Factors Involved in the Experimental Therapy of Metastatic Thyroid Cancer with I131

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The treatment of metastatic thyroid cancer by radioactive iodine presents features of interest and importance to investigators concerned with the control of cancer by systemic means. In terms of number of patients benefited, this subject does not 100m large in the general field of cancer since encouraging results have been obtained in only a very small percentage of cases of an already uncommon type of tumor. Nevertheless, the causes of its success and its limitations should be worthy of thorough study because arrest of the disease has been attained in the presence of odds insurmountable by other forms of treatment (8, 9) The character of these few but definite successes strongly supports the long held view that if means were found to localize radioe1ements in tumors, important progress would be made toward control of the disease. Efforts to localize radioe1ements in neoplastic cells have not been wanting in the past, but concentrations of practical value have not thus far been attained in cancer tissue per se. The premises upon which treatment of thyroid cancer by I13l is based are no exception to the rule, inasmuch as this therapy is possible only when cells of thyroid cancer share with the parent gland the pronounced avidity for the isotope. Under these conditions it is axiomatic to anticipate destruction of normal thyroid tissue in the process of tumor inactivation and to expect myxedema when good control of the disease is obtained. With this exception, and as in any other type of radiotherapeutic cancer treatment, the ultimate aim of the procedure is to achieve inactivation of the tumor with minimum damage to normal structures. Locally, radioactive iodine will suit this purpose on account of the limited range of the beta radiation emitted, but the therapist is faced from the outset with the necessity of reducing the radiation dose to body structures which may be spatially removed from the tumors, but are very much in the metabolic path of the iodine. The experience gained in the last decade in the treatment of hyperthyroidism with p3l or p30 cannot be utilized fully in the treatment of thyroid cancer. In the case of hyperthyroidism, remission of symptoms is sought by partial inactivation of the thyroid gland, whereas in the case of cancer, radical devitalization of extensive tumor masses is attempted under conditions of differential uptake which as a rule are not as favorable (2, 4, 6). Among tumors, only metastasizing struma approaches in magnitude and uniformity the avidity of the thyroid gland in Graves' disease. Other tumors, either primary or metastatic, which exhibit moderate preference for the isotope are those showing orderly cellular structure in follicular arrangement containing colloid-like material. Thus far only one example of thyroid carcinoma of high-grade malignancy has been reported to retain I131, and this was a solid alveolar adenocarcinoma.