Exposure Time Effective = Exposure Time T = Sum over all exposure periods of effective exposure time in each period All exposure periods F(t)dt exposure period F(t ) = E (t) E 0 = A(t) A 0

7 Radiation Exposure Pattern Pattern Start p Pattern End Administration Delay τ Time C 1 C 2 C 3 C 4

8 Pattern #5 Partner or Spouse t2 t3 bcomp1 bcomp Time Period (Hour of Day) Contact Delay (hrs)* = Contact Delay (hrs)* = Time Period (Hour of Day)

9 Phase Shifting Pattern Reference Time T r Administration T a Delay τ Phase Shift φ p

10 I think you should be more explicit here in step two!

11 Calculation of Effective Exposure Time (Multi-exponential clearance, multiple exposure, single pattern phase locked) l T = n i =1 a i e λ i τ λ i m j =1 e λ i θ j ( 1 e λ i C ) j T = effective exposure time C j = duration of jth period of contact θ j = time lapse between start of contact pattern and jth period of contact τ = time delay between administration and start of close contact pattern m = number of periods of close contact λ i = decay constant for ith component of decay a i = fraction of decay occurring through ith component

12 Calculation of Time Delay Required to Limit Effective Exposure (and hence Dose) to a Specified Value (Inverse Calculations) Analytical solution possible for single exponential clearance Numerical techniques must be used for multi-exponential clearance

13 Approximate Calculation = & 0 t s { ae λ } 1t + e λ 2t D kd b dt D is the total accumulated dose D & is the initial dose rate at 1 metre 0 λ 1 and λ 2 are the two decay constants associated with the clearance a and b are the proportions of each clearance component t is the time at which exposure starts s k is an exposure factor which relates the actual accumulated exposure from a given exposure pattern to that received from continuous exposure at one metre

14 Exposure Factor, k k = Accumulated radiation dose from a given pattern of exposure Total dose resulting from exposure at 1 metre in the same time k D D = = a b { } 1t 2t D& λ λ ae e D t 0 b dt & e λ e λ λ λ t s s + 2 s 1 2 t

15 Exposure Factor, k - Note Note that for exposure patterns where there are periods of exposure at distances less than 1 metre, the exposure factor may have a value exceeding 1. It should not be confused with occupancy factor. Note also that the exposure factor, k, is not a constant for a given exposure pattern, and will, in fact, vary with the start time of the exposure t s as well as the clearance rate of the radionuclide from the body; however, this variation is small for radioactive materials which have an overall clearance rate which is slow compared with the period of the exposure pattern. For I-131 and In-111 therapy, therefore, an averaged exposure factor for each pattern can be utilized, allowing the approximate equation to be used in estimating accumulated doses

16 Standard Exposure Patterns Exposure Pattern Activity Public transport travel Return to work not involving prolonged close contact with others Return to work involving prolonged close contact with others Close contact with adult friends and family/carers Close contact with pregnant women Caring for infants (demanding or sick) Caring for infants (normal ) Close contact with 2-5 year old children Close contact with 5-15 year old children Sleeping with spouse or partner, or a child Work with radiosensitive materials

17 Calculated exposure factors for various clearance rates and patterns of exposure Exposure Pattern I-131 iodide I-131 iodide I-131 iodide I-131 iodide Ablation - Thyrotoxic Ablation - fast Euthyroid slow Patients clearance patients clearance In-111 octrocide "Normal" clearance rate Mean CV(%)

18 Exposure Pattern F L I N D E R S M E D I C A L C E N T R E, MEDICAL IMAGING DIVISION Calculated restriction times using approximate and rigorous models Set Dose Constraint (µsv) Calculated Restriction (hours) Approx Model Rigorous Model Discrepancy

19 Calculated doses using approximate and rigorous models Set Dose Exposure Constraint Pattern (µsv) Actual Dose Received (µsv) Approx Model* Rigorous Model Discrepancy

20 Data Input

21 Calculated Restriction Times

22 Patient Information Sheet

23 Measured Clearance Data

24 Incorporated Clearance Data (can be modified by user)

25 Issues to be Resolved Choice of appropriate data for the decay in exposure rate with time. Exposure rate versus time curves for each patient can be obtained, but (beware!) propagated errors in calculated restriction times and doses can be very large. Some consensus is needed on realistic exposure patterns for various patient activities. Some consensus is needed on the dose limits to be applied to various groups of persons. Effective dose conversion factors. Mean or maximum doses?

26 Published Clearance Data for Thyrotoxic Patients O Doh 1.0 m O Doh 0.5 m ICRP Hilditch O Doh 0.1 m Time lapse in hours Hilditch O'Doh0.1 Odoh1.0 Odoh0.5 ICRP

27 Errors in Using Fitted Clearance Data for Individual Patients Modelled using Monte Carlo methods Calculated restriction times for sleeping with spouse or partner MBq ablation dose Exposure rate measured at 0, 12, 24, 48 and 96 hours post administration Measurement Precision (%) Restriction Time (h) CV (%) Minimum Maximum Exposure rate measured at 0, 12, 24, 96 and 360 hours post administration Measurement Precision (%) Restriction Time (h) CV (%) Minimum Maximum

28 Effect of Exposure Pattern and Dose Limit on Calculated Restriction Times Calculated restriction times for sleeping with spouse or partner MBq ablation dose Exposure Pattern Dose Limit (microsieverts) Effective Exposure Time (hours) Required Restriction (days) Normally Used m 6 1 m Normally Used m 6 1 m ? More Realistic? m m m h at 1 m

29 Further Information Full paper and spreadsheets available from authors

30 Effective Dose Conversion Factors About 0.7 Sv per Gy air kerma Hp(10) a rough surrogate for effective dose

31 Mean or Maximum Dose Or Both? Regulations generally specify a maximum dose. However, there will always be a finite error in calculated doses. Maximum dose may be considerably larger than the mean dose. Use mean dose in conjunction with 95 th percentile value as regulatory criteria?