1

NUREG 1556-based Release ofPatients from Radiation Safety
Restrictions
Richard E. Wendt III
Department of Imaging Physics
UT M. D. Anderson Cancer Center
Houston, Texas 77030

The aim of this presentation is to discuss theguidance by Nuclear Regulatory Commission
on the release of radioactive patients, to review
the calculations that are described in theguidance and to discuss how one might build
upon the procedures in the guidance toaccomplish an even more effective patient
release program. This talk addresses only US
regulations. The rules and practices in the restof the world vary, and some of the references
for this talk mention the differences.

rwendt@mdanderson.org

2

Regulatory compliance is mandatory. The
regulatory guidance offers a number ofapproaches to compliance that we will workthrough. The regulatory guidance alsoestablishes a framework in which moreadvanced approaches may be considered bymodification of model parameters.
The rationale for a higher limit than that to thegeneral public is that the most exposed person
is presumed to derive benefit from his or her
exposure to the patient.

10 CFR 35.75●

A patient who has been administeredradioactive materials may be released fromcontrols provided that the total effective dose
equivalent to any other individual as aconsequence is unlikely to exceed

5 mSv (500 mrem).
●

NUREG 1556, Volume 9, Appendix U, Revision
2 (née RegGuide 8.39) offers regulatory
guidance for compliance.
2

3

5 mSv Porridge
Some
Politicians

SomeExperts
HPS

Too Cold

Just Right

Too Hot

Some influential politicians argue that 5 mSv is too
dangerous and that we should return to the old
30 mCi or 5 mR/hr @ 1 m release criteria.
Some influential physicists and physicians argue
that the 5 mSv limit is well below the thresholdof danger and could safely be relaxed.
The position of the Health Physics Society is thatthe 5 mSv limit does no discernible harm to the
public and balances safety concerns with
benefits to patients, their families and society.
The Health Physics Society position statement
accurately reflects the reality that there is great
benefit in releasing patients using the 5 mSv
limit.
The computations in the regulatory guidance arearguably quite conservative.

4

We must keep the dose to the most exposed
person under 500 mrem (5 mSv). To put this incontext, the typical annual background dose isaround 300 mrem (3 mSv, but it varies withelevation and the local level of environmental
radioactivity). Children and pregnant woman
should receive no more than 100 mrem (1 mSv)a year (ICRP 94). Since the dose limit formembers of the general public is 100 mrem (1mSv), it makes sense to keep the doses of
disinterested people, including fellow travelers,
below 100 mrem (1 mSv) also, as they derive
no benefit from exposure to the patient.
American Gothic is in the public domain:
http://en.wikipedia.org/wiki/American_Gothic
The clip art of the pregnant woman and children isfrom a free clip art Web site
http://www.openclipart.org/.
All other illustrations in this presentation are thework of the author, except for a section of a
Google map, which is copyright Google, 2012.
Many of these were made with the Poser Pro
software package.

5

The overall risk of fatal cancer at low doses has been
estimated to be 5% per sievert. That predicts one
excess fatal cancer in a cohort of 4000 people, eachmember of which is exposed to 5 mSv.
Compare this to the 50% likelihood of developing cancer
anyway, and the half of those cases that will be fatal.
The inner barrel shows the healthy (green), non-fatal
cancer (yellow) and fatal cancer (red) cases in acohort of 4000 people.
The outer barrel shows the same for a cohort of 4000
people, every one of whom has been exposed to 500
mrem (5 mSv). One green person has turned red.
It is almost impossible to tell the difference by looking at
this plot. Furthermore, it is impossible to know which
one among the 1001 cancers was cased byexposure to a radioactive patient.
Another way to look at this analysis is that the 1999
healthy people know with certainty that their 5 mSv
doses did not cause cancer. The 1001 people who
contract a fatal cancer after a 5 mSv dose have a
0.1% probability that their cancer was caused bytheir 5 mSv dose and a 99.9% probability that they
got cancer for some other reason.

6I-131 sodium iodide is the principal therapeutic
radiopharmaceutical in most nuclear medicine
departments. There are other approved
therapeutic agents with administered activities
for which a calculated release might be needed.
Note that P-32, Sr-89 and Y-90 patients are
immediately releasable regardless of theadministered activity on the basis that theseradionuclides are pure beta emitters.

Radionuclides●●

I-131 sodium iodide for thyroid therapy
I-131-labeled radiopharmaceuticals for treating
lymphomas or neuroendocrine tumors●

Sm-153-labeled bone-seeking agents●

Various brachytherapy sources●

Numerous research radiopharmaceuticals●

P-32, Sr-89 and Y-90, which are considered tobe pure beta emitters, have no restrictions.
6

7

Exposed Persons●●

Addressed by regulatory guidance●

Most exposed person●

Nursing children

Additional concerns●

Members of the public●Children (other than nursing)●

Pregnant women●

Fellow travelers
7

8

The regulatory guidance describes methods ofincreasing sophistication (and decreasingconservatism) for release calculations.
The guidance also addresses internalization by themost exposed person of a portion of theradioactive material that was administered tothe patient.

Release Bases●

Administered Activity and Physical Half-life●

Measured Exposure Rate and Physical Half-life●

Administered Activity and Effective Half-life●

Measured Exposure Rate and Effective Half-life●●

The most exposed person is allowed to receive 5 mSv(500 mrem) on the theory that that person derivessome benefit from the exposure. This person is often
a spouse, a parent, an adult child or friend or
someone else who helps to care for the patient.
The guidance also addresses nursing children andwhether breastfeeding should be discontinued orinterrupted for a period of time to keep the dose to
the nursing child below 1 mSv (100 mrem).
Although the regulatory limit is 5 mSv (500 mrem), in
keeping with ICRP 94 and the work of Jeff Siegel
and others, in our practice we set the dose limit tothose who are especially sensitive or who would
derive no benefit from exposure to the patient to 100
mrem. These include members of the general public,
children in the patient's household, pregnant womenand fellow travelers.

Administered Activity in a Three Compartment
Model (geared toward I-131)
Internalization
8

9

The theoretical whole body time-activity curves forthese various models demonstrate a wide
variation in the cumulated activity within thepatient (i.e., the total number of disintegrations
and hence the energy radiating from thepatient).
The area under these curves represents thenumber of disintegrations that are presumed totake place within the patient and thus that havethe potential to deliver energy to the most
exposed person.

Modeling the Clearance of I-131
Retained Activity under Various Models of I-131 Clearance
100

Percentage of the Administered Activity

90
80
70
60
50
40
30
20
10
0

0

24

48

72

96

120

144

168

192

216

240

Time Post-Administation (Hours)
A0 and Tphys

A0 and Teff

Hyperthyroidism

Post-thyroidectomy

910

Occupancy Factors
E

Conditions

When the half-life is shorter than a day
1.0 – 0.75
(18-24 hrs/day)
0.25(6 hrs/day)
0.125(3 hrs/day)
0.75; 0.25
(18 hrs/day;
6 hrs/day)

When the half-life is longer than a day and the patient
can comply with lifestyle instructions that areconsistent with this occupancy factor
When the half-life is longer than a day and the patient
can comply with lifestyle instructions that areconsistent with this occupancy factor
When a two compartment model has one compartment with a half-life shorter than a day and the otherwith a half-life longer than a day, one may use as asingle occupancy factor whichever one is associated10with the compartment that dominates the dose.

11

A0 and Tphys●●

Occupancy factor of 0.25 @ 1 m if Tphys > 1 day
and 1 @ 1 m if Tphys < 1 day.
No shielding by tissue (of the patient or of the
exposed person).

1
m

11

The occupancy factor is the fraction of the day thatan exposed person is at a specified distance(typically one meter) from the patient.
The regulatory guidance gives some occupancy
factors that may be used under the statedconditions.
One can design a custom occupancy factor if
specifics of the behavior or the patient and theexposed person are known.
It is important to interview the patient in order tounderstand his or her living conditions and toassess his or her ability to be an active partnerin tempering the dose to the exposed persons.
For efficiency, this interview can be conductedby a technologist or physician at the same timethat other matters are discussed with thepatient.

It is implicit here that the patient is modeled as apoint source and that the exposed person isconsidered to be a point target. Neither is an
accurate approximation. This tends tooverestimate the dose to the exposed person.
Furthermore, assuming that the entirety of theadministered activity is cleared from the patient
only by physical decay is contrary to our clinicalexperience and might vastly overestimate thedose to the exposed person.

12

A0 and Tphys●●●

Dose rate @ 1 m = gamma ray dose constant@ 1 cm times the activity divided by (100 cm)2.
Cumulative dose is the integral duringexposure, which we typically treat as continuing
indefinitely to infinity, times the occupancy
factor, E.
The integral to infinity will be the initial dose ratetimes 1.443 times the physical half-life.
𝐷 ∞ =Γ𝑄0 1.443 24 𝑇𝑝 𝐸 36.4Γ𝑄0 𝑇𝑝 𝐸=
100cm 2100cm 212

13

A0 and Tphys
𝐷 ∞ =34.6Γ𝑄0 𝑇𝑝 𝐸
100cm 2

Tc−99m: 𝑇1⁄2 = 0.251days,𝐸 = 1.0, Γ = 0.756R−cm2 /mCi−hr
𝑄0 = 30mCi 1.11GBq :𝐷 ∞ = 0.0197rem = 0.197mSv
𝐷 ∞ = 0.5rem = 5mSv:𝑄0 = 762mCi (28.2 GBq)●

I−131: 𝑇1⁄2 = 8.04days,𝐸 = 0.25, Γ = 2.2R−cm2 /mCi−hr
𝑄0 = 30mCi 1.11GBq :𝐷 ∞ = 0.459rem = 4.59mSv
𝐷 ∞ = 0.5rem = 5mSv:𝑄0 = 32.7mCi (1.21 GBq)

Activity limits for release are tabulated in Column
13
1 of Table U.1 for common radionuclides

14●

For Tc-99m, the short half-life calls for a highoccupancy factor, E. We use E=0.25 for I-131
with its 8 day half-life.
We could give a patient as much as 760 mCi of Tc99m and release. 30 mCi of Tc-99m would
impart a very modest dose to the most exposed
person.
On the other hand, 30 mCi of I-131 is close to the
500 mrem (5 mSv) limit and 32.7 mCi (1.21
GBq) would reach it.
The regulatory guide tabulates the maximum
administered activities for many common
radionuclides that would give the most exposed
person 5 mSv under these assumptions.

To adopt a bit more realism, the exposure rate at ameter is measured, but the radioactivity in thepatient is still assumed to disappear only byphysical decay. While this ignores biologicalclearance and it treats the exposed person as apoint target, it does take into accountattenuation by the patient's body.

Measured Exposure Rate and Tphys●

The factor 34.6 is not immediately obvious. It is the
1.443 factor that comes from integrating thesingle exponential to infinity times 24, whichconverts the half-life from days to hours.
Table U.5 in the regulatory guidance tabulates
physical half-lives in days and exposure rate2
constants in R-cm /mCi-hr, so assuming that 1
R = 1 rem (= 10 mSv) we can calculate thedose.

Instead of calculating the exposure rate from Γ,measure it, X1m, (with a calibrated exposure
rate meter, preferably an ionization meter).
The measured exposure rate implicitly includes
attenuation by the patient.

1
m
14

15

In this calculation, instead of estimating theexposure rate from the administered activity,
distance from the patient and gamma ray dose
constant, we use the measured exposure rate
at 1 meter, X1m.
The regulatory guidance tabulates the maximum
exposure rate that would deliver 5 mSv to themost exposed person for a variety of common
radionuclides assuming physical decay.

Measured Exposure Rate and Tphys
𝐷 ∞ = 34.6𝑋1m 𝑇𝑝 𝐸

Tc−99m: 𝑇1⁄2 = 0.251days,𝐸 = 1.0

𝐷 ∞ = 0.5rem 5mSv : 𝑋1m = 57.6mR/hr 0.576mSv/hr
I−131: 𝑇1⁄2 = 8.04days,𝐸 = 0.25

𝐷 ∞ = 0.5rem 5mSv : 𝑋1m = 7.19mR/hr 0.0719mSv/hr

Dose rate limits for release are tabulated inColumn 2 of Table U.1 for common radionuclides15●

16

If one can measure the effective half-life, then it ispossible to estimate the dose to the most
exposed person more accurately.
The administered activity that would deliver 5 mSv
to the most exposed person is 87 mCi with thiseffective half-life, not 33 mCi with the physicalhalf-life.
Note that the effective half-life of the rapidly
clearing compartment in these actual I-131
patient data is only 3.4 hours, not the 8 hours
that the NRC assumes in the threecompartment models that we will discussshortly.

A0 and Teff
𝐷 ∞ =

●

34.6Γ𝑄0 𝑇eff 𝐸
100cm 2

If we can measure the patient's actual clearance, we can estimate the effective half-life.
−0.693𝑡−0.693t

Exposure = 7.26𝑒 3.403 + 25.7𝑒 72.42
𝑇eff = 3.02days

𝑄0 = 87mCi 3.2GBq →500mrem (5 mSv)

16

17

In a similar fashion to an earlier example, we cansubstitute the measured exposed rate for the
exposure rate calculated from the administeredactivity. Combined with the effective half-life, itwould take an initial exposure rate of 19 mR/hr@ 1 m to deliver 5 mSv to the most exposed
person, instead of the 7 mR/hr @ 1m that wasestimated by using the physical half-life.

Measured Exposure Rate and Teff●

𝐷 ∞ = 34.6𝑋1m 𝑇eff 𝐸

With the same effective half-life as in theprevious example, 3.02 days,
X1m = 19.1mR/hr @ 1 m → 500 mrem (5 mSv)

17

18

The previous models make sense for permanent
implants and for radiopharmaceuticals thathave single exponential clearance
characteristics.
The regulatory guidance offers a more complicatedmodel for radioiodine used to treat
hyperthyroidism or thyroid cancer.
The model consists of a period during which all of
the administered activity circulates in thepatient's body, decaying only by physicaldecay. At the end of that interval, the remaining
activity is then divided between a rapidly
clearing extra-thyroidal compartment and amore slowly clearing thyroidal compartment.

Three Compartment Model●

Designed for I-131 thyroid procedures
34.6Γ𝑄0
𝐸 𝑇 0.8
100cm 2 � 1 𝑝

𝐷 ∞ =
+𝑒

−0.693∗0.33
𝑇𝑝⏟

𝐸2 𝐹1 𝑇1eff + 𝑒

𝐄𝐱𝐭𝐫𝐚−𝐭𝐡𝐲𝐫𝐨𝐢𝐝𝐚𝐥

1−𝑒⏟−0.693∗0.33
𝑇𝑝

𝐂𝐢𝐫𝐜𝐮𝐥𝐚𝐭𝐢𝐧𝐠−0.693∗0.33
𝑇𝑝⏟

𝐸2 𝐹2 𝑇2eff

𝐓𝐡𝐲𝐫𝐨𝐢𝐝𝐚𝐥�

E1 – Occupancy factor for E2 – Occupancy factor from 8 hours
first 8 hours
onward
0.8 – initial circulating
fraction

F1 – Extrathyroidal
uptake fraction

F2 – Thyroidal
uptake fraction18

19

We generally acquire a whole body scan about 24
hours after administration of a small activity ofeither I-123 or I-131. This allows us to estimate
the uptake of I-131 in the thyroidal bed and also
to visualize metastases. If there are visible
metastases, we include them in the uptake
fraction of the thyroidal component. The extrathyroidal fraction is 1 minus the thyroidal
fraction.

Model Parameters●

Tabulated values may be used if one has notmade patient-specific measurements.
Uptake Fraction and Effective Half-life for I-131 Treatments
Extra-thyroidal
Medical Condition

Thyroidal

F1

T1eff (days)

F2

T2eff (days)

Hyperthyroidism

0.20

0.32

0.80

5.2

Post-Thyroidectomy for
Thyroid Cancer

0.95

0.32

0.05

7.3

●

We use an image-derived 24-hour uptake in the
thyroid bed and any visible metastases as the
thyroidal fraction, F2, and set F1 = 1-F2.
19

20

A0 and Three Compartment Model●

Consider 30 mCi to treat hyperthyroidism withthe standard parameters.

𝐷 ∞ =
+𝑒

−0.693∗0.33
34.6 × 2.2 × 30 �
0.75 × 8.04 × 0.8 × 1 − 𝑒 8.04
100cm 2−0.693∗0.33−0.693∗0.33
8.04
0.25 × 0.2 × 0.32 + 𝑒 8.04 0.25 × 0.8 × 5.2�𝐷 ∞ = 266mrem 2.66mSv

𝑄0 = 56.5mCi 2.09GBq → 500mrem 5mSv

20

In the case of 30 mCi of I-131 administered to treat
hyperthyroidism, we enter the appropriate
parameters in the three compartment model,using the default uptake fractions and effective
half-lives for the extra-thyroidal (0.2 and 0.33
days) and thyroidal compartments (0.8 and 5.2days) along with the standard occupancy factorof 0.25.
The dose to the most exposed person is 2.66 mSv
and we could administer 56.5 mCi to thispatient and just reach the 5 mSv limit to themost exposed person.

21

A0 and Three Compartment Model●

Consider 200 mCi to treat post-surgical thyroid
cancer remnants with the standard parameters.

𝐷 ∞ =
+𝑒

−0.693∗0.33
34.6 × 2.2 × 200 �
0.75 × 8.04 × 0.8 × 1 − 𝑒 8.04
100cm 2−0.693∗0.33−0.693∗0.33
8.04
0.25 × 0.95 × 0.32 + 𝑒 8.04 0.25 × 0.05 × 7.3�Now look at a thyroid cancer patient treated postthyroidectomy with 200 mCi, again using thedefault extrathyroidal (0.95 and 0.32 days) andthyroidal (0.05 and 7.3 days) parameters andan occupancy factor of 0.25.
This administered activity comes fairly close togiving a 5 mSv dose to the most exposed
person.

𝐷 ∞ = 453mrem 4.53mSv

𝑄0 = 220.5mCi 2.20GBq → 500mrem 5mSv

21

22

A0 and Three Compartment Model●

Consider 200 mCi to treat post-surgical thyroid
cancer remnants with 1% thyroidal uptake.

𝐷 ∞ =
+𝑒

−0.693∗0.33
34.6 × 2.2 × 200 �
0.75 × 8.04 × 0.8 × 1 − 𝑒 8.04
100cm 2−0.693∗0.33−0.693∗0.33
8.04
0.25 × 0.99 × 0.32 + 𝑒 8.04 0.25 × 0.01 × 7.3�𝐷 ∞ = 350mrem 3.50mSv

𝑄0 = 285mCi 2.85GBq → 500mrem 5mSv

22

23

Internal Dose●●●

𝐷internal = 𝑄0 × 10−5 × DCF

This is conservative by an order of magnitude
since 1 ppm is the more likely maximum
internalization in most cases.
The reference for dose conversion factors(DCF) is an EPA document that is hard to find.
The URL of a barely legible scanned copy iscited in the references.
If Dinternal < 10% of the total, it may be ignored.
23

Although the regulatory guidance says that one
may assume a 5% thyroidal compartment
uptake in the absence of measured data, wefind that many of our patients have 1% or lower
thyroidal uptake and thus the dose to the most
exposed person is much lower for the sameadministered activity. In this example of 1%
thyroidal uptake, an administered activity of 200
mCi would deliver only 3.5 mSv to the most
exposed person.
It is important to note that we encounter a small,but appreciable number of post-thyroidectomy
patients with thyroidal uptakes higher than 5%
or with substantial metastases that combinewith the thyroid bed uptake to exceed 5%.
It is thus important to measure the patient's uptake.

Although the scant empirical evidence suggeststhat the most exposed person internalizes very
little radioactivity from the released patient, theregulatory guidance includes an extremely
conservative calculation of the internal dose.
It uses 10 ppm (rather than the more generally
accepted 1 ppm) of the administered activity
and a dose conversion factor from an EPA
document by Keith Eckerman. There is a link tothis document among the references in thehandout for this talk.
If the internal dose is less than 10% of the total
dose to the most exposed person, theregulatory guidance allows us to ignore it.

24Internal Dose●●●

𝐷internal = 𝑄0 × 10−5 × DCF

DCFI-131 is 1.44×10-8 Sv/Bq (53.3 rem/mCi)
𝑄0 = 30mCi → 𝐷internal = 15.9mrem 0.159mSv

This is less than 10% of 500 mrem (5 mSv) andhence may be ignored.
𝑄0 = 200mCi → 𝐷internal = 106mrem 1.06mSv

This is more than 10% of 500 mrem (5 mSv)
and hence must be included in the dose to themost exposed person.
24

25

DL North, Uptake of I-131 in Households of Thyroid
Cancer Patients, Health Physics, 104(4):434436, 2013. The authors draw the conclusionthat the internalization advice in the regulatory
guidance is reasonable, if not conservative, andthat internalization is not a major concern.

Internal Dose●

North, et al. recently surveyed 22 households(43 people and 7 dogs) of thyroid cancer
patients treated with I-131●●●

Only 7 read above background, among them akissing boyfriend and a licking puppy
The worst internalization factor was 2×10-5
However, note that all patients had been
hospitalized overnight, thereby excluding the timewith the greatest potential for contamination of thehousehold.

26

Instructions and Records●●

In the case of a 30 mCi hyperthyroidism treatment,
the internal dose is 160 uSv and it can beignored. However, for a 200 mCi postthyroidectomy treatment, the internal dose is
1.06 mSv and must be included in the total
dose to the most exposed person.
The 10% rule is insidious, for the more accurately
the dose to the most exposed person ismodeled (and hence likely lower), the more
likely it is that the estimate of the internalized
dose will exceed 10% of the external dose andthus would need to be included in the total
dose.

The patient must be instructed in how tominimize the dose to the various exposed
persons if she is breast-feeding or whenever
the most exposed person would receive morethan 1 mSv (100 mrem).
A record of the release must be kept if thepatient is nursing a child, if the release is on any
other basis than that of physical decay of theadministered activity or if the most exposed
person would likely receive more than 1 mSv.
26

It is important to give patients generic instructions
regarding radiation safety, such as sitting tourinate and then flushing the toilet three times.
The patient should be given specific instructions
that include the durations of behavioral
restrictions, including breastfeeding if thepatient is nursing a child.
Record keeping is required under any circumstancethat involves a dose to the most exposed
person that exceeds 1 mSv or where that dose
estimate depends on a patient-specificmeasurement. The record should identify the
personnel involved and the specific instrumentsthat were used.
The details are given in Table U.4 of the regulatory
guide.

27

The Need for Prediction●●●

The regulatory guidance tells how to calculate
D(∞) at the time of administration or exposure
measurement, but not how long one ought todelay a release in order to get D(∞) under 500
mrem (5 mSv) or 100 mrem (1 mSv).
Some facilities must know prior to treatmentthat a patient will be an out-patient because oflimited suitable in-patient rooms.
Patients need to make travel, housing andfamily care plans prospectively.

27

28This example is based upon extremely unrealistic
values in order to illustrate the point. In thiscontrived case, we would hold the patient for
two days in order to get the dose to the most
exposed person down to 5 mSv and would
instruct the patient to avoid children andpregnant women entirely for 20 days to get theirdoses below 1 mSv.
We need to be able to use the measurements fromthe patient's uptake scan in order to predictthese restrictions ahead of time as sometimes
patient's plans cannot accommodate therequirements of safety.

Prediction●●

We calculate D(∞) as a function of the start of
exposure and work backwards up the cumulative dose curve until the dose limit is reached.
That is the earliest allowable release time.
600

Dose (mrem)

500
400
300
200
100
0

0

7

14

21

28

35

42

49

56

63

Release Time (Days post-Treatment)
Limit 500 mrem
Limit 100 mrem28

Dose(TR to Infinity) < 100 mrem
Dose(TR to Infinity) > 100 mrem

29

Prediction●

The rapid clearance of the three compartment
model facilitates prompt release.
This is particularly effective with post-thyroidectomy patients.
600

500

Dose to Infinity (mrem)

●

400

300

200

100

0
0

1

2

3

4

5

6

7

Release after Treatment (Days)

8

9

10

29

For thyroid therapy patients, and particularly forpost-thyroidectomy patients, the rapid
clearance of the extra-thyroidal compartment
means that it takes only a few hours for thedose to the most exposed person to fall below 5
mSv.
We keep our I-131 out-patients for at least twohours as a matter of course, so they typicallyhave no restrictions with respect to the most
exposed person immediately upon release.
However, they will have restrictions with respect toother aspects of their lives, as evident in the
additional time that is required for the dose out
to infinity to fall below 1 mSv.

30

Prediction●

Sometimes searching for the answer is easiest.
D(TR→∞)

2 hours

5.04 mSv

5.5 days

1.03 mSv

One cannot easily calculate the release time
analytically for the more complicated models.
A straightforward approach is to develop a
spreadsheet that tabulates the cumulative
doses for various release times. One thensimply searches through the table to find therelease time at which the cumulative dose has
fallen to the desired level.
Another approach is to use the calculators on the
RADAR Web site and adjust the release time
until the dose is below the desired level.

30

31Modeling Behavior●●

The regulatory guidance does not address how
to handle any exposed person other than themost exposed.
We have adapted the models for members of
the public, fellow travelers, children andpregnant women, and sleeping partners that
Jeff Siegel developed for the I-131-labeled
radiopharmaceutical Bexxar.

31

32

Supposing that Mom has had radioiodine treatmentof her thyroid gland, Dad is likely the most
exposed person. He clearly derives benefit fromcaring for his wife in their home and thus the 5
mSv limit makes sense.
Dad's occupancy factor is based upon no exposure
between Mom's treatment and release and thensix hours a day at a distance of one meter.

Most Exposed Person…
No Exposure

6 hours a day @ 1 m

TR – Release Time

32

33

Traveler

No Exposure

X hours @ Y distance

Map data © Google, 201233

Conclusion of Travel

Start of Travel

34

The model of exposure to a sleeping partner iscurious. It assumes only 4.5 hours a day at a
distance of one meter after a period of no
exposure.
Following a period of sleeping apart, an exposureof six hours a day at 0.3 meters is added to the
4.5 hours a day at a meter.

Sleeping Partner…
No Exposure

4.5 hours a day @ 1 m

4.5 hours a day @ 1 m+

Another reason to interview patients regarding theirlifestyles is that some patients routinely sleep inclose proximity to their children. In such a
scenario, the use of a 1 mSv total dose rather
than a 5 mSv total dose might be warranted.

6 hours a day @ 0.3 m

34

TR – Release Time

Resume Close Sleeping

35

Mom and Dad have a daughter, a member of asensitive population, whose dose we would liketo limit to 1 mSv.
The model has three phases like the sleeping
partner model.
If little Suzy can be sent to Grandma's andGrandpa's for a few days, she could thenresume limited contact with Mom, which wemodel as 30 minutes a day at 1 m and 6
minutes a day at 0.3 m. After a period of limited
contact, Mom and Suzy can resume regular
activities, which we model as 6 hours a day at 1
m and 30 minutes a day at 0.3 m.

Children (and Pregnant Women)…
No Exposure

30 mins a day @ 1 m

6 hours a day @ 1 m++

6 mins a day @ 0.3 m

30 mins a day @ 0.3 m

TL – Limited Contact

If Mom will be flying home to rejoin her family, themost exposed traveler is likely to be a stranger.
It is hard to justify a dose to the stranger inexcess of 1 mSv, although the regulatory
guidance does not technically forbid it.
One can calculate the dose from an exposure forthe duration of the trip at typical traveling
distances (such as sitting cheek-to-jowl on mostcommercial airliners).
We sometimes have to tell patients not to travel fora day or two after their release (especially ifthey plan a long trip).
If Dad will be driving Mom home, the travel time
typically comes out of his 6 hours at a meter forthat day unless there is an extraordinary
circumstance.

Resume Normal Contact

35

36

Finally, members of the general public, such as
regular customers, co-workers, or others who
encounter the patient regularly, should be
limited to a dose below 1 mSv.
We model the patient's contact with these people
as a period of no exposure followed by six
hours a day at 1 m.

Member of the General Public…
No Exposure

6 hours a day @ 1 m

TR – Release Time

36

37

A hypothetical post-thyroidectomy patient with a 5%
uptake who received 200 mCi just a few
minutes ago could be released after 2-1/2
hours.
He should not start his four hour plane flight until
mid-morning tomorrow. He should sleep alone
for about a month. He should avoid his kids
entirely for two days and then limit time withthem for another ten days. He should not goback to work or spend time around strangers
until Friday morning.
Under the A0 and Teff model, this patient would
have been hospitalized for almost three weeks.

Instructions – 200 mCi, 5% Uptake

37

38

Although this approach is usually quite effective,
the RADAR experts argue that the regulatory
guidance is still too conservative.
They advocate including a factor of 0.6 to reflect
the attenuation by the patient and theelimination of the circulating component of thethree-compartment model. These refinements
are incorporated into their online calculators
mentioned earlier.
The regulatory guidance is just that – guidance –and other methods of release calculation, suchas the RADAR approach, are valid as long as
the basis of the release is correctly calculated
and thoroughly documented.

RADAR Approach
●

●

●

●

Incorporates a factor of 0.6 to account forpatient self-shielding.
Eliminates the circulating (“pre-voiding”)
component of the three-compartment model.
RADAR-based dose estimates are roughly half
those made by the method in the regulatory
guidance.
This is acceptable, provided that one
documents the details of the calculation.
38

39

Yi, et al., have reported that a shielded line source is agood model for post-thyroidectomy I-131 therapy
patients.
De Carvalho, et al., used voxelized phantoms for thepatient and the exposed person and calculated theeffective dose from three types of treatments. They
compared these to doses from point and line
sources. In general, the dose from the more realistic,
3D source was about 50-70% of that from the point
source at a distance of one meter. At a distance of
0.3 m, the 3D source doses were more like 25% of
those from a point source.
Y Yi, MG Stabin, MH McKaskle, MD Shone and AB
Johnson, Comparison of Measured and CalculatedDose Rates Near Nuclear Medicine Patients, Health
Physics, 105(2):187-191, 2013.
AB de Carvalho, MG Stabin, JA Siegel and J Hunt,
Comparison of Point, Line and Volume Dose
Calculations for Exposure to Nuclear Medicine
Therapy Patients, Health Physics, 100(2):185-190,
2011.

Better Physical Models●

Representing the patient and the exposed
person by points is very conservative.

40

Resources●●●

RAdiation Dose Assessment Resource Patient Release
Criteria: http://www.doseinfo-radar.com/RADAR-INT-NMRelease.html
RADAR Patient Release Calculators: http://www.doseinforadar.com/ExposureCalculator.html
SNM Guidelines for I-131 Patients:
http://www.snm.org/docs/Guidelines_for_Patients_Receivi
ng_Radioiodine_Treatment.pdf

40

The RAdiation Dose Assessment Resource at
http://www.doseinfo-radar.com/ has a wealth of
information on many aspects of radiation
protection and internal dosimetry. In particular,
RADAR members have been active in thedevelopment of the risk-based approach topatient release.

41

Conclusions
●

●
●

●

The patient release calculations in NUREG
1556 are straightforward.
The simplest ones tend to be very conservative.
They can be made more realistic by using
patient-specific parameters that are based on
direct measurements and on modeling of thepatient's lifestyle.
More realistic 3D modeling of sources andtargets yields doses that are even more
realistic.

41

42

References●●●

NUREG 1556, Volume 9, Revision 2, Appendix U,
http://www.nrc.gov/reading-rm/doccollections/nuregs/staff/sr1556/v9/r2/
Limiting Values of Radionuclide Intake and Air Concentration
and Dose Conversion Factors for Inhalation, Submersion andIngestion, http://www.epa.gov/radiation/docs/federal/520-1-88020.pdf
NUREG 1492: Regulatory Analysis on Criteria for the Releaseof Patients Administered Radioactive Materials, US NRC, 1997,
http://www.osti.gov/bridge/servlets/purl/453778p8OWG6/webviewable/453778.pdf

42

DB Howe, M Beardsley and SR Bakhsh, Model
Procedure for Release of Patients or Human
Research Subjects Administered Radioactive
Materials, in Consolidated Guidance about Materials
Licenses: Program-Specific Guidance about Medical
Use Licenses, Final Report, Nuclear Regulatory
Commission, NUREG 1556, Volume 9, Revision 2,
Appendix U, January 2008,
http://www.nrc.gov/reading-rm/doccollections/nuregs/staff/sr1556/v9/r2/sr1556v9r2final.pdf#app-u
KF Eckerman, AB Wolbarst and ACB Richardson,
Limiting Values of Radionuclide Intake and Air
Concentration and Dose Conversion Factors forInhalation, Submersion and Ingestion, Federal
Guidance Report No. 11, Environmental Protection
Agency 520/1-88-020, September 1988,
http://www.epa.gov/radiation/docs/federal/520-1-88020.pdf.
S. Schneider and S.A. McGuire, NUREG 1492:
Regulatory Analysis on Criteria for the Release ofPatients Administered Radioactive Material, US
Nuclear Regulatory Commission, February, 1997,
http://www.osti.gov/bridge/servlets/purl/453778p8OWG6/webviewable/453778.pdf .

43

JA Siegel, et al., A Practical Methodology for Patient
Release after Tositumomab and (I-131)Tositumomab Therapy, Journal of Nuclear Medicine
43(3):354-363, 2002,
http://jnm.snmjournals.org/content/43/3/354.full.pdf
JA Siegel, CA Marcus, and MG Stabin, Licensee OverReliance on Conservatisms in NRC Guidance
131
Regarding the Release of Patients Treated with I,
Health Physics, 93(6):667-677, 2007.
D Fisher, et al., Patient Release Report, Advisory
Committee on the Medical Use of Isotopes, Nuclear
Regulatory Commission, 2010,https://rpop.iaea.org/RPOP/RPoP/Content/Documents/Whitepapers/NRC-release-of-patients.pdf

References●●●

A Practical Methodology for Patient Release after
Tositumomab and (I-131)-Tositomomab Therapy, JA
Siegel, et al.,
http://jnm.snmjournals.org/content/43/3/354.full.pdf
Licensee Over-Reliance on Conservatisms in NRC
Guidance Regarding the Release of Patients Treated with
131I, JA Siegel, et al., Health Physics 93(6):667-677,
2007.
NRC ACMUI Patient Release Report,
https://rpop.iaea.org/RPOP/RPoP/Content/Documents/Whi
tepapers/NRC-release-of-patients.pdf
4344

References
●

●

●

●

●

Release of Patients after Radionuclide Therapy, IAEA, 2009,http://www-pub.iaea.org/MTCD/Publications/PDF/pub1417_web.pdf
Release of Patients Treated with Therapeutic Quantities ofRadiopharmaceuticals and Sealed Sources, Health Physics Society,PS027-0, http://hps.org/documents/patientrelease_ps027-0.pdf
Comparison of Measured and Calculated Dose Rates near Nuclear
Medicine Patients, Health Physics, 105(2):187-191, 2013.
Comparison of Point, Line and Volume Dose Calculations for
Exposure to Nuclear Medicine Therapy Patients, Health Physics,100(2):185-190, 2011.
Uptake of I-131 in Households of Thyroid Cancer Patients, Health
Physics, 104(4):434-436, 2013.
44

International Atomic Energy Agency, Release of Patients
after Radionuclide Therapy, Report 63, Safety
Reports Series, Vienna: IAEA, 2009, http://wwwpub.iaea.org/MTCD/Publications/PDF/pub1417_web.
pdf .
Release of Patients Treated with Therapeutic Quantities
of Radiopharmaceuticals and Sealed Sources:
Position Statement of the Health Physics Society,
March 2012,
http://hps.org/documents/patientrelease_ps027-0.pdf
A.B. de Carvalho, MG Stabin, JA Siegel, and J Hunt,
Comparison of Point, Line and Volume Dose
Calculations for Exposure to Nuclear Medicine
Therapy Patients, Health Physics, 100(2):185-190,
2011.
Y Yi, MG Stabin, MH McKaskle, MD Shone and AB
Johnson, Comparison of Measured and CalculatedDose Rates Near Nuclear Medicine Patients, Health
Physics, 105(2):187-191, 2013.
D.L. North, Uptake of I-131 in Households of Thyroid
Cancer Patients, Health Physics, 104(4):434-436,
2013.