Re: Radiation exposure levels in family members of Omani patients with thyrotoxicosis treated with radioiodine (I-131) as outpatients

To the Editor We write in response to the above mentioned article which appeared in the August 2009 issue of SQUMJ.1 Radiation regulations in most countries permit treatment of thyrotoxicosis using radioactive iodine (I-131) as an outpatient procedure. Hospitals provide instructions to the patients receiving these treatments about the precautions to be observed in their homes in terms of avoiding local radioactive contaminations and also protect other inmates restricting the radiation exposures ‘as low as reasonably achievable’. Therefore it is of interest to quantify the radiation exposures incurred from these patients to spouse, children and attendants. Early works- addressed different related issues in this regard.2–4 From this objective, we appreciated the work of Al-Maskery and Bererhi reported in your August 2009 issue. The following are the highlights of their work. Their study involved 22 thyrotoxic patients treated on an out-patient basis. Thermo-luminescent dosimeters (TLD) monitoring was carried out on 86 inmates in their homes (29 children and 57 adults). The quantity of I-131 radioactivity administered was 610 ± 79 MBq in the range of 520–862 MBq. Measured mean radiation levels around these patients immediately post-administrations was 23.4 ± 6.3 μSv/h in the range of 13–42 μSv/h. The cumulative radiation doses by other inmates in the homes of these thyrotoxic treated patients during 10 days are shown in two categories. Spouses (n = 11) received a mean radiation dose of 105 ± 152 μSv in the range of 7–425 μSv. Other relatives or attendants received a mean radiation dose of 206 ± 440 μSv in the range of 0–2921μSv. In Oman, our hospital is another centre where radioactive I-131 treatments for thyrotoxicosis and carcinoma thyroid are offered. From the data of measured exposure rates around these patients, we have obtained the following results.5 The mean activities administered were 4.19 GBq (in the range of 2.04–9.3 GBq) and 574.7 MBq (in the range of 479–627 MBq) for thyroid cancer and hyperthyroidism treatments respectively. Mean exposure rates immediately after administration of the I-131 therapy doses were 88 μSv/h (n = 69 in the range of 34–184 μSv/h) for thyroid cancer patients, and 14.9 μSv/h (n = 49 in the range of 4.5–34 μSv/h) for thyrotoxic patients. Our study revealed that the radioactive body burden for post-operative thyroid carcinoma had a tri-exponential clearance pattern with T½eff values 14.4h, 22.0h and 41.3h. The body burden of the treated patients for hyperthyroidism cleared with an effective half life T½eff =111.4h. Figure 1 shows the clearance pattern of I-131 with time elapsed post administration in these thyrotoxic patients. With our data accrued from the Omani patients, taking the effective half life (T½eff) of clearance in the patients with hyperthyroidism viz. T½eff = 111.4h, we tried to explain the results of cumulative radiation exposures received by the inmates observed in the recent study as below.2 Figure 1 Clearance of I-131 in thyrotoxic patients (n = 49) Method of estimation of radiation level around the patients (for administered I-131 activity 610 MBq): Dose rate constant for I-131, at 1metre, Γ in mSv.m2/MBq.hr (Ref. 6) = 7.467 x10−5 mSv/h=7.467x10−2μSv/h = 0.07467 μSv/h. Total emitted dose at 1m from the patient administered with 610 MBq of I-131 = (Activity administered in MBq) x (1.44 times T½eff in hours x (Exposure rate in μSv/h) = 610 MBq x (1.44 x 111.4 hours) x (0.07467μSv/h/MBq) μSv = 7307 μSv Therefore, if we assume that in 10 days the administered activity of I-131(610MBq) has fully emitted all its emissions, this emitted radiation 7307 μSv at 1m should be taken to represent the cumulated dose received by the individuals moving around the patient in the house. This can be safely assumed because 10 days (240 h) elapsed time duration is nearing 2.5 T½eff eliminating 83% of administered activity.