, Volume 5, Issue 1, pp 75-82

External radiotherapy and radioiodine in the treatment of thyroid cancer

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Abstract

Surgery remains the most effective treatment for thyroid cancer. The indications for radiotherapy and radioiodine treatment should take into account the nature and extent of the surgical excision. There is not yet convincing evidence of the effectiveness of prophylactic postoperative radiotherapy. For patients under 45 years of age, the results obtained by complete surgical excision are excellent, and the degree of improvement that may be obtained from radiotherapy is not sufficient to justify the sequelae associated with beam therapy and the long-term hazards induced by radioiodine. On the other hand, for patients over 45 years of age and, in particular, when the cancer is of the moderately differentiated follicular type, the prognosis is clearly much less favorable and there is an indication for additional treatment. Furthermore thyroid ablation by radioiodine facilitates the early detection of local recurrence or metastases by whole body scintiscans or serum thyroglobulin immunoassay. After grossly incomplete surgery, the efficacy of high dose (over 5000 rads) beam therapy has been demonstrated by a low incidence of local recurrence. Combination of external radiation therapy and radioiodine is indicated and well tolerated. For inoperable tumors the results of radiotherapy are limited; although complete remissions are sometimes obtained the incidence of local recurrence is high. However, the palliative effect of radiotherapy should not be overlooked. Patients with lung metastases that take up radioiodine have a high survival rate; about 50% of them survive for 15 years after the first administration of radioiodine. Cure is probably achieved in a large proportion of these patients. The survival of patients without significant radioiodine uptake after thyroid ablation is much shorter. The results obtained in treatment of bone metastases are less satisfactory. Combination of radioiodine and external therapy is advocated. External radiotherapy is effective in medullary carcinoma despite the slow shrinkage of the tumor after irradiation. Calcitonin assay can help to select those patients who need postoperative irradiation. In undifferentiated carcinoma, radiotherapy is recommended even after complete surgical excision although the results are generally poor.