Radiation Risk Estimates in Indian Patients Due to Nuclear Cardiac Investigations

Radiation dose estimation due to administration of Tc-MIBI, 99m Tc-Tetrofosmin and TlCl is carried out using MIRD Scheme on anatomical consideration and tissue weighting factors for the Indian adult. The estimated radiation dose from these radiopharmaceuticals is about 12-14 % more as compared to ICRP adult on anatomical consideration alone and is attributed to smaller body weight and smaller organ weights. When risk factors of Indians are also considered, the estimated radiation dose is only marginally more. However, if the activity of the radiopharmaceutical is administered proportional to body weight, the estimated radiation dose is about 15% lower on anatomical consideration and about 25% lower when risk factors of Indians is also considered. It may be concluded that Indians are at lower risk of radiation health detriment in comparison to ICRP adult on administration of various cardiac-imaging radiopharmaceuticals. INTRODUCTION Radionuclide imaging is routinely done in nuclear medicine centres to assist the physician in diagnosis of clinical status of the patient. The various cardiac studies aimed to assess the extent of ischemia, regional blood flow and myocardium viability. TlCl (a cyclotron produced radionucliude) is one of the radiopharmaceutical of choice to characterise normal ischemic but viable and infracted muscle. The desirability to have a Tclabelled myocardiac agents were long felt and three agents were soon found, they are teboroxime, MIBI and tetrofosmin (Myoview). Because of Tc’s superior imaging characteristics over thallium Tl, as well as favourable dosimetry, technetium Tc-labelled cardiac-imaging agents are preferred. All the three compounds are perfusion agents and their uptake into myocardium is related to blood flow. Tc-MIBI is cleared more rapidly than teboroxime and thus is preferable from radiation dosimetric point of view and now largely used in nuclear medicine centres. From the dosimetric point of view, the new agent Tc-tetrofosmin is comparable to MIBI (1). In this present study radiation dose estimates from Tc MIBI, Tc-tetrofosmin and Tl are considered. The dose coefficients to various organs are available for these compounds (2-4). The Medical Internal Radiation Dose Scheme was used by ICRP to estimate radiation dose from administration of various radiopharmaceuticals to ICRP Reference Man. The ICRP Reference Man represents a typical Caucasian Adult of West European or North American origin. An average Indian adult differs significantly from his ICRP counterpart as regards to anatomical, physiological and metabolic characteristics, Jain et al (5) and Dang et al (6) have reported less body weight and weight of different organs for Indian adult from postmortem studies. The radiation dose (or effective dose) is estimated using tissue-weighting factors as given by ICRP-60 for ICRP Reference Man. However, for Indians, Mehta and Sarangpani (7) suggested new tissue weighting factors (called risk factors) based on cancer morbidity and mortality. The tissue risk factors are more for the organs namely bone marrow, testes, ovaries and breast but lower for other organs including those under remainder. In the present study available anatomical data of Indians and their risk factors have been utilized to estimate the radiation dose from administration of various cardiac agents namely Tc –MIBI, Tctetrofosmin and TlCl. MATERIALS AND METHODS The radiation dose to an ICRP adult is computed from organ absorbed doses using tissue-weighting factors as per ICRP-60. The weight of organs (brain, heart, lung, liver, kidneys, pancreas, spleen, thyroid) available from other studies were used to estimate dose coefficients in Indian adults by computing dose transformation factors (DTFs) using the method of Yamaguchi (8) for principle gamma energies. It is presumed that the radiopharmaceutical distribution in Indian adult is similar to that of ICRP adult & chemical composition etc. are identical to the one considered for ICRP adult. For the other organs weighing more than 20g, DTF has been computed cosidering the organ masses to be proportional to the body weight. For organs weighing ≤ 20g, DTF is taken as one. Using these DTFs and tissue weighting factors as given by ICRP-60, effective dose to Indian adult is estimated for Tc-MIBI & Tctetrofosmin (during rest and exercise) and for Tl thallous chloride and compared with the corresponding values for ICRP adult. The radiation dose to Indian adult is also estimated from these radiopharmaceuticals taking into account the risk factors for Indian population. In addition, radiation dose is estimated when the activity of the radiopharmaceutical is administered taking into cognizance of body weight of Indians. For comparison, the relative difference between radiation doses to ICRP adult to that of estimated