Activity-normalized survey-meter assessment around patients undergoing ⁹⁹ᵐTc-pertechnetate thyroid scintigraphy: Implications for radiation protection

Introduction/objective Patients undergoing diagnostic nuclear medicine procedures temporarily become sources of external radiation, potentially contributing to exposure of staff, accompanying persons, and other patients. This study aimed to evaluate activity-normalized survey-meter readings around patients undergoing routine ⁹⁹ᵐTc-pertechnetate thyroid scintigraphy as a function of distance, anatomical level, and post-injection measurement time under real clinical conditions. Methods A prospective observational study was conducted in eight patients. Free-in-air survey-meter readings were obtained using a NEB.211 dose-rate meter equipped with an energy-compensated Geiger–Müller detector. Measurements were performed at four anatomical levels — head, thorax, abdomen, and pelvis — and five distances — 25, 50, 100, 150, and 200 cm — on three post-injection measurement occasions. Because measurement timing varied under routine workflow conditions, analyses were based on recorded actual times, with mean post-injection times of 19.6 ± 14.4 min, 54.4 ± 17.1 min, and 91.5 ± 14.8 min. Readings were normalized to administered activity and interpreted as comparative survey-meter readings rather than calibrated H*(10) or Hp(10) dose-equivalent quantities. The distance–reading relationship was evaluated using log–log regression. Results Activity-normalized survey-meter readings decreased consistently with increasing distance at all anatomical levels and across all three measurement occasions. Log–log regression demonstrated a strong inverse relationship between distance and activity-normalized readings, with empirical distance exponents ranging from −0.746 to −0.975 and corresponding R² values ranging from 0.993–0.999 . Abdominal and thoracic levels generally showed higher readings than head and pelvic levels, whereas temporal changes within the observed interval were modest compared with the distance effect. Discussion The findings indicate that the patient-related radiation field under routine clinical conditions is strongly distance dependent and influenced by anatomical measurement level. Because the instrument did not have a traceable H*(10) calibration certificate, the results should be interpreted as comparative activity-normalized survey-meter data rather than absolute ambient dose-equivalent measurements. Conclusion Increasing distance was the dominant practical factor for reducing measured patient-related radiation levels around patients undergoing ⁹⁹ᵐTc-pertechnetate thyroid scintigraphy. These findings support simple distance-based workflow optimization measures in nuclear medicine departments.