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Grigsby PW, Siegel BA, Baker S, Eichling JO. Radiation Exposure From Outpatient Radioactive Iodine (131I) Therapy for Thyroid Carcinoma. JAMA. 2000;283(17):2272–2274. doi:10.1001/jama.283.17.2272
Context In May 1997, the US Nuclear Regulatory Commission (NRC) revised its
patient release regulations, allowing for outpatient administration of larger
activities of sodium iodide 131I than previously permitted.
Objective To measure the radiation exposure to household members from patients
receiving outpatient 131I therapy for thyroid carcinoma in accordance
with the new regulations.
Design Consecutive case series from October 1998 to June 1999.
Setting and Patients Thirty patients who received outpatient 131I therapy following
thyroidectomy for differentiated thyroid carcinoma were enrolled, along with
their 65 household members and 17 household pets.
Main Outcome Measure Radiation exposure to household members and 4 rooms in each home, as
monitored with dosimeters for 10 days following 131I administration.
Results The patients received 131I doses ranging from 2.8 to 5.6
GBq (mean, 4.3 GBq). The radiation dose to 65 household members ranged from
0.01 mSv to 1.09 mSv (mean, 0.24 mSv). The dose to 17 household pets ranged
from 0.02 mSv to 1.11 mSv (mean, 0.37 mSv). The mean dose to the 4 rooms ranged
from 0.17 mSv (kitchen) to 0.58 mSv (bedroom).
Conclusion In our study, 131I doses to household members of patients
receiving outpatient 131I therapy were well below the limit (5.0
mSv) mandated by current NRC regulations.
In May 1997, the US Nuclear Regulatory Commission revised its patient
release regulations.1 Under the previous rule,
patients receiving sodium iodide 131I therapy could not be released
from medical confinement until the exposure rate was less than 12.9 ×
10−7C/kg/h (5 mR/h) at a distance of 1 m from the patient
or until the patient's radionuclide activity was less than 1.1 GBq. Accordingly,
patients treated with large doses of 131I for thyroid cancer typically
were hospitalized under virtual isolation conditions for up to several days
The new rule allows patients to be released from control by the licensee
if the total effective dose equivalent (TEDE) to any other individual resulting
from exposure to the treated individual is not likely to exceed 5.0 mSv. US
Nuclear Regulatory Commission regulatory guide 8.39 describes 3 options for
patient release after 131I therapy in accordance with the new regulatory
requirement2: release of patients based on
administered activity (<1.2 GBq); release of patients based on measured
dose rate (<18.1 × 10−7C/kg/h [7 mR/h] at 1 m);
and release of patients based on a patient-specific calculation of the likely
exposure to the maximally exposed individual (TEDE <5.0 mSv).
The first and second options represent default values and are conservative,
chiefly because they assume that elimination of 131I occurs only
by physical decay. The objective of this study was to measure the radiation
dose to household members from patients who received outpatient 131I
therapy for thyroid carcinoma with administered activities exceeding these
default criteria, in accordance with these revised regulations.
Thirty consecutive patients willing to participate in the study and
all of their household members were entered in this study from October 1998
to June 1999. All patients signed a study-specific consent form approved by
the Washington University Human Studies Committee. All patients previously
had undergone a total thyroidectomy for papillary or mixed papillary-follicular
The estimated TEDE to the maximally exposed person was calculated using
the formula given in equation B-5 of regulatory guide 8.39.2
The TEDE calculated by this method depends on several different variables,
including the fractional uptake of 131I in thyroid tissue, the
effective half-lives of 131I in thyroid and extrathyroidal tissues,
and the occupancy factor (ie, the fraction of time the exposed person resides
at a distance of 1 m from the patient). We also used the 131I effective
half-life values and the occupancy factors recommended in the guide.2
To estimate the fractional uptake of 131I in thyroid tissue
before therapeutic administration of 131I, we performed a 48-hour
total-body 131I-retention study. Patients scheduled for their first
postthyroidectomy 131I treatment received 37 MBq of 131I
for the retention study. These patients subsequently underwent whole-body
imaging 3 to 5 days after the therapeutic 131I administration.
Patients undergoing follow-up evaluation for possible 131I treatment
of residual or recurrent cancer received 185 MBq of 131I to allow
for both whole-body 131I scintigraphy and the retention study.
For the retention study, the patient's total-body counts were measured with
a sodium iodide probe at a distance of 3.1 m from the xiphoid process. The
measurements were obtained 15 minutes after 131I administration
and again at approximately 48 hours. The ratio of the 48-hour activity to
the 15-minute activity, corrected for background and decay, was calculated.
This fractional whole-body 131I retention was conservatively assumed
to represent the thyroidal fraction. This value was used in a patient-specific
calculation to determine whether administration of the prescribed activity
would permit release of the patient. If the calculated radiation dose to the
maximally exposed person was less than 5.0 mSv, the patient qualified for
The patients, their household members, household pets, and 4 rooms in
their homes (bedroom, bathroom, living room, and kitchen) were continuously
monitored with optically stimulated luminescence dosimeters for the first
10 days after outpatient therapeutic 131I administration.
Patients were instructed to sleep alone, drink fluids liberally, and
avoid prolonged close personal contact with others for the first 2 days after 131I administration. Patients and family members were told that they
could resume normal activities thereafter. All participants were instructed
to wear the dosimeters 24 h/d for the 10-day period.
The patient population consisted of 22 females and 8 males, ranging
in age from 9 to 76 years old (mean, 42 years). Sixty-five household members
participated in this study: 41 males and 24 females, ranging in age from younger
than 1 year to 78 years old (mean, 28 years). Thirty household members were
younger than 19 years (range, 1-18; mean, 9.4; median, 9.5 years). Doses also
were monitored in 17 household pets.
The 48-hour whole-body 131I retention ranged from 0.7% to
21.5% (mean, 8.4%). The patients were treated with 2.8 to 5.6 GBq of 131I (mean, 4.3 GBq). The estimated TEDE to the maximally exposed person
(spouse, parent) ranged from 1.63 to 4.83 mSv (mean, 3.12 mSv).
The measured radiation dose to all household members ranged from 0.01
mSv to 1.09 mSv (mean, 0.24 mSv) (Figure 1 and Table 1). The dose
to household pets was of similar magnitude, ranging from 0.02 to 1.11 mSv
(mean, 0.37 mSv). The measured radiation in the patients' homes was greatest
in their bedrooms (Table 1).
Radiation exposure to household members from patients treated with 131I for thyroid carcinoma has been measured or estimated in earlier
studies but under different circumstances than in our study.3,4
Because of the previous regulatory restrictions, none of the prior studies
performed in the United States directly measured household members' exposure
during the first few days after outpatient administration of 131I.
Our study is unique because our patients received therapeutic quantities of
sodium iodide and were immediately released.
One limitation of our study was that thyroid bioassays were not performed
and internal doses to household members from ingested 131I were
not evaluated. However, other investigators have shown that internal doses
resulting from contamination and intake of 131I are likely to be
much smaller than external exposure to radiation from patients.5,6
The measured exposures in this study reflect the first 10 days after treatment.
However, the 10-day cumulative exposure represents most of the theoretical
dose; for the average thyroid uptake in this study, the 10-day cumulative
exposure accounts for 84% of the exposure, to infinity. Another limitation
of our study was the potential for noncompliance of family members: if they
did not wear their dosimeters as instructed, the reported absorbed doses would
be underestimates. One can assume that the doses recorded in the 4 living
areas reflect 100% compliance and that household pets were 100% compliant.
As expected, among the doses to the living areas, the bedroom doses were the
greatest. The bedroom doses and the doses to the pets are of the same magnitude
as the doses to the household members, and, therefore, we believe that recorded
doses in household members are reasonably accurate. Finally, this is a small
series; thus, it is possible that larger exposures than we observed might
be encountered in some household members of patients treated with 131I.
Our method of estimating the TEDE to the maximally exposed person is
very conservative, because we assume that the total-body 131I retention
at 48 hours is the thyroidal component with a long half-life. This study demonstrates
that patients can be administered outpatient 131I therapy for thyroid
carcinoma and that the resultant radiation exposure to household members is
well below the limit mandated by the new US Nuclear Regulatory Commission
regulations.1 Advantages of outpatient 131I therapy for thyroid carcinoma likely include reduced expense of
treatment and less psychological strain on patients and their families.