Dosimetry in radioisotope placentography

Radionuclide investigation of the placenta is being widely used in recent years for the diagnosis and management of vaginal bleeding in the third trimester of pregnancy. One is, therefore, concerned about the radiation exposure to the foetus during such procedures. In the present communication a precise method of estimation of radiation doses is presented. A concept termed 'effective absorbed dose constant' is utilized to enable the absorbed fractions and equilibrium absorbed dose constants to be more easily employed in radiation dose estimations. Tables of the effective absorbed dose constants for radionuclides like 131I, 123I, sup(113m)Tc, sup(99m)Tc, 67Ga, and 51Cr, are given for different masses and shapes. Masses of different organs of both mother and foetus at different periods of pregnancy and the biological turnover data for different radiopharmaceuticals are reviewed and typical values are presented. Radiation doses to different organs of both mother and foetus at the 30th week of pregnancy are finally estimated for 131I-HSA, 123I-SHA, sup(99m)Tc-HSA and sup(113m)In-chloride. The advantage of the effective absorbed dose constants in radiation dosimetry in general is discussed. The relative merits of different radiopharmaceuticals for placental investigations are brought out in comparison with antenatal pelvimetric and abdominal X-ray investigations, from the point of view of radiation doses. (author)