Paper A NEW PROPOSAL FOR MONITORING PATIENTS IN NUCLEAR MEDICINE J. Willegaignon,* Maria I. C. Guimara˜es, † Marcelo T. Sapienza, †‡ Michael G. Stabin, § Luiz F. Malvestiti, ‡ Marı ´lia Marone, ‡ and Gian-Maria A. A. Sordi* Abstract—The measurement of exposure rates is fundamen- tally important in the release of patients given radioactive materials and for keeping the exposures of others as low as reasonable achievable. Similar measurement methodologies have generally been used for point and extended sources, but this approach may lead to methodological errors in calculating radiation dose estimates. In this study, nuclear medicine patients who received high activities of Na 131 I for therapy were monitored using different measurement methodologies, and the results showed that the usual measurement performed at 1.0 m in front of the body resulted in a mean error of 40% between experimental and theoretical exposure rates. The best measurements were obtained when performed at 2.0 m in front of the patients. With this approach, the error was about 2% between experimental and theoretical values. These findings suggest a new methodology for patients’ measurement in nuclear medicine and could be useful for personal monitoring in cases of radiological emergencies involving 131 I ingestion. Health Phys. 91(6):624 – 629; 2006 Key words: 131 I; monitoring, personnel; nuclear medicine; radiation therapy INTRODUCTION IN NUCLEAR medicine, internal and external radiation exposure to family members or others near patients may result from the use of  (gamma) and  (beta) emitters. Radioprotection precautions must consider the specific characteristics of these emitters, and concerns are natu- rally greater in therapeutic than in diagnostic procedures. Patients who receive radiopharmaceuticals for clinical exams or therapy are radioactive for some period after activity administration and may expose other individuals and the environment during this period (Wagner et al. 1995; Siegel et al. 2002a; NCRP 1970). Conventionally, the dose estimate from radioactive patients has been performed considering the patient to be a point source and measuring the exposure rates or dose rates at a distance of 1 m from their body surface and at 1 m above the floor (NCRP 1970). The same methods have been used for point and extended sources to estimate other radiological data (e.g., activity retained in the patient). However, the procedure is not always accurate, leading to errors that may influence medical decisions, such as patient discharge based on activity retained or absorbed dose estimate to others after admin- istration of radiopharmaceuticals (NCRP 1970; Bar- rington 1996; U.S. NRC 1997; Rutar et al. 2001; ICRP 2004). When the source dimensions are small compared with the distance to the point of interest (about five to ten times less than the distance from the source), the source may be considered to be a point, and the inverse square law may be used in dose and exposure calculations (Wagner et al. 1995). The administration of radioiodine (Na 131 I) for the treatment of hyperthyroidism or thyroid cancer is a common practice. After Na 131 I ingestion, the activity is rapidly absorbed into the systemic circulation and is distributed throughout the body (Nicolau and Medeiros 1977). Neglecting self-shielding by the patient, the activity distribution is not well treated as a point source, since the activity is distributed throughout many tissues in the body, which is about 2 m in height. The point source model should only be used if the distance of the interest point from the source is 10 –20 m, and thus point source conditions rarely occur in practical situa- tions (Wagner et al. 1995). The accumulated dose rate obtained until time t at a distance r from the surface of patients has been given by eqn (1). This expression considers that the patient may be represented as a point gamma source and that no atten- uation occurs (NCRP 1970; U.S. NRC 1997): * Radiological Protection Service, Instituto de Pesquisas Ener- ge´ticas e Nucleares, Av. Prof. Lineu Prestes, 2242, Cidade Universi- taria, CEP 05508-000, Sa˜o Paulo, Brazil; † Nuclear Medicine Center, Hospital das Clı ´nicas da Faculdade de Medicina da Universidade de Sa˜o Paulo, Brazil; ‡ Nuclear Medicine Department, Sociedade Hospi- tal Samaritano, Sa˜o Paulo, Brazil; § Vanderbilt University Medical Center, Nashville, TN. For correspondence contact: J. Willegaignon, Radiological Pro- tection Service, Instituto de Pesquisas Energe´ticas e Nucleares, Av. Prof. Lineu Prestes, 2242, Cidade Universitaria, CEP 05508-000, Sa˜o Paulo, Brazil, or email at willegaignon@hotmail.com. (Manuscript accepted 25 April 2006) 0017-9078/06/0 Copyright © 2006 Health Physics Society 624