|
A careful total or near total thyroidectomy represents an essential step
in the treatment of thyroid cancer. The availability of a skilful surgeon
is mandatory. Without taking any special surgical risk, the completeness
of the thyroidectomy will be easily achieved by the administration of radioiodine
131I, 4-6 weeks later.
The ablative efficiency of 131I is well recognized for more than 40 years. It remains essential to point out that several conditions have to be met for a successful result in the treatment of thyroid cancer: (a) It is necessary to wait 4-6 weeks after surgery before the administration of a radioiodine therapeutic dose. The reason for this is to benefit from the development of thyroid insufficiency which in turn, is associated with an enhanced TSH level in the serum which last but not least, will stimulate the 131I uptake by the thyroid remnant. (b) A dose of 3,700 MBq (100 mCi) 131I is given during a short hospitalization of the patient (in agreement with the legal requirements for radioprotection). In some countries like Germany, the radioprotection requirements are much more difficult to be met and require a long hospitalization. This may raise social and professional problems for the patient. The opportunity of the administration of a therapeutic 131I dose is used to perform a whole-body scan, 5 days later. (c) A new thyroid scan is to be repeated 6 months later, after stopping for 4 weeks the TSH suppressive thyroid hormone therapy. This will ensure at the time of the scintigraphic control, an optimal stimulation of the remaining thyroid tissue or metastases. It is important to stress that the TSH enhancement after discontinuation of thyroid hormone therapy has not only to reach a certain level (at least 30 mU/L) as reflected by the serum TSH but this for several days (8-10 days) in order to reinforce the thyroid uptake and thus, the sensitivity of the 131I scintigraphy. The tracer dose has to be in order of 185 MBq 131I in order to maximize the chance of detection of the remaining tissue. However depending on the iodine environment, this "tracer” dose may vary between 74 to 3700 MBq (2 to 10 mCi). Strictly speaking, there is advantage not to give more than 185 MBq (5 mCi) 131I in order to reduce the radiation thyroiditis which, secondarily, may diminish the accumulation of the therapeutic 131I dose in the thyroid tissues, that will be subsequently administered. Thus, the enhancement of thyroid tissue detection by a high dose of the therapeutic 131I dose has to be balanced against a reduced efficiency of the treatment itself in case of post-diagnostic thyroiditis. Generally speaking, Europe is characterized by a low iodine environment and therefore, the detection sensitivity is higher than in other countries, especially in the USA. In the follow-up of thyroid cancer, a 185 MBq (5 mCi) 131I diagnostic doses should be advised. (d) A thyroid scan after stopping of thyroid medication is performed every 6 months until the scintigraphic investigation and/or the thyroglobulin becomes undetectable in the serum (under conditions of withdrawal of thyroid hormones). A positive scan is followed by 131I treatment. Once a scan becomes negative, the controls will be progressively spaced at 12 months intervals, 2 and 4 years intervals as long as they remain negative. (e) Advantage will be taken of the enhanced TSH level at the time of the thyroid scan, to measure the circulating serum thyroglobulin. Thyroglobulin measurement is an important tool in the cancer follow up, probably best in conditions of thyroid hormones withdrawal. In our hands, a value higher than 2 ng/ml indicates the persistence of active thyroid tissue, even if the thyroid scan is negative. (g) Ideally and as already stressed, the 131I thyroid scan should be interpreted in conjunction with the results of the serum thyroglobulin determination in the context of a high TSH level, at the time of the diagnostic 131I thyroid scan. There should be an agreement between a negative thyroid scan and a serum thyroglobulin level below 2 ng/ml, or preferably undetectable. False negative scans resulting from an iodine overload should be excluded by measuring the iodine concentration of a urinary spot sample at the time of administration of the diagnostic 131I dose. A negative thyroid scan associated with a serum thyroglobulin level higher than 2-5 ng/ml (depending on the sensitivity of the methodology) indicates the persistence of thyroid tissue. In contrast, a positive thyroid scan associated with undetectable levels of thyroglobulin will a few months later usually be followed by a negative thyroid scan. (h) Thyroid hormone therapy is maintained within the intervals between the thyroid scans. Appropriate doses of T4 maintain the serum TSH level undetectable or in the very low range. TSH has a mitotic action which has to be avoided in thyroid cancer. In fact the patient is not treated with a substitution dose but with a suppression dose of T4. This is the least dose of T4 that suppresses TSH below normal levels. A strong policy is recommended in order to maximize the chances to definitely cure a patient suffering from thyroid cancer. We do have all the tools in our hands ... In this respect, 131I undoubtly plays a key role. The requirement of stopping thyroid suppression therapy for an appropriate time represents a decision that often makes the patient's life uncomfortable but a high sensitivity of the thyroid scan and of the thyroglobulin measurement is the reward. The use of recombinant h-TSH may circumvent these discomforts provided its cost-effectiveness is clearly demonstrated. |
