DOSE RELATIONSHIP DURING RADIOABLATION OF THYROID RESIDUAL IN PATIENTS WITH THYROID DIFFERENTIATED CANCER

Objective: to study relationship between the focal adsorbed doses (FAD) in the residual thyroid tissue (RTT) at radioiodine therapy (RIT) in patients with differentiated thyroid cancer (DTC) and efficacy of radioablation (RA). Materials and methods. FAD in RTT in 221 patients aged 25–74 years were analyzed. Scintigraphic investigations were executed using gamma-camera «МВ 9200» and single photon emission tomograph «Е. САМ 180». FAD in RTT were determined according to recommendation of the Committee on Medical Internal Radiation Dose. Results. Mean FAD in RTT was consisted (125.9 ± 8.6) Gy with range 1.8–1373.4 Gy. Positive effect after first course of RIT was in 77.4 % of patients complete RA of 78 % of RTT was achieved. Mean FAD in cases with complete RA was (147.0 ± 13.3) Gy with range 1.8–1373.4 Gy. Efficacy of RIT constantly increased from 68.2 to 93.7 % with increasing mean FAD from 30 to 370 Gy. Feather enlarging of FAD did not lead to any significant increasing of efficacy of RIT: enlarging mean FAD from 370 to 776 Gy increased efficacy of RA on 0.4 % only. Repeated RIT decreased FAD in RTT after each course. Using regression analysis was established type of relationship between efficacy of RA and FAD in RTT, according it efficacy of first RA consists at 40 Gy 72.8 %, 150 Gy – 84.9 %, 300 Gy – 91.2 %, 400 Gy – 93.7 %, 500 Gy – 95.7 %. Conclusions. FAD in RTT at RIT differ in large range, it can be explained different anatomical and functional condition of iodine accumulate thyroid tissue after surgical treatment and its radioresistance. Efficacy of RA increases with enlarging of FAD, this relationship is nonlinear. Efficacy of RA has the upper dose threshold in range 350–400 Gy. The frequency of thyroid tissue with high radioresistance is approximately 6 %. Key words: differentiated thyroid cancer, thyroid residual tissue, radioiodine therapy, radioablation, focal adsorbed dose, iodine-131.