The radioiodine therapy is the most well-known branch of radionuclide therapy. The therapy composed of thyroidectomy and iodine-131 radiopharmaceutical is the “gold standard” for treat-ment of differentiated thyroid cancer patients (DTC). Given metastases in lymph nodes such therapy is absolutely exclusive treatment mode. At the same time, despite the targeted effect of radioiodine on pathological foci a side internal exposure of healthy organs and tissues is noted in the course of radioiodine therapy. Therefore, a reliable assessment of the side exposure is nec-essary taking into account the individual characteristics of patients considering that the side dose varies significantly from patient to patient. For this reason, the identification of significance of personal clinic-diagnostic factors determined the side whole body exposure is the vital and im-portant task. In this aspect, the cytogenetic examination of DTC patients by means of the analysis of chromosomal aberrations in peripheral blood lymphocytes is the goal of essential importance. The certain types of chromosomal aberrations are specific radiation markers that unequivocally indicate radiation exposure. Using the frequency of radiation markers the side absorbed dose can be reliably estimated. In the present work, the statistical analysis of the significance of vari-ous clinic-diagnostic factors in relation to the induction of radiation markers in the blood lympho-cytes was performed. For that end the results of the cytogenetic examination of the group com-prised of 38 DTC patients have been used. The examined patients underwent a course of radio-iodine therapy in the department of radionuclide therapy of the A.F. Tsyb MRRC, Obninsk. The slide preparation and cytogenetic analysis were carried out in the laboratory of radiation cytoge-netic of the same Center. The performed analysis of the correlation matrix with respect to the dependence of the increased frequency of stable and unstable markers resulted from a one-time course of radioiodine therapy showed no meaningfulness at the level R>0.3 for all the studied factors. For the absolute value of the frequency of radiation markers, both before and after radioiodine therapy, the meaningful relationship was found at the level R>0.7 for unsta-ble and at the level R>0.8 for stable markers with a total administered activity of radioiodine dur-ing all the previous courses of radioiodine therapy.

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. How-ever, orally administered radioiodine may be also absorbed into the bloodstream and can dam-age 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 addi-tion, 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 mark-ers 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 radia-tion 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 tis-sues 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 Radia-tion 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 thera-py 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 I-131 activity (Σact) exceeds 1200 mCi. The absolute value of the total focal radiotherapy dose is probably ir-relevant. In general, patients assigned to both radioiodine therapy and external beam radiothera-py require personalized approach to treatment.