Estimation of effective half life of clearance of radioactive Iodine (¹³¹I) in patients treated for hyperthyroidism and carcinoma thyroid

INTRODUCTION

Treatments for ablation of functioning thyroid for management of hyperthyroidism and for the elimination of post-surgical residual thyroid tissues in differentiated carcinoma of thyroid by administration of radioactive iodine (RA¹³¹ I) are well-established procedures in clinical nuclear medicine. The administered radioactive iodine is absorbed in the functioning thyroid tissue and gets eliminated mainly from the bowel, resulting in whole body dose in addition to being taken up in the glandular tissues of thyroid. The effective half life for clearance of radioactive body burden for normal human subjects is about 5.5 days (about 132 h) which is useful in health physics applications, to calculate the radiation dose received by the thyroid and total body, from the beta and gamma emissions originating from the radioactive iodine. In case of patients treated for carcinoma thyroid, because of surgical removal of thyroid tissues, elimination of administered I-131 radioactivity clears at a faster rate, thereby allowing treatments with large amounts of I-131 as high as 1.85 GBq to 5.5 GBq (50-150 mCi).

The assessment of external radiation exposures from patients at known geometry using radiation monitors is one of the accepted standard methods for estimation of radioactive body burden.[12345] Present regulations in Oman[6] indicates the need for hospitalization of patients above 400 MBq administered activities of RA¹³¹ I. We have only two isolation rooms connected with delay tank facility for I-131 treatments. This led to a waiting list for treatments of thyroid cancers in the oncology center. A recent report from Oman[7] has indicated that with good radiation safety instructions to patients and relatives, thyrotoxic patients may be treated as outpatients, provided the patient is physically and mentally fit enough to comply with given instructions. To address the isolation of Omani patients treated with RA¹³¹ I in the wards, justifying out-patient treatments for patients with thyrotoxicois and making necessary recommendations regarding their retention period of Ca.thyroid patients, we have analyzed the records of external exposure rates measured from patients who underwent radio-iodine therapy both for differentiated carcinoma thyroid and thyrotoxicosis.

MATERIALS AND METHODS

Patients

The patients who received RA¹³¹ I were referred from endocrinology department of the Royal Hospital based on accepted protocols [Table 1]. Patients were accepted for their treatments after obtaining informed consent after these patients were properly explained by the nuclear medicine consultants about the procedure, and possible associated risks. The patients treated during the period 2006 to 2009 were taken up for the study.

Table 1 Standard operating procedure of nuclear medicine department

Administration of RA ¹³¹I

The radioactive iodine is in the form of capsules supplied by GE Healthcare, Amersham, UK. The administrations were carried out on Mondays, and the activities were ordered with reduced activities with reference dates on Thursdays. The radioactive strengths (activity) of the capsules were assayed in Mark V, CalRad Isotope Calibrator (Model 34-164, Nuclear Associates, USA) immediately after arrival of consignment. The containers are labeled with suitable identifications, and activity certificates are prepared in the department. The work flow relating to RA¹³¹ I administrations followed in the department is shown in Table 2.

Table 2 Work Flow regarding radioiodine administration

RESULTS

Table 3 shows the number of patients who received treatments for Ca.thyroid and thyrotoxicosis along with the activities of Ra¹³¹ I administered. The mean activities administered were 4.19 GBq and 574.7 MBq for these groups of patients respectively. Table 4 shows the measured mean exposure rates and standard deviation. The number of patients is indicated in brackets. The semi-log plot of these mean exposure rates [Figure 1] gave two distinct patterns of clearance of radioactive body burdens. For patients with post-operative Ca.thyroid, the variation of body burden with time post-administration indicated a tri-exponential pattern. The three slopes of the plots yielded T_½eff values 14.4 h, 22.0 h and 41.3 h. For patients treated for thyrotoxicosis, the body burden showed slow rate of clearance with a T_½eff =111.4 h, and the exposure rate at 1 m did not show appreciable change after 48 h [Table 4].

Table 3 Details of administered activity

Table 4 Measured Exposure rates in ìSv/h at 1m from patients post administration of I-131

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Figure 1 Patterns of Clearance of body burden with administration of RA¹³¹I

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DISCUSSION

Our mean exposure rates at 1 m immediately following administration of activity were 88.0 μSv/h (SD 37.6) and 14.9 μSv/h (SD 6), respectively, for the Ca.thyroid and thyrotoxic patients. The results of 1 m exposure rates for thyrotoxic patients 23.4 ± 6.3 μSv/h as reported by Ibtisaam and Bererhi[7] are of the same magnitude. The mean activity of thyrotoxic patients is 574.7 MBq in the present work, to the mean activity of 609.8 MBq reported by these authors. For the patients with thyrotoxicosis, the mean half time of clearance of body burden in our work is T_½eff = 111.4 h which is of the order of magnitude quoted by Meredith and Massey,[8] which is 5.5 days (about 136 h).

In our study, the exposure rate at 1 m is averaged over measured values and represented along with the range. This is because of many variables, such as administered activities [Figure 2], various age groups of patients ranging from 18 to 73 years, both sex and different patient weights. For health physics purposes, these values still give an estimate of the risk involved in terms of radiation levels encountered during isolation of these patients.

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Figure 2 Details of administered activities in Ca Thyroid patients

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In the Ca.thyroid patients in the present work, an estimated effective half time of clearance of 14.4 h for body burden is in good agreement with the earlier reported value 11.2-15.4 h[5] in Indian population, and the values reported by Barrington et al,[4] viz. 9.4 h, 13.4 h and 12 h. Thomas et al,[1] compared three methods of body burden estimation viz. 1) urine assay 2) prediction based on a pre-therapy diagnostic work up method using 74 MBq (2 mCi) 3)external exposure rate measurements during actual treatments. They opined that direct external exposure rate surveys showed the potential for being an accurate, reliable and relatively safe method of monitoring patient′s I-131 body burden. From Table 4, it is clearly seen that the exposure rate around the patient treated for thyrotoxicosis remains unchanged after 48 h, seldom reaching the value of 10 μSv/h (1 mR/h) at 1 m because of retention of activity by toxic thyroid.

The Nuclear Regulatory Commission (NRC) specifies patient′s discharge when the body burden falls below about 1100 MBq (30 mCi). However, based on the social and cultural reasons and living conditions in their home, 1100 MBq I-131 appears to be a higher activity limit in Oman. A lower limit of 400 MBq for the patients to be hospitalized (as per prevailing MOH recommendations)[6] will indicate need for hospitalization of all the patients treated for thyrotoxic disorders (above 370MBq). This will make the need for creation of more isolation rooms, which is not practical. Our body burden measurements in thyrotoxic patients treated in the last four years clearly indicate an effective half time of clearance around 114 h (about five days) and no further clearance after 48 h post administration. In this context, it appears that there is no justification in hospitalizing these thyrotoxic patients post administration of I-131 to reduce their body burden to permissible limits, with the presence of healthy thyroid, concentrating bulk of administered activities because of high uptakes. Hospitalization of a few patients for thyrotoxicosis treatments may therefore be selectively restricted to some cases who have infant children around or many inmates living in small homes, justifying reduction of public exposures to permissible levels.