Reproducibility of flow cytometric assessment of follicular tumours of the thyroid.

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Abstract
The reproducibility of the DNA index of paraffin wax sections from 44 follicular tumours of the thyroid (18 follicular adenomas and 26 follicular carcinomas), which had been assessed by flow cytometry was analysed in two laboratories, using consecutive sections of the same specimens and two different commercially available flow cytometers. Two slightly different cell preparation and staining techniques were used in the two laboratories. Using strictly defined criteria the histograms were classified blind as diploid, peritetraploid, aneuploid, or inadequate and insufficient by two independent investigators. Both the concordance between the two different flow cytometers and the agreement of duplicate assessments within the same flow cytometers were assessed. The mean coefficient of variation of the G0/G1 peak of the diploid tumours in the PARTEC flow cytometer was 5.5 (range 2.3-9.8) and in the FACS flow cytometer 5.2 (range 3.7-8.3); this difference was not significant. There was concordance of classification between the two laboratories in 35 of 36 cases. In 25 cases (18 diploid, seven aneuploid) the intralaboratory variation showed a 100% concordance in histogram classification. It is concluded that flow cytometer DNA index assessment of follicular tumours of the thyroid is reproducible and can be used to evaluate the discriminating and prognostic value of this feature.