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The Natural Thyroid Diet

The Natural Thyroid Diet

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For diagnostic purposes the gamma emissions are important. The distribution of the radiopharmaceutical inside the body can be externally measured through imaging with gamma cameras. For therapeutic purposes the beta energy emission is important because of the destructive character in tissue. The path length of the beta particle depends on the energy and ranges from several mm to 1 cm.

Iodine-131

An important property of is that it can both be used for imaging purposes (high energy gamma ray) as well as for therapeutic purposes (medium-energy beta emission) while most of other radionuclides only have diagnostic utility. has however suboptimal imaging characteristics, including high energy gamma ray (364 keV), which is not optimal for most gamma cameras. The long half-life (8.04 days) and high beta emission limit the administered dose for diagnostic purpose only. The path length of the beta particle is about 0.5 mm, the toxic effects are limited to the thyroid tissue, with therefore sparing of adjacent normal tissue. The normal biodistribution of iodine includes salivary glands, stomach and renal tract including the bladder.

The long half-life is advantageous in the detection of functioning metastatic thyroid cancer lesions, because imaging can be done for many days after administration. This enables long take up periods in metastatic tissue and adequate clearance of background activity.

The radiation dose delivered by 1311 concentrated in a tissue depends on two factors: the radioactive concentration (the ratio between total uptake and the volume of functioning tissue) and the effective half-life (time after which the radioactivity in the tissue has decreased by a factor of 2). The effective half-life is related to the physical half-life and the biological half-life, which is related to the elimination of from the concentrating tissue.

In normal thyroid tissue the concentration is about 1 to 2% of the administered activity per gram and effective half-life is about 8 days. Functional thyroid cancer tissues concentrate under favourable condition about 0.1 to 0.5% of the administered activity per gram and the effective half-life is shorter than 3 days (9).

Iodine-123

Like 1311 the chemical behavior of 123I is identical to that of stable iodide. The halflife of 123I is 13.2 hours. 123I decays by electron capture and is a lower energy gamma emitter (159 keV) compared with 1311. Therefore the resulting imaging quality is better than In addition, delivers a lower radiation dose to the thyroid tissue due to the absence of beta particle emission which may prevent a possible 'stunning' effect (discussed below). The major disadvantages of are the high cost due to the facts that it is produced by cyclotron, the limited availability and furthermore, the short half-life.

Iodine-124

While the radioisotopes and especially are used on a wide scale in diagnosis and treatment of all thyroid disorders, the positron emitting isotope which is suitable for PET, has received little attention. Chemically identical to non-radioactive iodine, this isotope would allow thyroid cancer imaging using the high resolution PET technique (10). I24l,however, is difficult to obtain and only available at specific research centers, as it is produced in a cyclotron. The isotope has a relatively low yield ofradiation (positron yield 23%) suitable for imaging, but also emits other high-energy gamma radiation that increases the radiation to the thyroid (when present) almost to the (therapeutic) level of1311. In addition, the high-energy byproducts may deteriorate image quality. For these reasons clinical use has been minimal. has been used for dosimetric purposes or thyroid volume measurements (11, 12, 13, 14, 15). Recent development of combined PET-CT scanners with a single gantry, may increase clinical application in thyroid cancer patients, as detailed anatomical information is combined with the location of iodine positive tissue (16). The clinical value, for example, as compared to scintigraphy, is currently unknown.

Iodine-125

1251 decays by electron capture and gamma emission. The very low energy gamma emission (28-35 keV) and the long half-life (60 days) of 1231 make this radionuclide less suitable for in-vivo application. The very low energy gamma ray is to weak to be detected by gamma cameras. However, 1251 is extremely suitable for in-vitro application. It is a common agent for use in radioimmunoassay.

Scan method

Patient preparation

Thyroid stimulating hormone (TSH), produced by the pituitary is essential for stimulation of thyroid cells for optimal imaging with radioiodine. There are two ways to prepare a patient for radioiodine imaging: thyroid hormone withdrawal or administration ofrecombinant human TSH (rhTSH) during thyroid hormone therapy. Standard thyroid hormone medication (l -thyroxine, T4) withdrawal is usually 4-6 weeks until the serum TSH is greater than 30 mU/l to permit maximum stimulation of thyroid tissue. L-triiodothyronine (T3, Cytomel) replacement therapy BID or TID)

can be given the first 4 weeks of a 6 weeks withdrawal due to the short half-life and the immediate effects of L-triiodothyronine. The transient thyroid hormone suppletion withdrawal is associated with morbidity of hypothyroidism and therefore decreases the quality of life and diminishing productivity (17).

Recombinant human TSH (rhTSH) prevents the profound symptoms of hypothyroidism as a consequence of thyroid hormone withdrawal. rhTSH increases serum TSH concentration sufficiently to stimulate thyroidal uptake and release of thyroglobulin (Tg) while patients are still taking thyroid hormone medication. The recommended protocol of rhTSH is two intramuscular injections of 0.9 mg given on 2 consecutive days followed by 148 MBq (4 mCi) 1311 on the third day and a WBS and Tg measurement on the fifth day. Whole body images were acquired after 30 minutes of scanning or after 140,000 counts. This is necessary because 4 mCi 1311 after rhTSH has about the same effect as 2 mCi given in the hypothyroid state with reduced renal clearance and raised 1311 body retention (18,19).

However, it must be emphasized that there is so far few experience concerning this issue especially on the long term effects on outcome, so the application of rhTSH in the diagnostics still a matter of discussion (20).

Diagnostic131/ WBS

techniques of scanning. Imaging is performed using high-energy collimator. The bladder must be emptied before imaging. Supine anterior and posterior images of the neck, chest, abdomen and pelvis are acquired. Anatomic landmark or transmission scans using cobalt marker can be helpful in the interpretation of the images. Additional or delayed images can be obtained in patients with atypical findings on the scans.

interpretation. The correct interpretation of the radioiodine images is crucial in the therapy management of thyroid cancer. It requires knowledge and understanding of the normal biodistribution of radioiodine. Radioiodine uptake in the choroid plexus, nasal mucosa, salivary glands, mammary glands, gastric mucosa, gastrointestinal tract and urinary tract including bladder should be considered as physiological. These tissues contain like thyroid tissue NIS-transporter. Diffuse iodine uptake in the liver can also be seen on the post-treatment scans when there is functioning thyroid due to the incorporation of radioiodine into thyroid hormones which are degraded in the liver by de-iodination and conjugation. Uptake ofradioiodine outside the above mentioned organs should be considered as residual and/or metastatic thyroid tissue (true positive) or as contamination (false positive) (21,22).

Clinical application

Pretherapeutic diagnostic scintigraphy

The goal of the diagnostic scan after total or near-total thyroidectomy is to quantify the residual thyroid and detect metastatic disease. It is also included as part ofthe follow-up procedures. The ablative or therapeutic dose of 1311 used for treatment can be based on the results of the diagnostic scan. The diagnostic WBS is usually acquired 48-72 hours after administration of a diagnostic dose of1311 during hypothyroid state.

performing diagnostic scans before ablation therapy (23) or during follow-up, up is controversial (24).

The reason to perform no pre-ablative diagnostic scintigraphy is, that it is known that nearly all patients show residual neck uptake after (near) total thyroidectomy. And some believe that low diagnostic dose of1311 may impair the thyroid remnants uptake of the subsequent ablative dose of the so-called stunning effect. This issue will be discussed further on. Carlisle et al. (22) support performing diagnostic scans prior to therapy for several reasons. First, patients with undectectable Tg and a normal diagnostic scan after total thyroidectomy need not to be treated with 1311. Second, a correct treatment 1311 dose can be determined when the extent of the disease is known.

Discussions are continuing concerning performing diagnostic scans before 1311 treatment in patients with elevated serum Tg. Cailleux et al. (24) suggest that diagnostic scanning need not to be done when serum Tg is higher than 5 ng/ml and one rather should considered therapy and posttherapeutic scan after thyroid hormone withdrawal.

Figure 1.1. Pre-therapeutic diagnostic 131I-WBS 1 day after 40 MBq in a 37-year-old patient with papillary thyroid carcinoma with elevated serum Tg after total thyroidectomy. It shows intense uptake in the neck (arrow) and uptake in the lung (arrow). Normal biodistribution in the gastrointestinal tract and bladder. This patient was subsequently treated with ablative dose of 1850 MBq 1311.

Figure 1.1. Pre-therapeutic diagnostic 131I-WBS 1 day after 40 MBq in a 37-year-old patient with papillary thyroid carcinoma with elevated serum Tg after total thyroidectomy. It shows intense uptake in the neck (arrow) and uptake in the lung (arrow). Normal biodistribution in the gastrointestinal tract and bladder. This patient was subsequently treated with ablative dose of 1850 MBq 1311.

1 31 -1 scintigraphy 3day(s) after 7EM8q(2mCi)

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