X-ray fluorescence analysis of lead in human skeleton in vivo.

AHLGREN, L., LIDEN, K., MATTSSON, S. and TEJNING, S. X-ray fluorescence analysis of lead in human skeleton in vivo. Scand. j. work environ. & health 2 (1976) 82-86. The lead concentration in the skeleton of living man was measured by X-ray fluorescence analysis. Five former workers from a metal industry were studied. The mean lead concentration in their skeletons was estimated to be 62 p,g/g with a standard error of ± 5 p,g/g. A comparison with the "normal" skeletal concentrations of lead in people from southern Sweden showed the skeletal concentrations of the men studied to be about three to nine times higher.

X-ray fluorescence analysis is a. well esta:blished technique used in determining the elemental composition of various laboratory samples. The introduction of solid state detectors with high energy resolution has made it possible to use this technique for in vivo studies (3). In in vivo studies the tissue volume is irradiated with X-or y-radiation and the resulting characteristic X rays are measured with a Ge(Li) or a Si(Li) spectrometer. Except for the unique occurrence of a high iodine Concentration in normal thyroid tissue (400-600 ltg/g), the stable elements studied in living man by this technique have been administered as a tracer (2).
The present work introduces the X-ray fluorescence method for in vivo determina-Reprint requests to: Dr. Soren Mattsson, Radiation Physics Department, Lasarettet, S-221 85 Lund, Sweden. 82 tion of the lead content in the skeleton of workers in the lead industry.

EXPERIMENTAL TECHNIQUE
A 20 mei 57CO source, mainly emitting 122 keY 'Y rays, was used for the generation of the characteristic X-radiation of lead. The collimator and the radiation shield consisted of high purity tin. Moreover, we eliminated the small contribution of characteristic X rays from lead impurities in the collimator by covering its opening with high purity gold.
A W mm (diameter) X 5.2 .mm Ge(Li) detector was used (energy resolution FWHM = 750 eV at 75 keY; entrance window = 0.13 mm Be). The angle between the incident and measured radiation was 90°.

IN VIVO MEASUREMENTS
Five males were studied. They had formerly worked in a metal industry and had handled much lead. They had retired 0.5--4 years before the measurements (table 1). Dudng a period of several years they had shown a high lead content in their blood.
X-ray fluorescence measurements were made at three to four sites on every man, namely, on the left and/or right forefinger ( fig. 1), on the tibia 8 cm from the ankle (position 1), on the tibia at about 20 cm from the ankle {position 2), and, in two cases, on the tibia at about 28 cm from the ankle (position 3). Fig. 2 shows a typical pulse height dis-triJbution from a measurement of a forefinger. The distribution is dominated by pulses due to the Compton scattering of 122 keV (and 136 keV) photons at about 90°. One can also recognize a peak due to

RADIATION DOSIMETRY
In the present experiment the absorbed dose at the surface of the skin was estimated to be less than 800 mrad for a 30~min irradiation period. Since the irradiated area was very small, about 3 cm 2 , the integral dose was insignificant in comparison with that of an ordinary X-ray examination.

RESULTS AND DISCUSSION
For all measuring sites on all five subjects a statistically significant net number of pulses was found in the Kat and the K a2 regions of the registered pulse height distribution {table 2).
Measurements on finger phantoms filled with distilled water or measurements on one of the authors gave no net number of pulses in the .corresponding region.
In order to estimate the lead content in the irradiated tissue seen by the detector, we measured known concentrations of lead in different phantoms for comparison.
The forefinger measurements were compared with measurements on cylindrical Perspex tubes (wall thickness = 1.5 mm) of various diameters filled with a known concentration of lead nitrate in a water solution. As expected, the counting rate in the K a peaks varied considerably with tUibe diameter. This counting rate was proportional to the irradiated water vol-  ume seen by the detector. For instance, if the inner diameter of the tube was increased from 15 to 25 mm, the counting rate increased by a factor of 2.4. In addition the counting rate in the Compton "peak" varied with tube diameter. If the outside diameter of the tube was increased from 15 to 25 mm, the counting rate increased by a factor of 2.2. The number of Compton interactions is proportional to the electron density of the scattering material. As the number of electrons per mass unit is almost independent of the atomic number, the counting rate in the Compton "peak" is proportional to the mass of the volume examined. The attenuation of the incoming primary photons and the generated characteristic lead X rays is dominated by Compton interactions both in bone and in water. Therefore, as a first approximation, the ratio between the counting rates in the lead K a peaks and the Compton "peak" is dependent only on the lead concentration and is independent of the atomic number and the density of the surrounding matrix.
In the finger measurements the "effective diameter" of the finger was estimated from the counting rate in the Compton "peak" of the phantom measurements. The relationship between counting rate in the Compton "peak" and the K a peaks obtained from the finger phantom measurements was then used to estimate the concentration in the finger.
The tibia measurements were compared with measurements on cylindrical Perspex tubes filled with a known concentration of lead nitrate in a water solution and placed in a paraffin phantom. The ratio .between the counting rate in the K a peaks and the Compton "peak" was studied for the actual geometries and used for the estimation of the lead concentration in the tibia. The tibia measurements were less accurate than those of the forefinger mainly because of certain difficulties in defining the geometry. A comparison of the tibia and the finger measurements, however, strongly indicated that there could be no significant external contamination of the fingers. Table 1 summarizes the estimated lead concentrations in the forefingers and tibia of the five persons studied. Table 1 also shows the blood lead content determined by atomic absorption spectrophotometry of blood samples obtained on the day the in vivo ·measurements were made. The mean value of the blood lead concentrations during the last 5 years of employment is also shown. According to table 1, on the average, no statistically significant difference in lead contamination between the forefinger and tibia was observed for the persons studied, and the individual lead concentrations did not differ significantly. The mean lead concentration determined for all the skeletal measurements, I.e., three to four sites on each of the five subjects, was 62 [lg/g with a standard error of ± 5 Jlg/g.
According to the unpublished results of Schiitz the "normal" blood lead concentration of people from southern Sweden is between 0.05 and 0.13 [lg/g. Using a ratio estimated from Barry and Mossman (1), between the concentration of lead in bone and in blood for males between 60 and 79 years of age, we found the "normal" concentration in the skeleton of people of the same age from southern Sweden to be between 7 and 18 [lg/g. Therefore the five men studied in this investigation had a lead content in the studied parts of their skeleton which was about three to nine times higher than the calculated "normal" mean value.