6.5. IODINE-131 THERAPY IN THYROID CANCER
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Iodine-131 is given orally; neck uptake and imaging is carried out between 24
and 72 hours after administration. Whole body imaging at 72 hours should also
be carried out, especially when the results of neck imaging are negative. Many
centres, however, proceed directly to ablative therapy.
A form signed by the patient giving their informed consent for therapy is
required.
6.5.8. Therapy
Ablative therapy is defined as that given immediately following definitive
surgery. In most centres, a fixed dose of between 1 and 4 GBq (25–100 mCi)
131
I
is given. When the mass of thyroid remnant can be estimated, for example
using ultrasound, a dose of
131
I calculated to deliver 30–50 Gy to the thyroid
remnant may also be used. Ablative therapy should be given to all patients with
iodine-avid thyroid/malignant tissue in the neck or elsewhere, or in those
patients who, immediately after surgery, have no evidence of iodine-avid
thyroid tissue 72 hours after oral administration of
131
I tracer but who have
elevated serum thyroglobulin levels.
Patients should be evaluated not earlier than six months after ablative
131
I
therapy for evidence of residual or recurrent disease. This evaluation is carried
out not less than four weeks after cessation of thyroid hormone replacement or,
if the patient cannot tolerate hormone withdrawal, by the following regimen:
—Stop levothyroxine and substitute with a comparable dose of T3 for two
weeks.
—Stop T3 for at least two weeks prior to
131
I.
Where available, patients may be evaluated using rhTSH, which is given
as an intramuscular injection of not more than 0.9 mg daily for two days,
followed a day later by not more than 185 MBq
131
I taken orally.
Therapy for recurrent or metastatic disease is given to patients who show
evidence of uptake of a tracer amount (80–185 MBq) of
131
I in such sites.
Anterior and posterior whole body imaging should be carried out at least
72
hours after administration of the tracer, using high energy collimation. An
alternative to whole body imaging is static anterior and posterior imaging of
the relevant areas (head, neck, chest, abdomen, pelvis and lower extremities),
taken for at least 10 min each.
If there is evidence of iodine-avid disease from scintigraphy and/or if the
serum thyroglobulin level is elevated, the patient should be treated with
131
I.
The dose of
131
I is usually between 5 and 7 GBq. The maximum safe dose of
131
I
may also be calculated using clearance data, obtained by measurement of