Keywords radioiodine therapy - thyrotoxicosis - ablation - outpatient - radiation exposure
Introduction
Since 1940s, radioiodine therapy (I-131) has been the most widely used method for
the treatment of various benign and malignant thyroid disorders like hyperthyroidism
and thyroid carcinoma.[1 ] Although the radioiodine therapy with I-131 is safe and has fewer chances of side
effects, there is a major risk of external radiation hazard due to its gamma emission.[2 ] Because of the strong penetration power and relatively high energy of gamma radiation
emitted from I-131, radiation safety becomes a major concern. Because the high energy
gamma radiation can penetrate through the patient's body and can cause external radiation
hazards to the persons around the patient especially to the children and pregnant
females as they are more sensitive to radiation.[3 ]
[4 ] In radioiodine therapy, proper radiation safety practices and precautions are mandatory
to minimize unnecessary radiation exposure to the family of the patient, occupational
workers treating the patient, and general public.[5 ]
Iodine-131 undergoes beta minus decay with the emission of gamma radiation of 364
keV along with the emission of high-energy beta particles, a maximum energy of 0.61
MeV, and an average energy of 0.192 MeV. Because of its long physical half-life of
8.1 days and trapping as well as organification capacity by thyroid, it is the ideal
physiological radioisotope of choice for the treatment.[1 ] Radioiodine therapy is indicated for patients suffering from hyperthyroidism mainly
due to Grave's disease, nontoxic multinodular goiter, and thyroid carcinoma. Severe
uncontrolled thyrotoxicosis and in female patients pregnancy and breastfeeding are
common contraindications for radioiodine therapy.[1 ] Therapeutic doses of I-131 commonly range from 185 MBq (5 mCi) to 11,100 MBq (300
mCi). Lower radioactivity is usually recommended to treat Grave's or toxic Goiter.
Higher doses are used to ablate the remnant thyroid tissue and treat metastatic disease
in patients with differentiated thyroid carcinoma. Higher doses require hospitalization
of patients in specially designed isolation wards for a period until the radiation
exposure rate of the patient comes under permissible discharge limits prescribed by
the competent authority. These criteria for the discharge limit of the patients vary
from country to country. As per the International Atomic Energy Agency (IAEA), patients
undergoing radioiodine therapy could be discharged if I-131 activity is given up to
1,110 MBq (30 mCi). Earlier, according to the competent authority of India, Atomic
Energy Regulatory Board (AERB), radioiodine patients could be discharged if I-131
activity given was up to 555 MBq (15 mCi) and the radiation level at the distance
of 1 m from the patient did not exceed 20–30 µSv/h (2.5 mR/h) at the time of discharge.[6 ] In 2011, the competent authority of India revised its criteria for radioiodine patient's
discharge, allowing the discharge of patients for higher I-131 activity up to 1,110
MBq (30 mCi) and the radiation level at the distance of 1 m from the patient should
not exceed 50 µSv/h (5 mR/h).[7 ]
The use of a radiation monitor is the most accepted method for the measurement of
external radiation exposure of patients.[8 ] The aim of this study is to determine the radiation exposure rate as the function
of time in outpatient administration of radioiodine I-131 in the treatment of thyrotoxicosis
and remnant ablation using a radiation monitor in the region of the Himalayan range
of Uttarakhand and to study the impact of revised discharge criteria for radioiodine
therapy enforced by regulatory authority AERB India.
Material and Methods
Patients
The study was performed at the Nuclear Medicine Department, Cancer Research Institute,
Swami Rama Himalayan University (SRHU) Dehradun, Uttarakhand. The patients who received
low-dose radioiodine (I-131) therapy were referred from Himalayan Hospital, Swami
Rama Himalayan University. Written informed consent was taken from all patients undergoing
radioiodine therapy following the detailed verbal and written explanation of the whole
procedure and possible associated risks involved. This study population was categorized
into two groups: (group A) patients with thyrotoxicosis disease and (group B) patients
for the ablation of residual thyroid tissue after total thyroidectomy.
Patient Preparation
Antithyroid drugs like neomercazole, carbimazole were stopped 1 week before the therapy
for patients suffering from thyrotoxicosis. For remnant ablation, therapy was given
after 1 month of thyroidectomy. In female patients of the reproductive age group,
pregnancy was ruled out and patients were counseled to avoid pregnancy for 6 to 12
months after the therapy. Pregnant females and lactating mothers were excluded from
the study. Patients were instructed to take lemon in any form (juice, candy, Vitamin
C tablets, etc.) to minimize the radiation dose to salivary glands. In patients, increased
fluids/water intake was encouraged and frequent voiding was suggested after the therapy
to minimize the radiation dose to gonads, kidneys, and urinary bladder. Patients were
not allowed to eat 2 hours before and after administration of I-131 dose to minimize
the radiation dose to the stomach and reduce the chances of developing nausea. Patients
were also instructed to sleep alone and to avoid close contact with pregnant females
and children for a period of time specified in accordance to the radioactivity dose
given to the patient.
Oral Administration of Radioiodine
The radioiodine (I-131), sodium iodide solution supplied by BRIT Mumbai, India. I-131
doses were measured in well-calibrated dose calibrator (CAPINTEC- CRC 25 R) and properly
tagged with the details like activity, time, date, and patient's name. Before giving
the therapy, identify the patient to avoid the misadministration. The measured and
tagged I-131 dose was orally administered to the patient inside the well-ventilated
fume hood. All work surfaces were properly covered with absorbent sheets backed with
plastic sheets. All safety measures were taken during the handling and administration
of radioiodine. In group A, the administered activity of I-131 was 207.2 ± 55.5 MBq
(5.6 ± 1.5 mCi) with range 96.2 to 543.9 MBq (2.6–14.7 mCi), whereas in group B the
administered activity was 725.2 ± 188.7 MBq (19.6 ± 5.1 mCi) with range 447.7 to 1091.5
MBq (12.1–29.5 mCi).
Exposure Rate Measurement
After oral administration of radioiodine, the radiation exposure rate of the patients
was measured using a radiation survey meter (Victoreen, United States, 451B-RYR) based
on the ionization chamber. This is a handheld-type radiation detector with digital
reading facility. The measuring range of detector was 0.1 μSv/h to 100 Sv/h. The radiation
exposure rate of patient was measured in mR/h at the distance of 5 cm distance from
stomach and neck levels, and at the distance of 1 m with patient standing after oral
administration of I-131 at 0, 1, and 2 hours. Patients were kept under observation
up to 2 hours of administration of I-131 in postadministration patient's waiting area
and then allowed to go home.
Results
A total of 134 patients (117 females and 17 males) were included in the study. The
mean age of the patients was 41 years with range 10 to 75 years. The group A comprised
102 patients receiving low-dose radioiodine (I-131) therapy for patients with thyrotoxicosis,
whereas group B included 32 patients receiving radioiodine therapy for the ablation
of residual thyroid tissue after total thyroidectomy. The I-131 dose given to Group
B was significantly higher compared to the dose given to Group A.
Group A includes 102 patients (88 females and 14 males) who underwent radioiodine
therapy for thyrotoxicosis. All readings taken for the exposure rate for every patient
included in the study were normalized with respect to the radioiodine activity administered
to the patient to get a clear picture of radioactivity distribution pattern. [Table 1 ] shows the normalized average exposure rate at neck, stomach, and distance of 1 m
at different time intervals for patients who underwent radioiodine therapy for thyrotoxicosis
(group A).
Table 1
Normalized average exposure rate for group A patients at neck, stomach, and distance
of 1 m from patients for different time intervals where T′0 is time of I-131 administration,
T′1 is 1 hour after I-131 administration, T′2 is 2 hours after I-131 administration
T′0 (mR/h/mCi)
T′1 (mR/h/mCi)
T′2 (mR/h/mCi)
Neck
1.2
6.07
7.57
Stomach
4.2
2.43
1.8
1 m distance from patient
0.23
0.19
0.17
However, [Fig. 1 ] summarizes the pattern of average activity or the average exposure rate at the level
of neck, stomach, and at 1 m distance from patient at the time interval of T′0, T′1,
T′2 in group A.
Fig. 1 Pattern of average activity or average exposure rate at level of neck, stomach, and
at the distance of 1 m from patient at the time interval of T′0, T′1, and T′2 in Group
A, where T′0 time of I-131 administration, T′1 1 hour after I-131 administration,
T′2 2 hours after I-131 administration.
Group B includes 32 patients (29 females and 3 males) underwent radioiodine therapy
for the ablation of residual thyroid tissue after total thyroidectomy. [Table 2 ] shows the normalized average exposure rate at neck, stomach, and distance of 1 m
from patients at different time intervals of radioiodine therapy given. However, [Fig. 2 ] summarizes the pattern of average activity or the average exposure rate at the level
of neck, stomach, and at the distance of 1m from patient at the time interval of T′0,
T′1, and T′2 in group B.
Fig. 2 Pattern of average activity or average exposure rate at the level of neck, stomach,
and at distance of 1 m from the patient at the time interval of T′0, T′1, T′2 in Group
B, where T′0 time of I-131 administration, T′1 1 hour after I-131 administration,
T′2 2 hours after I-131 administration.
Table 2
Normalized average exposure rate for group B patients at neck, stomach and distance
of 1 m from patients for different time intervals where T′0 is time of I-131 administration,
T′1 is 1 hour after I-131 administration, T′2 is 2 hours after I-131 administration
T′0 (mR/h/mCi)
T′1 (mR/h/mCi)
T′2 (mR/h/mCi)
Neck
1.1
2.21
2.5
Stomach
3.5
2.5
2.04
1 m distance from patient
0.16
0.13
0.11
This was observed that the normalized average exposure rate was lower for group B
as compare to group A.
Discussion
High-energy and penetration power of gamma emission of I-131 makes it a major source
of external exposure and the volatile nature of I-131 leads to the chances of internal
exposure too, by inhalation of radioactive vapors. Due to all these factors, radiation
protection becomes a very sensitive issue in radioiodine therapy to avoid unnecessary
exposure to occupational workers, patients, as well as general public. Guidelines
of safe handling and radiation safety aspects are well outlined by IAEA reports and
implemented by AERB in India.[9 ]
[10 ] According to the guidelines, after I-131 therapy radiation exposure should be measured
in all patients before their discharge from hospital to assure that the external exposure
rate from the patient underwent therapy is under permissible limits prescribed by
competitive authority of AERB, India and safe for patient's family and general public.
The administered radioiodine is absorbed in the functioning thyroid tissue and gets
washed out through bowels and kidneys, resulting in whole body radiation burden. The
present study was designed to study the pattern of radiation exposure in patients
belonging to the Himalayan region of Uttarakhand because this Himalayan region is
an iodine-deficient region where a small amount of radioiodine dose can attribute
to high-radiation exposure. However, the present study depicts that patients undergoing
radioiodine therapy with radioactivity dose up to 1,110 MBq (30 mCi) can be safely
managed in the outpatient clinic.
In group A, the initial exposure rate measured after the oral administration of I-131
at the surface of stomach was higher as compare to the exposure rate at the neck level
and distance of 1 m from patient. At the time interval of 1 and 2 hours of I-131 administration,
the exposure rate at the neck level was increasing and at surface of stomach the exposure
rate was decreasing. This shows that iodine was slowly taken up by the thyroid gland.
Similarly, the exposure rate at the distance of 1 m from patient was also decreasing,
which suggests that patient can be safely discharged after 2 hours of I-131 administration
as the exposure rate from patient becomes very minimal.
The same pattern of activity and exposure distribution was observed in group B as
was observed in group A. But in patients treated for ablations of residual thyroid
tissue after thyroidectomy in group B, radioiodine activity gets cleared from bowels
at the faster rate as compared to the patients treated for thyrotoxicosis resulting
into lesser radiation burden. Ravichandran et al quoted that after thyroidectomy,
because of the surgical excision of the thyroid gland, radioiodine (I-131) gets washed
out from the patient's body at a faster rate.[8 ] Hänscheid et al and Sisson et al also stated that the lack of thyroid gland and
higher renal clearance in thyroid cancer patient causes faster excretion of activity
and reduces the exposure rate more rapidly during radioiodine ablation.[11 ]
[12 ]
Immediately after the oral administration of I-131, the normalized average exposure
rate at the distance of 1 m distance was 0.23 and 0.16 mR/h, respectively, for thyrotoxicosis
and ca thyroid patient. Exposure rate at the distance of 1 m after administering the
maximum of 1,091.5 MBq (29.5 mCi) of I-131 was observed to be 4.8 mR/h which lowered
to 3 mR/h after 2 hours of I-131 administration. Maximum exposure rate observed at
the distance of 1 m of patient was 5.1 mR/h which lowered to 3.6 mR/h at 2 hours of
I-131 administration, this patient was administered with 917.6 MBq (24.8 mCi) of I-131.
These observations also emphasize practicing the 2 hours observation period posttherapy
to ensure that the exposure rate is under permissible limits of discharge criteria.
Also, suppose any kind of medical emergency happens post-radio-iodine dose administration.
Radiation professionals can better handle it inside the hospital premises, further
lowering the risk of unnecessary radiation burden to the patient's family and the
general public. Results of present study showed that exposure rates measured for all
patients were under permissible limits as per revised discharged limits prescribed
by AERB. So the revised permissible limits prescribed by AERB India, for discharge
criteria after radioiodine therapy can be implemented successfully.
Conclusion
As the exposure rate measured for patients treated with up to 1,110 MBq (30 mCi) of
I-131 was under permissible limits as per revised discharged limits prescribed by
AERB, based upon our results, it is suggested that with good work practice and thorough
radiation safety instructions to patients and their attendants, thyrotoxicosis patients
and patients for remnant ablation treating with radioactivity dose up to (1,110 MBq)
30 mCi of I-131 can be discharged safely after 2 hours of administration of I-131
as outpatients. This practice will reduce the need for additional isolation wards
and cost of treatment for hospital as well as for the patients too.
