P e r g a m o n Appl. Radiat..;sot. Vol. 46, No. 6/7, pp. 629-630, 1995

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TREATMENT OF DATA IN THE STUDY OF ELEMENTAL CONCENTRATION OF BIOLOGICAL

MATERIALS

K H Ng I, D A Bradley 3, L M Looi2:

iDepartment of Radiology & 2pathology, University of Malaya, Kuala Lumpur, Malaysia

3Asia Lab (M) Sdn Bhd, Cheras, Kuala Lumpur, Malaysia

Studies of elemental concentrations in biological materials generally exhibit a lack of application of appropriate data handling techniques. As a consequence results can be both invalid and misleading. In the present work on breast tissue, we detail the use of rigorous statistical methodology. We have carried out a multi-elemental analysis of 46 pairs of normal and malignant human breast tissue by instrumental neutron activation analysis (INAA). This technique was used because of the limited sample masses available (typically 50 rag) and because of the general high sensitivity of INAA. The TRIGA Mark II research reactor which was used ~rovided a thermal neutron flux of the order of 1012 neutrons cm -2 s -~. In this study 12 elements were ~etermined, namely, AI, Br, Ca, CI, Co, Cs, Fe, K, Mn, Na, Rb, and Zn2 These are classified accordingly as: essential elements: Ca, CI, Co, Fe, K, Mn, Na, Zn; non- essential elements: AI, Br, Cs, Rb. Dry weight concentrations ranged from 7 ng/g for Cs up to 3 mg/g for Na.

Sampling theory I was used to determine the minimum number of paired samples required to yield a given discrimination power; this required a- priori knowledge of the approximate concentrations 2. By way of example, for the element bromine, a sample size of 50 pairs is predicted to discriminate 5.6 ~g/g difference between the means, at 80% power while for iron the corresponding value is 28.1 ~g/g.

Probability plots were applied to illustrate the log-normal and quasi- normal behaviour of distributions for all of the elements which were examined. This latter observation provided objective support for the expectation that histologically normal tissue adjacent to malignant tissue are altered by the elemental demand-and-supply of the disease process. The accompanying figure (inset) provides an example of the log- normal behaviour of Fe. Descriptive statistics are given in the form of median values together with associated 25 and 75 percentiles, geometric means and associated geometric standard deviations. Also illustrated in the figure is the use of the box-and-whisker representation for the elements AI, Br, Rb and Zn.

629

630 K.H. Ng et al.

Statistically significant differences (p<0.0003) between malignant and normal tissue have been observed in respect of all 12 elements using the Wilcoxon signed-ranks test. Finally, Spearman ranked correlation has been used as a measure of correlation between pairs of variables since their distributions are skewed and contain outlying values. A number of significant correlations at the 95% confidence level have been observed, including Ca(n) :Br(n) , Ca(n) :CI (n) , Ca(n) :Na(n) , Co(n) :Co(t), Fe(n) :Fe(t), Mn(n) :Mn(t), Zn(t) :Rb(t), where n denotes normal tissue and t tumour tissue.

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References

I. Dawson-Saunders B, and Trapp R.G. (1990) Basic and clinical

biostatistics. Appleton & Lange, Norwalk. p 116.

2. Ng K.H., Bradley D.A., Looi L.M., Mahmood C.S. and Wood A.K. (1993) Differentiation of elemental composition of normal and malignant breast tissue by instrumental neutron activation analysis. Appl. Radiat. Isot.

44, 511.