Analysis of side effect in thyroid cancer patients received combined treatment with radioiodine and external beam radiotherapy

Radical surgery is the mainstay for differentiated thyroid cancer, radioiodine therapy followed by the surgery is the “gold standard” treatment modality indicated after the surgery. Radioiodine therapy becomes practically non-alternative treatment for patients with distant metastases. However, orally administered radioiodine may be also absorbed into the bloodstream and can damage surrounding normal organs and tissues and cause side effect. To test radioiodine therapy safety side effects of the radionuclide on somatic cells should be accurately assessed. In addition, external beam radiotherapy can also damage adjacent normal organs and tissues and thereby increases radioiodine therapy side effect. It is possible to evaluate patient-specific total radiation dose with the use of cytogenetic assay of chromosome aberrations level in peripheral blood lymphocytes because some types of chromosome aberrations are specific radiation markers clearly indicating that the cells have been exposed to radiation. Radiation absorbed dose can be estimated by the number of the aberrations occurrence frequency, and the value of side radiation dose greatly depends on a patient personality. In this case peripheral blood lymphocytes are thought of as a model of somatic and bone marrow cells. The paper present results of application of cytogenetic assay for estimating side effects caused by exposure of adjacent organs and tissues to external beam radiotherapy and surrounding somatic cells to radioiodine therapy. Results of statistical analysis of external beam radiotherapy related side effect contribution to the total side effect are present in the paper. The joint group of 43 patients was involved in the study, all patients received radioiodine therapy and 5 of them received also external beam radiotherapy in addition to radioiodine therapy. All patents received radioiodine therapy in the Radionuclide Therapy Department, A. Tsyb MRRC in Obninsk. Cytogenetic assay was carried out in the Radiation Cytogenetics Laboratory, A. Tsyb MRRC. Results of the study and examination of regularities of radioiodine therapy and external beam radiotherapy related side effects development make it evident that patients received combining radioiodine therapy and external beam radiotherapy should present special group with the treatment plan different from that for the radioiodine therapy exposed group. Exposure to external radiation may cause not only significant overestimation but also a false-positive underestimation of the number of radiation-specific markers because of lymphocytes inhibition following exposure to high total radiation dose. Inhibition effect may be a result of overestimating multiple radioiodine therapy administrations to achieve total 131I activity (Σact) exceeds 1200 mCi. The absolute value of the total focal radiotherapy dose is probably irrelevant. In general, patients assigned to both radioiodine therapy and external beam radiotherapy require personalized approach to treatment.