Af ter re ceiv ing high doses of radioiodine the pa tients have to re main iso lated within the "restricted area", un til the ra dio ac tiv ity of the body drops be low a cer tain level. The aim of this pa per was to pres ent our alarm ing sys tem de signed to dis cover pa tients who at tempt to abandon the "re stricted area" and in form med i cal staff about the event. The sys tem con sists of a sur vey-me ter with a pan cake probe di rected to wards the cor ri dor. The sur vey-me ter is connected to a trig ger cir cuit which gives a sig nal in the case when the mea sured count rate exceeds a pre-set value. This sig nal sets "on" the alarm de vice, blink ing light, pro gram ma ble siren and IP cam era, in or der to warn the pa tient and in form the per son nel when such a case oc curs. In or der to test the con sis tency and sen si tiv ity of our sys tem we mea sured ten times the am bi ent dose equiv a lent, H * (10), from the source of 925 MBq (25 mCi) 131 I, kept at a dis tance of 1 m. The av er age am bi ent dose equiv a lent was 77.73 ± 31.57 (0.084 mSvh-1 per MBq, or 3.1 mSvh-1 per mCi). We mea sured ten times the same source at var i ous dis tances (1-2.25 m) from the probe. In each po si tion, the sys tem was trig gered. Also we tested the system on 40 pa tients treated with radioiodine in structed to pass through the cor ri dor. Each of their at tempts trig gered the sys tem. Ac cord ing to our ex pe ri ence gained over the past few years, this alarm sys tem in tended for pa tients re ceiv ing radionuclide ther apy en sures a high level of safety for both the pa tients and med i cal staff.