A 68-year-old woman was referred to our hospital from Hospital A for postsurgical RAI therapy for papillary adenocarcinoma of the thyroid. About four months before visiting our hospital, she underwent total thyroidectomy and D2 lymph node dissection at Hospital A. A preoperative CT scan showed that there was no distal metastasis. The tumor had invaded into tracheal cartilage, cricoid cartilage, and esophageal serosa. A part of the tumor that had invaded into tracheal cartilage and cricoid cartilage may have remained. Pathological assessment revealed that the tumor had infiltrated into surrounding connective tissue, anterior cervical muscles, and the recurrent laryngeal nerve. Lymph node metastasis with extra-nodal extension was observed in 8 out of 18 lymph nodes. The final diagnosis was papillary adenocarcinoma (pT4aN1b-2, Stage III). She was a candidate for adjuvant RAI therapy. The patient had CKDG5d associated with diabetes mellitus and IgA nephropathy. Hemodialysis had been performed every Monday, Wednesday, and Friday at Clinic B for 3 years. Her other medical history included hypertension, cataracts, coronary arterial disease, cerebral aneurysm, femoral fracture, and colon cancer. The patient’s medications were levothyroxine sodium hydrate, linagliptin, aspirin, lansoprazole, atorvastatin calcium hydrate, alfacalcidol, bisoprolol fumarate, shakuyakukanzoto, lanthanum carbonate hydrate, acetaminophen, rebamipide, and nicorandil. Before performing RAI therapy, we confirmed the absence of contraindications to the treatment.

Treatment and hemodialysis schedule

One week before hospitalization, the patient had taken a low iodine diet. On the day of hospitalization, the patient was admitted to our hospital after undergoing hemodialysis at Clinic B. We followed the standard thyrotropin alfa regimen for patients with preserved renal function because the patient would undergo hemodialysis on the fourth hospital day. In a previous study, a single injection of thyrotropin alfa 48 h before 131I administration was used to avoid an excessive level of thyroid-stimulating hormone (TSH) [18]. We dissolved 0.9 mg of thyrotropin alfa in 1.2 ml of an injection solvent and injected it into the muscle at 48 h (on the second hospital day) and 24 h (on the third hospital day) before 131I administration. On the fourth hospital day, after hemodialysis, 131I (15 mCi, 0.56 GBq) was administered. On the seventh hospital day, hemodialysis was performed at our hospital. On the eighth hospital day, thyroid scintigraphy was performed. The patient was discharged on the ninth hospital day.

Hemodialysis

Four hours of hemodialysis was performed through an arteriovenous fistula (AVF) using a VPS-15HA with a Qb of 220 ml/min and a Qd of 500 ml/min. The dialysis staff and all visitors were required to wear protective overgarments and lead shielding; glass badges were worn to monitor radiation exposure. After hemodialysis, all the devices were collected and kept in a shielded room until the radiation dose rate dropped.

Dose rate measurement

During hospitalization, two types of radiation measurements were performed. First, the dose equivalent rate was measured every hour using a device (device 1). The device 1 was placed about 90 cm apart from the patient. Second, the dose equivalent rate was measured using a γ survey meter (device 2). The dose equivalent rate was measured about 1 m from the patient by a medical doctor with 4 years of experience. The doctor checked the dry battery in every measurement. The first measurement was made immediately after administration of 131I. After the administration, the measurements were performed 11 times during hospitalization.

Estimation of half-life

We estimated the half-life of 131I in the patient’s body from administration of 131I to hemodialysis as follows. In the estimation, we standardized the values measured by device 1 to values measured at 1 m apart from the patient.

i)

We assumed that the dose equivalent rate changes according to

$$D\left(t\right)=_\text\left(-\frac2}_}t\right),$$

where \(_\) and \(_\) represent dose equivalent rate at \(t=0\) and half-life, respectively.

ii)

A semi-log plot was applied to assumption i),and the slope (α) was estimated using the least square method. The estimated slope was statistically tested according to linear regression analysis.

$$lnD\left(t\right)=_-\frac2}_}t,$$

iii)

We obtained the half-life \(_\) as \(_=-\frac2}\).

Estimation of integrated dose for caregivers and the public

We estimated the integrated dose for caregivers and the public based on the guide for the appropriate use of 131I in RAI therapy [25]. In this guide, it is recommended that the integrated dose for a caregiver be under 5 mSv according to the ICRP Publication 73 [26]. As an exception, the integrated dose for a child should be under 1 mSv. Similarly, the guide recommends that the upper limit of the dose should be 1 mSv per year, according to the ICRP Publication 103 [27]. Based on these values, the guide sets the dose conversion coefficients for caregivers and the public as 0.5 and 0.25, respectively. In Japan, there is another guide for ablation therapy using 1,110 MBq of 131I: a guide for ablation therapy using 131I (1,110 MBq) at an outpatient clinic [28]. This guide sets the dose conversion coefficients for caregivers and the public as 0.25 and 0.25, respectively. The difference between the two guides is explained by the fact that a patient undergoes ablation therapy using 131I (1,110 MBq) at an outpatient clinic on the condition that the patient independently lives under some restrictions to avoid radiation exposure to caregivers and the public. Since it is controversial whether the value of 0.5 or 0.25 should be used for a patient undergoing RAI therapy and hemodialysis, we calculated caregivers’ exposure using the values of 0.25 and 0.5. Both guides set the coefficient for internal exposure due to inhaling as 1.045, and we used this value. The calculation was performed from the administration of 131I to hemodialysis and from the administration of 131I to discharge.