Fetal dosimetry from natural alpha-particle emitters

The alpha-particle dose to human fetal bone marrow from natural exposure was investigated. The rate of calcification and the (210)Pb activity levels in fetal vertebrae showed that (210)Pb follows the uptake of calcium into the skeleton. Lead-210-supported (210)Po activity concentrations of up to 0.18 Bq kg(-1) were found in fetal lumbar vertebra. The mean chord length of the trabecular spaces in lumbar vertebra were 110-320 micrometer at 20- 40 weeks, in rib 130-180 micrometer at 20-35 weeks, and in sternum 190 micrometer at 35 weeks of gestation. In lumbar vertebra, up to 80% of marrow was within alpha-particle range (37 micrometer) of a bone surface. This resulted in a gestational equivalent dose to marrow from (210)Pb-supported (210)Po in bone of 8 and 24 micrometerSv when the contribution from all major natural alpha-particle-emitting radionuclides was considered. The mean distance of CD34(+) cells from the nearest bone surfaces in lumbar vertebra was 61 and 46 micrometer at mid- and late gestation, respectively. The mean cellular and nuclear diameters of CD34(+) cells were 5.5 and 3.8 micrometer respectively, and remained constant with gestational age. Few stem cells were hit by alpha particles at natural exposure; however, those that were hit received doses of up to 1.3 Gy.